Prostate target volume variations during a course of radiotherapy

Application of dose compensation in image-guided radiotherapy of prostate cancer

In image-guided radiation therapy (IGRT), volumetric information on patient anatomy at treatment conditions is made available with in-room imaging devices capable of cone-beam CT. Setup error and inter-fraction rigid motion can be corrected online. The planning margin can therefore be reduced significantly. However, to compensate for uncertainties including organ deformation and intra-fraction motion, offline evaluation and replanning are necessary. The purpose of this study is to investigate the use of an offline dose compensation technique to further reduce the margin safely. In IGRT, online CT scan, rigid image registration and setup correction are performed at each fraction. Later the regions of interest are registered offline between treatment and planning CTs using a finite element method to account for non-rigid organ motion. Cumulative dose distribution is calculated and compared with the prescription dose. The discrepancy, if found significant, is repaired using the dose compensation technique, in which the cumulative dose distribution is incorporated in adaptive IMRT planning for future fractions. Two compensation schedules were tested in this study: single compensation at the end of the treatment course and compensation performed weekly. One patient with one planning CT and 16 treatment CTs were used in this simulation study. Due to the aggressive smaller planning margin used, severe underdose was observed in the clinical target volume. The size and magnitude of the underdose were reduced substantially with online guidance but were still significant. Both dose compensation strategies were able to reduce the dose deficit to an acceptable level without additional planning margin. Weekly compensation is more biologically beneficial and can spread the execution error into multiple fractions. The offline dose compensation technique allows further margin reduction and can complement the online guidance by compensating for uncertainties that cannot be reduced online, thereby increasing the confidence in IGRT delivery.

A review of prostate motion with considerations for the treatment of prostate cancer

The motion of the prostate gland can influence the efficacy of radiation therapy. This article examines the literature concerning prostate gland motion with considerations for the treatment of cancer. The objectives of this review include providing radiation oncologists, medical physicists, and dosimetrists with data to assist in determining the best treatment adaptation for individual patients. The prostate gland is not a static structure, but rather a dynamic structure and this should be a consideration in the treatment protocol. The treatment planning personnel must add a margin to the clinical treatment volume (CTV) radiation field to account for prostate motion and patient setup errors resulting in a planning treatment volume (PTV). The movement of the prostate in a radiation field with a small margin to protect the anterior rectum may allow the posterior aspect of the gland to escape the prescribed dose. Thus, an understanding of potential prostate movements in radiation therapy is critical to achieve tumor control and minimize radiation complications in patients.

Treatment planning system evaluation for mesothelioma IMRT

BACKGROUND AND PURPOSE

Malignant pleural mesothelioma (MPM) has been treated with extrapleural pneumonectomy (EPP) followed by IMRT. IMRT improved radiation dose distributions to the complex operative bed, and preliminary results suggested improved local control compared with conventional treatment planning. IMRT was initially developed on the Corvus treatment planning system. Other treatment planning systems are also IMRT-capable. Treatment plans from several systems were compared to determine the feasibility of using IMRT in a multi-institution trial.

PATIENTS AND METHODS

Treatment plans were generated on Corvus, Eclipse, and Pinnacle for a right-sided MPM after EPP using 6 MV X-rays. Tissue heterogeneity corrections were used in dose calculation. Plans were optimized such that the clinical target volume received 50 Gy in 25 fractions. Dose distributions to the target and normal structures were evaluated. The treatment time and delivery efficiency were estimated.

RESULTS

Treatment plans could be calculated by all three planning systems without system failure. Larger volumes received 60Gy in Corvus plans (40%, 17% and 8% for Corvus, Pinnacle and Eclipse, respectively). Corvus used the most monitor units (2786 versus 1451 and 1813 for Pinnacle and Eclipse), and treated the most segments (1050 versus 267 and 173 for Pinnacle and Eclipse). Doses to spinal cord, lung, heart, liver, and contralateral kidney were acceptable for all planning systems.

CONCLUSIONS

IMRT plans can be calculated for MPM targets by at least three commonly available treatment planning systems. Pinnacle- and Eclipse-based plans seem more efficient, and may be delivered in a shorter time than Corvus-based plans.

Increased risk of biochemical and local failure in patients with distended rectum on the planning CT for prostate cancer radiotherapy

PURPOSE

To retrospectively test the hypothesis that rectal distension on the planning computed tomography (CT) scan is associated with an increased risk of biochemical and local failure among patients irradiated for prostate carcinoma when a daily repositioning technique based on direct prostate-organ localization is not used.

METHODS AND MATERIALS

This study included 127 patients who received definitive three-dimensional conformal radiotherapy for prostate cancer to a total dose of 78 Gy at The University of Texas M. D. Anderson Cancer Center. Rectal distension was assessed by calculation of the average cross-sectional rectal area (CSA; defined as the rectal volume divided by length) and measuring three rectal diameters on the planning CT. The impact of rectal distension on biochemical control, 2-year prostate biopsy results, and incidence of Grade 2 or greater late rectal bleeding was assessed.

RESULTS

The incidence of biochemical failure was significantly higher among patients with distended rectums (CSA >11.2 cm(2)) on the planning CT scan (p = 0.0009, log-rank test). Multivariate analysis indicates that rectal distension and high-risk disease are independent risk factors for biochemical failure, with hazard ratios of 3.89 (95% C.I. 1.58 to 9.56, p = 0.003) and 2.45 (95% C.I. 1.18 to 5.08, p = 0.016), respectively. The probability of residual tumor without evidence of radiation treatment (as scored by the pathologist) increased significantly with rectal distension (p = 0.010, logistic analysis), and a lower incidence of Grade 2 or greater late rectal bleeding within 2 years was simultaneously observed with higher CSA values (p = 0.031, logistic analysis).

CONCLUSIONS

We found strong evidence that rectal distension on the treatment-planning CT scan decreased the probability of biochemical control, local control, and rectal toxicity in patients who were treated without daily image-guided prostate localization, presumably because of geographic misses. Therefore, an empty rectum is warranted at the time of simulation. These results also emphasize the need for image-guided radiotherapy to improve local control in irradiating prostate cancer.

Intensity-modulated radiation therapy for mesothelioma: Impact of multileaf collimator leaf width and pencil beam size on planning quality and delivery efficiency

PURPOSE

To compare treatment plans for multileaf collimators (MLCs) with different leaf widths and different finite pencil beam (FPB) sizes, to determine the planning quality and delivery efficiency of segmented MLC (SMLC) delivery of intensity-modulated radiation therapy (IMRT) for malignant pleural mesothelioma (MPM).

METHODS AND MATERIALS

Computerized tomography images of 10 right-side MPM patients were used for this planning study on a CORVUS treatment-planning system (NOMOS Corporation, Sewickley, PA) for a Varian Millennium 120-MLC (Varian Medical Systems, Palo Alto, CA). Three beam models were used. The first model forced two 0.5-cm MLC leaves to move in tandem to simulate a 1-cm leaf-width MLC and a FPB size of 1 x 1 cm2. The second model used 0.5-cm leaves with a FPB size of 0.5 x 1 cm2 (1 cm in the direction of leaf movement). The third model used 0.5-cm leaves, with a FPB size of 0.5 x 0.5 cm2. For optimization, the same dose constraints and beam parameters were used for each data set. Tissue heterogeneity corrections were used during optimization and dose calculation. Plans were optimized such that the clinical target volume received 50 Gy in 25 fractions. Dose distributions to the target and normal structures were evaluated. The number of monitor units, the number of segments, and delivery times were used to evaluate delivery efficiency.

RESULTS

All three beam models could be used for IMRT planning for MPM. The doses to clinical target volume, spinal cord, lung, liver, heart, and contralateral kidney were acceptable with all three beam models. The 0.5 x 0.5-cm2 beam model used the most monitor units (6883 +/- 974 vs. 3332 +/- 406 and 3407 +/- 443 for the 1 x 1-cm2 and 0.5 x 1-cm2 models, respectively) and treated the most segments (4297 +/- 802 vs. 1357 +/- 156 and 1767 +/- 212 for the 1 x 1-cm2 and 0.5 x 1-cm2 models, respectively). The plan generated with the 1 x 1-cm2 model required the least amount of time to deliver.

CONCLUSIONS

The quality of the MPM IMRT plans generated with the three beam models presented here was similar; however, the 1 x 1-cm2 model provided the most efficient delivery of MPM IMRT with the CORVUS planning system.

Complications of radiotherapy: improving the therapeutic index

For every course of radiotherapy treatment, the potential benefit has to be weighed against the risk of normal tissue damage. Radiation-induced proctitis during and after radical radiotherapy for prostate cancer can be decreased by reducing both the size of the target volume and the margins required around this volume. In the future, target volumes could be reduced by both CT/MRI co-registration and dose painting using MR spectroscopy of choline and citrate in the prostate. Improved immobilisation and image-guided radiotherapy should allow reduced margins without compromising the effectiveness of treatment. Similarly, in breast radiotherapy treatment, lung and cardiac complications can be reduced by better patient positioning and ensuring that doses to the heart and lung are minimised during radiotherapy treatment planning. Cosmesis can be improved by using 3D breast planning techniques rather than the conventional 2D approach. These ongoing improvements and developments in radiotherapy treatment planning are leading to treatments which offer both better tumour volume coverage, and are minimising the risk of treatment-related complications. In time, these changes should allow the escalation in dose delivered to the tumour volume with the potential for increased cure rates.

Target Definition in Prostate, Head, and Neck

Target definition is a major source of errors in both prostate and head and neck external-beam radiation treatment. Delineation errors remain constant during the course of radiation and therefore have a large impact on the dose to the tumor. Major sources of delineation variation are visibility of the target including its extensions, disagreement on the target extension, and interpretation or lack of delineation protocols. The visibility of the target can be greatly improved with the use of multimodality imaging. Both in the head and neck and the prostate, computed tomography (CT)-magnetic resonance imaging coregistration decreases the target volume and its variability. CT-positron emission tomography delineation is promising for delineation in head and neck cancer. Despite the better visibility, a different interpretation of the target extension remains a major source of error. The use of coregistration of CT with a second modality, together with improved guidelines for delineation and an online anatomical atlas, increases agreement between observers in prostate, lung, and nasopharynx tumors. Delineation errors should not be treated differently from other geometrical errors. Similar margin recipes for the correction of setup errors and organ motion should be adapted to incorporate the effect of delineation errors. A calculation of a 3-dimensional clinical target volume-planning target volume margin incorporating delineation errors for the head and neck is around 6.1 to 9.7 mm. Given the good local control of IMRT with smaller margins and smaller pathological specimens, it is likely that the delineated CTV frequently overestimates the actual volume.

Semiautomatic nonrigid registration for the prostate and pelvic MR volumes

RATIONALE AND OBJECTIVES

Three-dimensional (3D) nonrigid image registration for potential applications in prostate cancer treatment and interventional magnetic resonance (iMRI) imaging-guided therapies were investigated.

MATERIALS AND METHODS

An almost fully automated 3D nonrigid registration algorithm using mutual information and a thin plate spline (TPS) transformation for MR images of the prostate and pelvis were created and evaluated. In the first step, an automatic rigid body registration with special features was used to capture the global transformation. In the second step, local feature points (FPs) were registered using mutual information. An operator entered only five FPs located at the prostate center, left and right hip joints, and left and right distal femurs. The program automatically determined and optimized other FPs at the external pelvic skin surface and along the femurs. More than 600 control points were used to establish a TPS transformation for deformation of the pelvic region and prostate. Ten volume pairs were acquired from three volunteers in the diagnostic (supine) and treatment positions (supine with legs raised).

RESULTS

Various visualization techniques showed that warping rectified the significant pelvic misalignment by the rigid-body method. Gray-value measures of registration quality, including mutual information, correlation coefficient, and intensity difference, all improved with warping. The distance between prostate 3D centroids was 0.7 +/- 0.2 mm after warping compared with 4.9 +/- 3.4 mm with rigid-body registration.

CONCLUSION

Semiautomatic nonrigid registration works better than rigid-body registration when patient position is changed greatly between acquisitions. It could be a useful tool for many applications in the management of prostate.

Ultrasound-Based Localization

Ultrasound is a noninvasive, relatively easy, rapid, and real-time imaging technique for organ targeting for radiotherapy. Its application has been developed to a greater extent in prostate cancer than in other sites in which it has been shown to improve the accuracy of daily treatment delivery. With the move toward dose escalation and the need to maximally spare the adjacent critical structures through more conformal therapy and smaller field margins, an innovative technique for accurate and reproducible tumor targeting is mandatory. Basic ultrasound principles and organ location lend themselves well to the application of this modality in prostate cancer. Promising results using daily ultrasound-guided B-mode acquisition and targeting for patients with upper abdominal tumors suggest an area for additional trials and study. For breast cancer radiotherapy, ultrasound serves to define involved primary and nodal sites, especially in patients in whom surgical evaluation will not be the first therapeutic step.

Strategies to reduce the systematic error due to tumor and rectum motion in radiotherapy of prostate cancer

BACKGROUND AND PURPOSE

The goal of this work is to develop and evaluate strategies to reduce the uncertainty in the prostate position and rectum shape that arises in the preparation stage of the radiation treatment of prostate cancer.

PATIENTS AND METHODS

Nineteen prostate cancer patients, who were treated with 3-dimensional conformal radiotherapy, received each a planning CT scan and 8-13 repeat CT scans during the treatment period. We quantified prostate motion relative to the pelvic bone by first matching the repeat CT scans on the planning CT scan using the bony anatomy. Subsequently, each contoured prostate, including seminal vesicles, was matched on the prostate in the planning CT scan to obtain the translations and rotations. The variation in prostate position was determined in terms of the systematic, random and group mean error. We tested the performance of two correction strategies to reduce the systematic error due to prostate motion. The first strategy, the pre-treatment strategy, used only the initial rectum volume in the planning CT scan to adjust the angle of the prostate with respect to the left-right (LR) axis and the shape and position of the rectum. The second strategy, the adaptive strategy, used the data of repeat CT scans to improve the estimate of the prostate position and rectum shape during the treatment.

RESULTS

The largest component of prostate motion was a rotation around the LR axis. The systematic error (1 SD) was 5.1 degrees and the random error was 3.6 degrees (1 SD). The average LR-axis rotation between the planning and the repeat CT scans correlated significantly with the rectum volume in the planning CT scan (r=0.86, P<0.0001). Correction of the rotational position on the basis of the planning rectum volume alone reduced the systematic error by 28%. A correction, based on the data of the planning CT scan and 4 repeat CT scans reduced the systematic error over the complete treatment period by a factor of 2. When the correction was carried out later in the treatment (based on the data of more scans) the overall reduction was less. For the rectum, the first strategy performed best at the upper anterior side, where a reduction of the anterior-posterior displacement of 30% could be achieved. The systematic error could be reduced by 43% for the whole rectum by using the data of 4 repeat CT scans and the planning CT scan.

CONCLUSIONS

Both the pre-treatment as well as the adaptive correction strategy reduced the systematic error in the prostate position and rectum position and shape. A smaller systematic error makes it possible to safely reduce the margin around the clinical tumor volume, so that normal tissues can be spared or the prescription dose can be escalated.

IMRT versus conventional 3DCRT on prostate and normal tissue dosimetry using an endorectal balloon for prostate immobilization

This study was undertaken to compare prostate and normal tissue dosimetry in prostate cancer patients treated with intensity-modulated radiation therapy (IMRT) and conventional 3-dimensional conformal radiotherapy (3DCRT) using an endorectal balloon for prostate immobilization. Ten prostate cancer patients were studied using both IMRT and conventional 3DCRT at Houston Veterans Affairs Medical Center. For IMRT, the prescription was 70 Gy at 2 Gy/fraction at the 83.4% isodose line, allowing no more than 15% of the rectum and 33% of the bladder to receive above 68 and 65 Gy, respectively. For conventional 3DCRT, a 6-field arrangement with lateral and oblique fields was used to deliver 76 Gy at 2Gy/fraction, ensuring complete tumor coverage by the 72-Gy isodose line. Mean doses for prostate and seminal vesicles were 75.10 and 75.11 Gy, respectively, for IMRT and 75.40 and 75.02 Gy, respectively, for 3DCRT (p > 0.218). 3DCRT delivered significantly higher doses to 33%, 50%, and 66% volumes of rectum by 3.55, 6.64, and 10.18 Gy, respectively (p < 0.002), and upper rectum by 7.26, 9.86, and 9.16 Gy, respectively (p < 0.007). 3DCRT also delivered higher doses to femur volumes of 33% and 50% by 9.38 and 10.19 Gy, respectively, (p < 0.001). Insignificant differences in tumor control probability (TCP) values between IMRT and 3DCRT were calculated for prostate (p = 0.320) and seminal vesicles (p = 0.289). Compared to 3DCRT, IMRT resulted in significantly reduced normal tissue complication probability (NTCP) only for upper rectum (p = 0.025) and femurs (p = 0.021). This study demonstrates that IMRT achieves superior normal tissue avoidance, especially for rectum and femurs compared to 3DCRT, with comparable target dose escalation.

Theoretical foundation for real-time prostate localization using an inductively coupled transmitter and a superconducting quantum interference device (SQUID) magnetometer system

Real‐time, 3D localization of the prostate for intensity‐modulated radiotherapy can be accomplished with passively charged radio frequency transmitters and superconducting quantum interference device (SQUID) magnetometers. The overall system design consists of an external dipole antenna as a power source for charging a microchip implant transmitter and SQUID magnetometers for signal detection. An external dipole antenna charges an on‐chip capacitor through inductive coupling in the near field region through a small implant inductor. The charge and discharge sequence between the external antenna and the implant circuit can be defined by half duplex, full duplex, or sequential operations. The resulting implant discharge current creates an alternating magnetic field through the inductor. The field is detected by the surrounding magnetometers, and the location of the implant transmitter can be calculated. Problems associated with this system design are interrelated with the signal strength at the detectors, detector sensitivity, and charge time of the implant capacitor. The physical parameters required for optimizing the system for real‐time applications are the operating frequency, implant inductance and capacitance, the external dipole current and loop radius, the detector distance, and mutual inductance. Consequently, the sequential operating mode is the best choice for real‐time localization for constraints requiring positioning within 1 s due to the mutual inductance and detector sensitivity. We present the theoretical foundation for designing a real‐time, 3D prostate localization system including the associated physical parameters and demonstrate the feasibility and physical limitations for such a system.

PACS number: 87.53.‐j

Implementation and validation of a three-dimensional deformable registration algorithm for targeted prostate cancer radiotherapy

PURPOSE

Daily prostate deformation hinders accurate calculation of dose, especially to intraprostatic targets. We implemented a three-dimensional deformable registration algorithm to aid dose tracking for targeted prostate radiotherapy.

METHODS AND MATERIALS

The algorithm registers two computed tomography (CT) scans by iteratively minimizing their differences in image intensity. For validation, we measured the accuracy in registering (a) a pelvic CT set to its mathematically deformed counterpart, (b) CT scans of a deformable pelvic phantom with and without an endorectal balloon inflated, to simulate intraprostatic targets, 23 CT-opaque seeds were embedded in the prostate, and (c) two pelvic CT scans of a patient obtained on 2 separate days.

RESULTS

The mean (SD) error in registering the pelvic CT set to its transformed set was 0.5 mm (1.5), with correlation coefficient improvement from 0.626 to 0.991. Using the deformable pelvic phantom, the correlation coefficient improved from 0.543 to 0.816 after registration. The mean (SD) error in tracking the intraprostatic seeds was 0.8 mm (0.5). The correlation coefficient improved from 0.610 to 0.944 after registration of the two patient CT sets.

CONCLUSION

The algorithm had an accuracy of about 1 mm. It could be used for optimizing dose calculation and delivery for prostate radiotherapy.

Impact of setup uncertainty in the dosimetry of prostate and surrounding tissues in prostate cancer patients treated with Peacock/IMRT

The purpose of this paper was to assess the effect of setup uncertainty on dosimetry of prostate, seminal vesicles, bladder, rectum, and colon in prostate cancer patients treated with Peacock intensity-modulated radiation therapy (IMRT). Ten patients underwent computed tomography (CT) scans using the "prostate box" for external, and an "endorectal balloon" for target immobilization devices, and treatment plans were generated (T1). A maximum of +/-5-mm setup error was chosen to model dosimetric effects. Isodose lines from the T1 treatment plan were then superimposed on each patient's CT anatomy shifted by 5 mm toward the cephalad and caudal direction, generating 2 more dosimetric plans (H1 and H2, respectively). Average mean doses ranged from 74.5 to 74.92 Gy for prostate and 73.65 to 74.94 Gy for seminal vesicles. Average percent target volume below 70 Gy increased significantly for seminal vesicles, from 0.53% to 6.26%, but minimally for prostate, from 2.08% to 4.4%. Dose statistics adhered to prescription limits for normal tissues. Setup uncertainty had minimum impact on target dose escalation and normal tissue dosing. The impact of target dose inhomogeneity is currently evaluated in clinical studies.

Quantification of shape variation of prostate and seminal vesicles during external beam radiotherapy

PURPOSE

The prostate is known to translate and rotate under influence of rectal filling changes and many studies have addressed the magnitude of these motions. However, prostate shape variations also have been reported. For image-guided radiotherapy, it is essential to know the relative magnitude of translations, rotations, and shape variation so that the most appropriate correction strategy can be chosen. However, no quantitative analysis of shape variation has been performed. It is, therefore, the purpose of this article to develop a method to determine shape variation of complex organs and apply it to determine shape variation during external beam radiotherapy of a GTV (gross tumor volume) consisting of prostate and seminal vesicles.

METHODS AND MATERIALS

For this study, the data of 19 patients with prostate cancer were used. Each patient received a planning computed tomography (CT) scan and 8-12 (11 on average) repeat CT scans that were made during the course of conformal radiotherapy. One observer delineated the GTV in all scans, and volume variations were measured. After matching the GTVs for each patient for translation and rotation, a coverage probability matrix was constructed and the 50% isosurface was taken to determine the average GTV surface. Perpendicular distances between the average GTV and the individual GTVs were calculated for each point of the average GTV, and their variation was expressed in terms of local standard deviation (SD). The local SDs of the shape variation of all 19 patients were mapped onto a reference case by matching and morphing of the individual average GTVs. Repeated delineation of the GTV was done for 6 patients to determine intraobserver variation. Finally, the measured shape variation was corrected for intraobserver variation to estimate the "real" shape variation.

RESULTS

No significant variations in GTV volume were observed. The measured shape variation (including delineation variation) was largest at the tip of the vesicles (SD = 2.0 mm), smallest at the left and right side of the prostate (SD = 1.0 mm), and average elsewhere (SD = 1.5 mm). At the left, right, and cranial sides of the prostate, the intraobserver variation was of the same order of magnitude as the measured shape variation; elsewhere it was smaller. However, the accuracy of the estimated SD for intraobserver variation was about half of the accuracy of the estimated SD for the measured shape variation, giving an overall uncertainty of maximum 0.6 mm SD in the estimate of the "real" shape variation. The "real" shape variation was small at the left, right, and cranial side of the prostate (SD <0.5 mm) and between 0.5 mm and 1.6 mm elsewhere.

CONCLUSIONS

We developed a method to quantify shape variation of organs with a complex shape and applied it to a GTV consisting of prostate and seminal vesicles. Deformation of prostate and seminal vesicles during the course of radiotherapy is small (relative to organ motion). Therefore, it is a valid approximation in image-guided radiotherapy of prostate cancer, in first order, to correct only for setup errors and organ motion. Prostate and seminal vesicles deformation can be considered as a second-order effect.

Magnetic resonance assessment of prostate localization variability in intensity-modulated radiotherapy for prostate cancer

PURPOSE

To measure prostate motion with magnetic resonance imaging (MRI) during a course of intensity-modulated radiotherapy.

METHODS AND MATERIALS

Seven patients with prostate carcinoma were scanned supine on a 1.5-Tesla MRI system with weekly pretreatment and on-treatment HASTE T2-weighted images in 3 orthogonal planes. The bladder and rectal volumes and position of the prostatic midpoint (PMP) and margins relative to the bony pelvis were measured.

RESULTS

All pretreatment positions were at the mean position as computed from the on-treatment scans in each patient. The PMP variability (given as 1 SD) in the anterior-posterior (AP), superior-inferior (SI), and right-left (RL) directions was 2.6, 2.4, and 1.0 mm, respectively. The largest variabilities occurred at the posterior (3.2 mm), superior (2.6 mm), and inferior (2.6 mm) margins. A strong correlation was found between large rectal volume (>95th percentile) and anterior PMP displacement. A weak correlation was found between bladder volume and superior PMP displacement.

CONCLUSIONS

All pretreatment positions were representative of the subsequent on-treatment positions. A clinical target volume (CTV) expansion of 5.3 mm in any direction was sufficient to ascertain a 95% coverage of the CTV within the planning target volume (PTV), provided that a rectal suppository is administered to avoid rectal overdistension and that the patient has a comfortably filled bladder (<300 mL).

Intensity Modulated Radiation Therapy (IMRT) in the Management of Prostate Cancer

Intensity modulated radiation therapy (IMRT) is gaining widespread use in the radiation therapy community. Prostate cancer is the ideal target for IMRT due to the growing body of literature supporting dose escalation and normal tissue limitations. The need for dose escalation and the limits of conventional radiation therapy necessitate precise patient and prostate localization as well as advanced treatment delivery. The treatment of prostate cancer has been dramatically altered by the introduction of technology that can focus on the target while avoiding normal tissue. IMRT is evolving as the treatment of the future for prostate cancer.

Ultrasound probe pressure as a source of error in prostate localization for external beam radiotherapy

PURPOSE

A pelvic phantom was constructed to evaluate the effect of ultrasound probe pressure during performance of bipolar acquisition technique (BAT) for prostate localization for radiotherapy.

METHODS AND MATERIALS

A pelvic phantom of a gelatin mold with a water-filled balloon representing the bladder and rectum and a central encapsulated clay sphere representing the prostate was constructed. This phantom was then scanned using planning computed tomography (CT). The geometric information of the phantom was outlined in two planes. The phantom was then scanned using the BAT system with mild and moderate ultrasound probe pressure. Differences in prostate depth between the CT and BAT systems were displayed.

RESULTS

A difference of 1 cm between the phantom surface and the prostate could be produced with moderate ultrasound probe pressure. The differences were similar between the CT- and BAT-generated contours and were dependent on the ultrasound probe pressure.

CONCLUSION

Care must be taken not to cause any alteration in prostate localization with increasing ultrasound probe pressure when using BAT localization. Increased probe pressure may introduce errors in prostate localization and under dose the target.

Prostate movement during simulation resulting from retrograde urethrogram compared with “natural” prostate movement

PURPOSE

Retrograde urethrography (UG) is commonly used at the time of simulation to assist in defining the prostate apex. Some investigators have reported that performing the UG introduces error by causing prostate displacement. We investigate the movement of the prostate caused by the retrograde UG.

METHODS AND MATERIALS

Twenty-four patients treated with three-dimensional conformal radiotherapy for prostate cancer who had gold marker seeds placed into their prostates were studied. Marker seed locations at the time of simulation and on the portal images acquired just before the treatment were compared with the locations on digitally reconstructed radiographs (DRR). Movement in the superior-inferior and anteroposterior directions as seen on lateral images was measured from 402 portal images by offline customized imaging software and evaluated using analysis of variance methods for continuous variables and chi-square statistics for categoric variables.

RESULTS

"Natural" nonrandom movement of the prostate around an "origin" as defined by markers on DRR was observed. This movement tends to be in a superior and anterior direction, with the average shift being 1 mm and 0.82 mm, respectively. The magnitude of movement in the superior direction averaged 2.88 mm compared with 1.64 mm in the inferior direction (p = 0.04). There was slightly greater movement after the UG compared with mean "natural" movement but the difference was less than 3 mm in either direction on average (difference: superior-inferior = 2.64 mm, p = 0.004; anteroposterior = 2.24, p = 0.035).

CONCLUSIONS

Use of the UG induces a small but clinically insignificant displacement of the prostate when "natural" movement is taken into account.

On-line aSi portal imaging of implanted fiducial markers for the reduction of interfraction error during conformal radiotherapy of prostate carcinoma

PURPOSE

An on-line system to ensure accuracy of daily setup and therapy of the prostate has been implemented with no equipment modification required. We report results and accuracy of patient setup using this system.

METHODS AND MATERIALS

Radiopaque fiducial markers were implanted into the prostate before radiation therapy. Lateral digitally reconstructed radiographs (DRRs) were obtained from planning CT data. Before each treatment fraction, a lateral amorphous silicon (aSi) portal image was acquired and the position of the fiducial markers was compared to the DRRs using chamfer matching. Couch translation only was used to account for marker position displacements, followed by a second lateral portal image to verify isocenter position. Residual displacement data for the aSi and previous portal film systems were compared.

RESULTS

This analysis includes a total of 239 portal images during treatment in 17 patients. Initial prostate center of mass (COM) displacements in the superior, inferior, anterior, and posterior directions were a maximum of 7 mm, 9 mm, 10 mm and 11 mm respectively. After identification and correction, prostate COM displacements were <3 mm in all directions. The therapists found it simple to match markers 88% of the time using this system. Treatment delivery times were in the order of 9 min for patients requiring isocenter adjustment and 6 min for those who did not.

CONCLUSIONS

This system is technically possible to implement and use as part of an on-line correction protocol and does not require a longer than standard daily appointment time at our center with the current action limit of 3 mm. The system is commercially available and is more efficient and user-friendly than portal film analysis. It provides the opportunity to identify and accommodate interfraction organ motion and may also permit the use of smaller margins during conformal prostate radiotherapy. Further integration of the system such as remote table control would improve efficiency.

Assessment of effective dose from concomitant exposures required in verification of the target volume in radiotherapy

The requirement of the Ionising Radiation (Medical Exposure) Regulation 2000 [IR(ME)R] of justifying all exposures to ionizing radiation includes those from radiotherapy double exposure portal images resulting in exposure to normal tissues outside the treatment volume. Typical effective doses were calculated for a range of common sites using CT data to outline those parts of specific organs subject to concomitant radiation and generate dose-volume histograms. The product of the mean dose and the relative probability of inducing a fatal cancer in specific organs was used to determine a representative total effective dose in mSv per monitor unit for each site. A table of representative effective doses, ranging from 0.32 mSv to 2.56 mSv per monitor unit, was produced, which may be used to monitor cumulative effective doses of individual patients from double exposure portal images, in addition to those received from localization procedures.

Clinical and physical quality assurance for intensity modulated radiotherapy of prostate cancer

The implementation of intensity modulated radiotherapy (IMRT) for patients with prostate cancer in daily routine has been elaborated at our department. Our quality assurance (QA) concept is one method to pave the way for initiating IMRT treatments for starting institutions. A clinical quality assurance (CQA) procedure has been set-up for all patients before and throughout the course of radiotherapy. Simultaneously medical physicists established a physical quality assurance (PQA) concept that has been followed for all patients as well. Alternative CQA and PQA procedures are discussed. The literature is reviewed and discussed with special respect to quality assurance in IMRT of prostate cancer patients.

A randomized trial of supine vs. prone positioning in patients undergoing escalated dose conformal radiotherapy for prostate cancer

BACKGROUND AND PURPOSE

The optimal treatment position for patients receiving radical radiation therapy for prostate cancer has been a source of controversy. To resolve this issue, we conducted a randomized trial to evaluate the effects of supine and prone positioning on organ motion, positioning errors, and dose to critical organs during escalated dose conformal irradiation for localized prostate cancer and patient and therapist satisfaction with setup technique.

PATIENTS AND METHODS

Twenty eight patients were randomized to commence treatment immobilized in the supine or prone position and were subsequently changed to the alternate positioning for the latter half of their treatment. Patients underwent CT simulation and conformal radiotherapy planning and treatment in both positions. The clinical target volume encompassed the prostate gland. Alternate day lateral port films were compared to corresponding simulator radiographs to measure the isocentre positioning errors (IPE). Prostate motion (PM) and total positioning error (TPE) were measured from the same films by the displacements of three implanted fiducial markers. Dose volume histograms (DVHs) for the two treatment positions were compared at the 95, 80 and 50% dose (D%) levels. The patients and radiation therapists completed weekly questionnaires regarding patient comfort and ease of setup.

RESULTS

Seven patients, who started in the supine position, subsequently refused prone position and received their whole treatment supine. Small bowel in the treatment volume, not present in the supine position, prevented one patient from being treated prone. PM in anterior posterior direction was statistically significantly less in the supine position (P<0.05). There was no significant difference in superior inferior PM for the two treatment positions. No statistically significant difference between supine and prone positioning was observed in isocentre positioning error (IPE) or total positioning error (TPE) due to a policy of daily pre-treatment correction. However, more pre-treatment corrections were required for patients in the prone position. The DVH analysis demonstrated larger volumes of the bladder wall, rectal wall and small bowel within the D95, D80 and D50% when comparing the planning target volumes (PTVs) actually treated for prone positioning. When the prone PTV was expanded to account for the greater PM encountered in that position, a statistically significant difference (P<0.007) was observed in favour of the supine position at all dose levels. In the prone position, four patients had small bowel within the 60 Gray (Gy) isodose and in the supine position, no patients had small bowel in the 60 or 38Gy volumes. Supine position was significantly more comfortable for the patients and setup was significantly easier for the radiation therapists. The median patient comfort score was 0.79 (Standard deviation (SD) 0.03) supine and 0.45 (SD 0.05) prone (P<0.001) The therapist convenience of setup was 0.80 (SD 0.016) supine and 0.54 (SD 0.025) prone (P<0.005). No statistically significant difference was seen for the other parameters studied.

CONCLUSIONS

We demonstrated significantly less PM in the supine treatment position. There was no difference for either treatment position in IPE or TPE, however, more pre-treatment corrections were required in the prone position. Prone position required a larger PTV with resulting increased dose to critical organs. There were statistically significant improvements at all dose levels for small bowel, rectal wall and bladder wall doses in the supine position once corrections were made for differences in organ motion. Linear analogue scores of patient comfort and radiation therapist convenience demonstrated statistically significant improvement in favour of the supine position. Supine positioning has been adopted as the standard for conformal prostatic irradiation at our centre.

Interfractional variation in position of the uterus during radical radiotherapy for cervical cancer

BACKGROUND AND PURPOSE

This study was conducted to investigate the positional change of the uterus during radiotherapy which can degrade the accuracy of three-dimensional conformal radiotherapy (3DCRT) and intensity modulated radiotherapy (IMRT).

PATIENTS AND METHODS

Sixty-six patients received radical radiotherapy for cervical cancer in Samsung Medical Center. For each patient, two MRI scans were taken; one was before beginning radiotherapy and the other was in the third or fourth week of radiotherapy. In T2-weighted MRI images, the positional change of the uterus was quantified by measuring six parameters; the distance from the external uterine opening to the isthmus of the uterus (Dcx), the distance from the isthmus of the uterus to the uterine fundus (Dco), the perpendicular distance of the uterine body to the uterine corpus (Dco-per), the angle between the vertical line and the cervical canal in sagittal images (Acx), the uterine corpus angle from the vertical line in sagittal plan (Aco), the angle between the uterine corpus from an arbitrary bony landmark and a vertical mid line in axial images (Aco-axi)

RESULTS

Mean value of change in Dcx+Dco of tumor size during treatment was 8.0 mm in small tumors and 17.9 mm in large tumors. Among 44 anteflexed uterus patients, 5 changed into a retroflexed position. 12 patients (18%) had a greater than 30 degrees variation in any angle. For patients under 60 years, the difference in Acx was statistically significant.

CONCLUSIONS

Positional changes of the uterus during radiotherapy should be considered in the treatment planning of 3DCRT or IMRT, particularly in patients under 60 years or those with tumor size greater than 4 cm in diameter.

Evaluation of therapeutic potential of heavy ion therapy for patients with locally advanced prostate cancer

PURPOSE

To investigate the feasibility of raster scanned heavy charged particle therapy in the treatment of prostate cancer (PCa,) with special regard to the influence of internal organ motion on the dose distribution.

METHODS AND MATERIALS

The CT data of 8 patients with PCa who underwent three-dimensional conformal radiotherapy (RT) were chosen. In addition to the routine treatment planning scan, three to five additional positioning control CT scans were performed. The organs at risk and the target volumes were defined on all CT scans. Primary and boost carbon ion plans were calculated to deliver 66 Gy to the clinical target volume/planning target volume, with an additional 10 Gy to the gross tumor volume (GTV). To estimate the influence of internal organ motion on plan quality, the dose was recalculated on the basis of the control CT scans. The comparative analysis was based on the dose-volume histogram-derived physical parameters.

RESULTS

The average 90% target coverage was 99.1% for the GTV. The maximal dose to the rectum was 71.8 Gy. The average rectal mean dose was 19 Gy. The volume of the rectum receiving 70 and 68 Gy was 0.1 and 0.3 cm3. The average difference in the 90% coverage for the GTV on control CT cubes was 3.6%. The maximal rectal dose increased to 76.2 Gy. The deviation in the mean rectal dose was <1 Gy on average. The rectal volume receiving 70 and 68 Gy increased to 2.5 and 3.3 cm3.

CONCLUSION

The investigation demonstrated the feasibility of raster scanned carbon ions for PCa RT. Excellent coverage of the target volume and optimal sparing of the rectum were acquired. The combination of photon intensity-modulated RT and a carbon ion boost to the GTV is the most rational solution for the gain of clinical experience in heavy ion RT for PCa patients.

Intrafractional stability of the prostate using a stereotactic radiotherapy technique

PURPOSE

To evaluate the stability of the prostate during stereotactic radiation therapy.

MATERIALS AND METHODS

Forty-seven patients underwent placement of three fiducial markers into the prostate as part of a pilot study of hypofractionated stereotactic radiotherapy. Portal images before and subsequent to 227 radiotherapy fractions were analyzed for prostate movement. Six patients also underwent localizing radiographs at 6-min intervals for 24 min. Relative motion of the bony landmarks and prostate markers was calculated.

RESULTS

Analysis of portal images revealed the undirected average prostate movement of 2.0 mm (superior/inferior), 1.9 mm (anterior/posterior), and 1.4 mm (right/left) with maximum standard deviation (SD) of 2.0. Analysis of radiographs at 6-min intervals showed the greatest undirected average prostate motion between 0-6 min; 1.5 mm (superior/inferior), 1.4 mm (anterior/posterior), and 0.4 mm (right/left). Beyond 6 min, movements decreased to 0.4, 0.9, and 0.8 mm, respectively. Bony landmark motion was 0.9 mm (superior/inferior), 0.9 mm (anterior/posterior), and 0.4 mm (right/left) between 0-6 min. Beyond 6 min, motion decreased to less than 0.5 mm in any direction.

CONCLUSIONS

Stereotactic prostate radiotherapy, utilizing fiducial marker localization, resulted in average intrafractional prostate movement of 2.0 mm or less. Most patient and organ movement occurs early and a settling-in period is advisable before treatment.

Predictors of Extracapsular Extension and Its Radial Distance in Prostate Cancer

PURPOSE

Tightly constricted isodose lines are generated using brachytherapy or intensity-modulated radiation therapy (IMRT) treatment planning systems for prostate cancer. Definition of margins that encompass subclinical disease extension is important to maximize dose escalation while attemptingto adhere to normal tissue dose tolerances. In this study, we attempted to find predictors of extracapsular extension (ECE) and its radial distance.

MATERIALS AND METHODS

Pathological assessment of ECE and its radial distance was performed on 712 radical prostatectomy specimens. Preoperative data (initial prostate-specific antigen, clinical stage, ultrasound volume, and biopsy Gleason score) were evaluated for their ability to predict the presence of ECE and its radial distance.

RESULTS

Measurable disease was noted outside the prostatic capsule in 185 of 712 (26.0%) specimens. All preoperative parameters except ultrasound volume were able to predict the presence of ECE. However, none of them was predictive of the radial ECE distance. In this group, the median and the range of the maximum depth of invasion (radial extension from the capsule) were 2.00 and 0.5-12.00 mm, respectively. The mean radial distance from the capsule was 2.93 mm, SD +/- 2.286 mm. All subgroups had some patients with radial extension ranging from 0-2 mm, 2-5 mm, to > 5 mm. Only patients with a prostate-specific antigen of 0-4 ng/mL had no extension > 5 mm.

CONCLUSIONS

This is the largest series in the literature thus far that quantitatively assesses radial extracapsular extension. Coverage of subclinical disease must be addressed carefully before successful implementation of intensity-modulated radiation therapy, brachytherapy, or prostatectomy in order to avoid geographical miss.

Finding dose–volume constraints to reduce late rectal toxicity following 3D-conformal radiotherapy (3D-CRT) of prostate cancer

BACKGROUND AND PURPOSE

The rectum is known to display a dose-volume effect following high-dose 3D-conformal radiotherapy (3D-CRT). The aim of the study is to search for significant dose-volume combinations with the specific treatment technique and patient set-up currently used in our institution.

PATIENTS AND METHODS

We retrospectively analyzed the dose-volume histograms (DVH) of 135 patients with stage T1b-T3b prostate cancer treated consecutively with 3D-CRT between 1996 and 2000 to a total dose of 76 Gy. The median follow-up was 28 months (range 12-62). All late rectal complications were scored using RTOG criteria. Time to late toxicity was assessed using the Kaplan-Meyer method. The association between variables at baseline and > or=2 rectal toxicity was tested using chi(2) test or Fisher's exact test. A multivariate analysis using logistic regression was performed.

RESULTS

Late rectal toxicity grade > or=2 was observed in 24 of the 135 patients (17.8%). A 'grey area' of increased risk has been identified. Average DVHs of the bleeding and non-bleeding patients were generated. The area under the percent volume DVH for the rectum of the bleeding patients was significantly higher than that of patients without late rectal toxicity. On multivariate analysis the correlation between the high risk DVHs and late rectal bleeding was confirmed.

CONCLUSIONS

The present analysis confirms the role of the rectal DVH as a tool to discriminate patients undergoing high-dose 3D-CRT into a low and a high risk of developing late rectal bleeding. Based on our own results and taking into account the data published in the literature, we have been able to establish new dose-volume constraints for treatment planning: if possible, the percentage of rectal volume exposed to 40, 50, 60, 72 and 76 Gy should be limited to 60, 50, 25, 15 and 5%, respectively.

Use of portal images and BAT ultrasonography to measure setup error and organ motion for prostate IMRT: implications for treatment margins

PURPOSE

Traditionally, portal images have been used for verification of patient setup. More recently, direct prostate localization using ultrasound imaging has become available. The aim of this study was to use both modalities to measure daily setup error and prostate organ motion and their respective contributions to the overall uncertainty of prostate target localization.

METHODS AND MATERIALS

Thirty-five patients treated for prostate cancer with intensity-modulated radiotherapy (IMRT) between February 6 and July 2, 2001 underwent daily B-mode acquisition and targeting (BAT) ultrasound localization and weekly orthogonal portal imaging.

RESULTS

A total of 243 pairs of orthogonal portal films and the corresponding daily BAT images were reviewed. The mean shift +/- standard deviation in the right-left (RL), AP, and superinferior (SI) directions was 0.035 +/- 2.8 mm, -0.23 +/- 3.0 mm, and -0.013 +/- 2.0 mm, respectively, for portal films and -0.82 +/- 3.2 mm, -1.4 +/- 6.4 mm and -1.7 +/- 6.4 mm, respectively, for BAT images taken on the same day as the portal films. The mean prostate organ motion measurements were -0.89 +/- 3.3 mm (RL), -1.3 +/- 5.7 mm (AP), and -1.6 +/- 6.4 mm (SI). Without BAT localization, organ motion would have caused the clinical target volume to move outside the planning target volume margin in 23.3-41.8% of the treatments. Margins necessary to achieve complete coverage of the clinical target volume > 95% of the time without BAT would have been 5.3, 10.4 and 10.4 mm in the RL, AP, and SI dimensions, respectively.

CONCLUSIONS

Prostate organ motion appears to predominate over setup error as the major component of variation in target localization. Without the use of BAT ultrasound prostate imaging, misses of the prostate can occur in a high percentage of treatments, despite patient setup verification with portal images. Relatively large planning target volume margins in the AP and SI dimensions may be necessary to overcome this.

Prostate localization using transabdominal ultrasound imaging

PURPOSE

Adding margin around a target is done in an attempt to ensure complete coverage of the target. The B-mode acquisition and targeting (BAT) system allows ultrasound imaging of the prostate in patients with a full bladder. This provides a setup tool for patients with localized prostate cancer that takes into account real-time prostate position and may make it possible to decrease tumor margins. Prostate localization using the conventional setup verification method and daily isocenter shifts recommended by the ultrasound imaging system (BAT) were compared and analyzed.

METHODS AND MATERIALS

Daily treatment isocenter shifts for patients with localized adenocarcinoma of the prostate, obtained from two different imaging modalities, electronic portal imaging (EPI) and BAT, were calculated. We studied the difference in patient setup error calculated using BAT contour alignment and measured from EPI; the reproducibility of BAT contour alignment; intrafraction prostate motion; and how the BAT imaging procedure itself affected the prostate position. Prostate motion relative to its position during simulation was calculated by subtracting the EPI-measured isocenter shifts from the corresponding BAT-defined isocenter shifts. BAT reproducibility was measured by taking a verification BAT image after the patient was moved according to the initial BAT-defined isocenter shifts. Intrafraction prostate motion was measured by repeating BAT imaging at the end of a treatment fraction. The BAT imaging effect on prostate position was studied by examining the effect of suprapubic pressure on seed position in patients after a seed implant.

RESULTS

The mean BAT isocenter shifts for prostate motion were 0.32 +/- 0.46 cm in the lateral, 0.31 +/- 0.73 cm in the superoinferior, and 0.32 +/- 0.56 cm in the AP directions. Isocenter shifts obtained from EPI measurements were significantly smaller, with a mean of 0.05 +/- 0.24 cm in the lateral, 0.01 +/- 0.11 cm in the superoinferior and -0.11 +/- 0.29 cm in the AP directions. This larger shift seen by BAT was due to prostate motion. For BAT reproducibility, the results showed that for BAT verification images, 90% of the lateral shifts were <0.2 cm, 93% of the superoinferior shifts were <0.3 cm, and 83% of the AP shifts were <0.2 cm. The mean isocenter shift (intrafraction localization error) during patient treatment fraction was 0.02 +/- 0.28 cm in the lateral, 0.04 +/- 0.48 cm in the superoinferior, and 0.0 +/- 0.32 cm in the AP direction. The BAT procedure itself induced an average motion of 1 mm in the AP and superoinferior directions.

CONCLUSIONS

Prostate patient setup verification on the basis of bony anatomy position does not reflect the actual prostate position. BAT ultrasound target alignment provides a real-time prostate localization system that may make it possible to measure prostate position variations and reduce margins.

A comparative study of warping and rigid body registration for the prostate and pelvic MR volumes

A three-dimensional warping registration algorithm was created and compared to rigid body registration of magnetic resonance (MR) pelvic volumes including the prostate. The rigid body registration method combines the advantages of mutual information (MI) and correlation coefficient at different resolutions. Warping registration is based upon independent optimization of many interactively placed control points (CP's) using MI and a thin plate spline transformation. More than 100 registration experiments with 17 MR volume pairs determined the quality of registration under conditions simulating potential interventional MRI-guided treatments of prostate cancer. For image pairs that stress rigid body registration (e.g. supine, the diagnostic position, and legs raised, the treatment position), both visual and numerical evaluation methods showed that warping consistently worked better than rigid body. Experiments showed that approximately 180 strategically placed CP's were sufficiently expressive to capture important features of the deformation.

Commissioning and clinical implementation of a sliding gantry CT scanner installed in an existing treatment room and early clinical experience for precise tumor localization

The primary objective of the present study is to demonstrate that a unique computed tomography (CT)-linear accelerator combination can be used to reduce uncertainties caused by organ motion and setup inaccuracy. The acceptance, commissioning, and clinical implementation of a sliding gantry CT scanner installed in an existing linear accelerator room are reported in this paper. A Siemens CT scanner was installed directly opposite to an existing accelerator. The scanner is movable on a pair of horizontal rails mounted parallel to the longitudinal axis of the treatment couch replaced with a carbon fiber tabletop. Acceptance and commissioning of the CT scanner were verified with phantom studies. For clinical implementation, quality assurance (QA) procedures have been instituted to ensure the integrity of the CT gantry axis alignment and the accuracy of its movement using a phantom designed in house. A clinical example employing the CT-Linac combination to correct the isocenter positioning caused by organ motion and setup inaccuracy was presented for a prostate irradiation. Dose calculations were performed to study the effects on tumor coverage without the adjustments of the isocenter. A summary of the isocenter adjustments for the first 30 patients is also presented. The geometric accuracy of the CT scanner is < or =1 mm. An isocenter deviation of > or =2 mm from the original plan can be detected. For the clinical example of a prostate patient, the average movement of the prostate gland was found to be approximately 3mm in the anterior-posterior (AP/PA) direction and 5 mm in the cephalic-caudal direction. Variations in the isocenter position may result in underdosage of the PTV if correction is not made for the change in the isocenter position. Our experience with the first 30 patients indicates that while the left-right adjustment of the isocenter is minimal, in the AP/PA direction, about 33% of treatments required an adjustment of 3-5 mm, and about 18% required a 5.1-mm to 10-mm adjustment. In the caudal-cephalic direction, about 26% required an adjustment of 3-5 mm, and 8% required a 5.1-mm to 10-mm adjustment. Retrofitting a CT scanner in an existing linear accelerator room requires careful planning and well-coordinated efforts from all personnel involved. Special QA procedures are needed to ensure the mechanical integrity and imaging accuracy of the CT scanner. A CT scan of the patient prior to irradiation provides valuable information on organ motion. Any deviations from treatment plan can be corrected before dose delivery. Significant deviation from the planning isocenter may occur due to daily variations in the rectal filling. The CT-Linac combination has significant implications for the treatment of prostate cancer.

Experience of ultrasound-based daily prostate localization

PURPOSE

The NOMOS (Sewickley, PA) B-mode Acquisition and Targeting System (BAT) ultrasound system provides a rapid means of correcting for interfraction prostate positional variation. In this investigation, we report our experience on the clinical issues relevant to the daily use of the BAT system and the analysis of combined setup error and organ motion for 3509 BAT alignment procedures in 147 consecutive patients treated with IMRT for prostate cancer.

METHODS AND MATERIALS

After setup to external skin marks, therapists performed the BAT ultrasound alignment procedure before each IMRT treatment. In this study, a single physician (A.C.) reviewed all BAT images and classified image quality and accuracy of image alignment by the therapist. On a scale of 1-3, near-perfect image quality or alignment was given a 1, fair image quality or misalignment > or = 5 mm (likely within the PTV) was given a 2, and unacceptable image quality or misalignment >5 mm (potential to violate the PTV) was given a value of 3. The distribution of shifts made was analyzed in each dimension and for all patients. The time required to perform the BAT alignment was also assessed in 17 patients.

RESULTS

Among the 3509 attempted BAT procedures, the image quality was judged to be poor or unacceptable in 5.1% (181). Of the remaining 3328 BAT images, with quality scores of 1-2, alignments were unacceptable (>5 mm misalignment as judged by the reviewing physician) in 3% (100). The mean shift in each direction, averaged over all patients, was 0.5-0.7 mm. Interfraction standard deviation (1 SD) of prostate position based on combined setup error and internal organ motion is 4.9 mm, 4.4 mm, and 2.8 mm in the anteroposterior (AP), superior-inferior (SI), and lateral (RL) dimensions, respectively. The distribution of the shifts was a near-random Gaussian-type in all three major axes, with greater variations in AP and SI directions. The percent of BAT procedures in which the shift was >5 mm was 28.6% in AP, 23% in SI, and 9% in RL directions. The average BAT procedure took extra 5 min out of a 20-min time slot in a typical eight-field IMRT treatment.

CONCLUSIONS

The quality of the daily ultrasound images was deemed acceptable in 95%. Major alignment errors by therapists were only 3%. The BAT system is clinically effective and feasible in a matter of 5 min. Although the accuracy of the BAT was not addressed in this investigation, we found a significant percentage of large shifts being made from the initial alignment position.

Interfractional and intrafractional accuracy during radiotherapy of gynecologic carcinomas: a comprehensive evaluation using the ExacTrac system

PURPOSE

To evaluate positioning uncertainties with an infrared body marker-based positioning system (ExacTrac) compared with conventional laser positioning in patients treated for gynecologic carcinomas, and to investigate patient movement during therapy.

MATERIALS AND METHODS

Ten patients were positioned both with a conventional laser system and with the ExacTrac system. Positioning accuracy was evaluated using repeated electronic portal images. Average displacements and overall, systematic, and random errors were calculated and compared for the two positioning methods. Further, inter- and intrafractional patient movement including time trends in positioning displacements, respiratory amplitudes, and breathing frequencies were analyzed by online documentation of body marker movement with the ExacTrac system.

RESULTS

Average displacements ranged between -3.6 and 6.7 mm for the three coordinates. Mean systematic and random errors ranged from 1.6 to 3.7 mm and 2.2 to 3.7 mm, respectively, with no significant differences between conventional and ExacTrac positioning (p > 0.07). The main breathing direction was from dorsocaudal to anterocranial in 9 of 10 patients. The mean 3D breathing amplitude in the pelvis was 2.4 mm (0.49-6.96 mm). Significant interfractional and intrafractional time trends were observed concerning breathing amplitudes and positioning displacements.

CONCLUSIONS

The observed displacements did not vary significantly between the two evaluated positioning systems. The analysis of registered body marker positions revealed a wide variation in respiratory frequencies, breathing amplitudes, and patient displacements with interfractional and intrafractional time trends. Systems that allow the measurement of each patient's motion characteristics are a necessary requirement for all efforts at individually tailored radiation therapy.

Mindless or mindful? Radiation oncologists' perspectives on the evolution of prostate cancer treatment

The evolution of radiation therapy treatment for prostate cancer has been striking over the last 10 years. Advances in brachytherapy (BT), external beam radiotherapy (EBRT), and the combination of EBRT + BT have led to improved biochemical and clinical results. This article describes these advances in the context of the treatment decision process. Key to this process is the assignment of patient risk, which is based on the results of conventional radiation dose and techniques. Using the 1992 AJCC palpation staging system, Gleason score, and pretreatment prostate-specific antigen, two different risk assessment algorithms were compared. Both gave comparable approximations of risk, although the single factor high-risk model was superior in differentiating those patients with the highest probability of failing treatment after radiotherapy. Such criteria are the foundation for treatment selection. Objective findings support BT alone or EBRT alone for low-risk patients, high-dose EBRT or EBRT + BT for intermediate-risk patients, and EBRT + androgen deprivation for high-risk patients. In summary, advances in radiation oncology have led to significant gains in prostate cancer control. Clinical prognostic factor-based patient selection is central to the optimization of outcome.

IMRT for prostate cancer: defining target volume based on correlated pathologic volume of disease

PURPOSE

The intensity-modulated radiation therapy (IMRT) treatment planning system generates tightly constricted isodose lines. It is very important to define the margins that are acceptable in the treatment of prostate cancer to maximize the dose escalation and normal tissue avoidance advantages offered by IMRT. It is necessary to take into account subclinical disease and the potential for extracapsular spread. Organ and patient motion as well as setup errors are variables that must be minimized and defined to avoid underdosing the tumor or overdosing the normal tissues. We have addressed these issues previously. The purpose of the study was twofold: to quantify the radial distance of extracapsular extension in the prostatectomy specimens, and to quantify differences between the pathologic prostate volume (PPV), CT-based gross tumor volume (GTV), and planning target volume (PTV).

MATERIALS AND METHODS

Two related studies were undertaken. A total of 712 patients underwent prostatectomy between August 1983 and September 1995. Pathologic assessment of the radial distance of extracapsular extension was performed. Shrinkage associated with fixation was accounted for with a linear shrinkage factor. Ten patients had preoperative staging studies including a CT scan of the pelvis. The GTV was outlined and volume determined from these CT scans. The PTV, defined as GTV with a 5-mm margin in all dimensions, was then calculated. The Peacock inverse planning system (NOMOS Corp., Sewickley, PA) was used. The PPV, GTV, and PTV were compared for differences and evaluated for correlation.

RESULTS

Extracapsular extension (ECE) (i.e., prostatic capsular invasion level 3 [both focal and established]) was found in 299 of 712 patients (42.0%). Measurable disease extending radially outside the prostatic capsule (i.e., ECE level 3 established) was noted in 185 of 712 (26.0%). The median radial extension was 2.0 mm (range 0.50-12.00 mm) outside the prostatic capsule. As a group, 20 of 712 (2.8%) had extracapsular extension of more than 5 mm. In the volumetric comparison and correlation study of the GTV and PTV to the PPV, the average GTV was 2 times larger than the PPV. The average PTV was 4.1 times larger than the PPV.

CONCLUSIONS

This is the largest series in the literature quantitatively assessing prostatic capsular invasion (i.e., the radial extracapsular extension). It is the first report of a comparison of PPV to CT-planned GTV and PTV. Using patient and prostate immobilization, 5 mm of margin to the GTV in this study provided sufficient coverage of the tumor volume based on data gathered from 712 patients. In the absence of prostate immobilization, additional margins of differing amounts depending on the technique employed would have to be placed to account for target, patient, and setup uncertainties. The large mean difference between CT-based estimates of the tumor volume and target volume (GTV+PTV) and PPV added further evidence for adequacy of tumor coverage. Target immobilization, setup error, and coverage of subclinical disease must be addressed carefully before successful implementation of IMRT to maximize its ability to escalate dose and to spare normal tissue simultaneously and safely.

Feasibility of insertion/implantation of 2.0-mm-diameter gold internal fiducial markers for precise setup and real-time tumor tracking in radiotherapy

PURPOSE

To examine the feasibility and reliability of insertion of internal fiducial markers into various organs for precise setup and real-time tumor tracking in radiotherapy (RT).

MATERIALS AND METHODS

Equipment and techniques for the insertion of 2.0-mm-diameter gold markers into or near the tumor were developed for spinal/paraspinal lesions, prostate tumors, and liver and lung tumors. Three markers were used to adjust the center of the mass of the target volume to the planned position in spinal/paraspinal lesions and prostate tumors (the three-marker method). The feasibility of the marker insertion and the stability of the position of markers were tested using stopping rules in the clinical protocol (i.e., the procedure was abandoned if 2 of 3 or 3 of 6 patients experienced marker dropping or migration). After the evaluation of the feasibility, the stability of the marker positions was monitored in those patients who entered the dose-escalation study.

RESULTS

Each of the following was shown to be feasible: bronchoscopic insertion for the peripheral lung; image-guided transcutaneous insertion for the liver; cystoscopic and image-guided percutaneous insertion for the prostate; and surgical implantation for spinal/paraspinal lesions. Transcutaneous insertion of markers for spinal/paraspinal lesions and bronchoscopic insertion for central lung lesions were abandoned. Overall, marker implantation was successful and was used for real-time tumor tracking in RT in 90 (90%) of 100 lesions. No serious complications related to the marker insertion were noted for any of the 100 lesions. Using three markers surgically implanted into the vertebral bone, the mean +/- standard deviation in distance among the three markers was within 0.2 +/- 0.6 mm (range -1.4 to 0.8) through the treatment period of 30 days. The distance between the three markers gradually decreased during RT in five of six prostate cancers, consistent with a mean rate of volume regression of 9.3% (range 0.015-13%) in 10 days.

CONCLUSIONS

Internal 2.0-mm-diameter gold markers can be safely inserted into various organs for real-time tumor tracking in RT using the prescribed equipment and techniques. The three-marker method has been shown to be a useful technique for precise setup for spinal/paraspinal lesions and prostate tumors.

A study of the effects of internal organ motion on dose escalation in conformal prostate treatments

BACKGROUND AND PURPOSE

To assess the effect of internal organ motion on the dose distributions and biological indices for the target and non-target organs for three different conformal prostate treatment techniques.

MATERIALS AND METHODS

We examined three types of treatment plans in 20 patients: (1) a six field plan, with a prescribed dose of 75.6 Gy; (2) the same six field plan to 72 Gy followed by a boost to 81 Gy; and (3) a five field plan with intensity modulated beams delivering 81 Gy. Treatment plans were designed using an initial CT data set (planning) and applied to three subsequent CT scans (treatment). The treatment CT contours were used to represent patient specific organ displacement; in addition, the dose distribution was convolved with a Gaussian distribution to model random setup error. Dose-volume histograms were calculated using an organ deformation model in which the movement between scans of individual points interior to the organs was tracked and the dose accumulated. The tumor control probability (TCP) for the prostate and proximal half of seminal vesicles (clinical target volume, CTV), normal tissue complication probability (NTCP) for the rectum and the percent volume of bladder wall receiving at least 75 Gy were calculated.

RESULTS

The patient averaged increase in the planned TCP between plan types 2 and 1 and types 3 and 1 was 9.8% (range 4.9-12.5%) for both, whereas the corresponding increases in treatment TCP were 9.0% (1.3-16%) and 8.1% (-1.3-13.8%). In all patients, plans 2 and 3 (81 Gy) exhibited equal or higher treatment TCP than plan 1 (75.6 Gy). The maximum treatment NTCP for rectum never exceeded the planning constraint and percent volume of bladder wall receiving at least 75 Gy was similar in the planning and treatment scans for all three plans.

CONCLUSION

For plans that deliver a uniform prescribed dose to the planning target volume (PTV) (plan 1), current margins are adequate. In plans that further escalate the dose to part of the PTV (plans 2 and 3), in a fraction of the cases the CTV dose increase is less than planned, yet in all cases the TCP values are higher relative to the uniform dose PTV (plan 1). Doses to critical organs remain within the planning criteria.

Neoadjuvant androgen deprivation and prostate gland shrinkage during conformal radiotherapy

PURPOSE

The shrinking effect of 3-month neoadjuvant androgen deprivation (NAD) on preradiotherapy prostate gland volume is well documented. However, recently, it has been shown that the cancerous prostate gland keeps shrinking up to 12 months after NAD start. Thus, if such a reduction is not taken into account, a larger than planned portion of the surrounding normal tissues might shift in the high-dose region during conformal radiotherapy (3DCRT) course. The present study was undertaken to quantify this issue.

MATERIALS AND METHODS

Prostate gland volume reduction between planning CT (plCT) and the last week of 3DCRT (tmtCT) was prospectively assessed in 33 consecutive patients with localized prostate carcinoma. The median time interval between plCT and tmtCT was 2.5 months (2.1-2.7 months). A single observer was asked to draw on each slice prostate gland volume as appropriate. The observer was 'blind' to the timing of CT (plCT vs. tmtCT). In order to estimate intra-observer variability, prostate gland delineation was repeated twice for each data set. Mean prostate gland change, plCT and tmtCT cumulative dose volume histogram (DVH) calculations for the rectum were analyzed for each patient. Results were correlated to AD status and its duration before plCT. Means were compared by non-parametric rank tests.

RESULTS

Based on an internal protocol, 14 patients (42%) did not receive AD, while 19 patients (58%) had undergone neoadjuvant and concomitant AD. The median duration of AD before plCT ranged from 0.2 to 6 months (median: 2.9 months). Although individual data were highly variable, compared to plCT volume, mean prostate gland volume change at the end of 3DCRT was similar for patients receiving (-7.3%) or not (-7%) androgen deprivation (P=0.77). However, within the group of patients treated with hormones, patients starting AD within 3 months from plCT had a significantly larger reduction in prostate volume (-14.2%) than patients with longer NAD duration (-1.1%, P=0.03). At tmtCT, on average, patients undergoing 3DCRT within 3 months from AD start showed an increase of the amount of rectum receiving 40-75 Gy compared to plCT values. At 40 Gy (V40) the mean difference between tmtCT and plCT was +7.5%. In the other two groups, average variations of V40-70 were within +/-2% of plCT values. However, these differences are not significant.

CONCLUSION

For patients who undergo plCT and 3DCRT shortly after AD start, prostate gland shrinkage may be substantial. In some of these patients, this might lead to an unexpected increase of the percentage of rectal wall exposed to intermediate doses.

Optimal stochastic correction strategies for rigid-body target motion

PURPOSE

To derive optimal correction strategies for setup errors, including the uncertainty in their measurement, and to analyze their impact on treatment margins.

METHODS AND MATERIALS

New concepts like image-guided radiotherapy aim to provide an increasing amount of targeting information during treatment. Future treatment devices incorporating imaging capabilities will facilitate frequent correction of treatment setup errors. It is, therefore, possible to design new correction protocols that reduce not only systematic but also random setup errors. A novel, very general approach to developing optimal correction strategies in the presence of measurement uncertainties is derived from linear systems theory. In the simplest approach, the state variable of the system, which represents the patient, is the spatial displacement of the center-of-mass of the clinical target volume with respect to the planning CT. This displacement is the sum of a systematic and a random component. Uncertainties in the measured value of the state variable due to the measurement process, image processing technique, or organ deformation are naturally incorporated into a linear system. The true value of the displacement can be estimated from the noisy measurements with a stochastic filter (Kalman filter). These estimates provide an optimal control law for the system and therefore optimal values for the setup corrections. In the case of unknown systematic and random error variances, an adaptive version of the filter was implemented. The statistical properties of the filter were investigated by performing simulations of the state space model and assessed for individual patients and a large patient population subject to different action criteria.

RESULTS

Over a patient population, the corrections by the Kalman filter estimates are always advantageous compared with the corrections by the measured values themselves. For a small percentage of individual patients, however, the Kalman corrections worsen the results. For large measurement error, the residual standard deviation of the random setup errors can be reduced by approximately 28% for over 90% of the patients. The uncertainty in the measured value impairs the ability to completely account for uncertainties.

CONCLUSIONS

The Kalman estimates provide an effective means to perform daily setup corrections in the presence of measurement errors. The linear system approach is very versatile and can be extended to more general state variables.

Entropy-based dual-portal-to-3-DCT registration incorporating pixel correlation

For patient setup verification in external beam radiotherapy (EBRT) of prostate cancer, we developed an information theoretic registration framework, called the minimax entropy registration framework, to simultaneously and iteratively segment portal images and register them to three-dimensional (3-D) computed tomography (CT) image data. The registration framework has two steps, the max step and the min step, and evaluates appropriate entropies to estimate segmentations of the portal images and to find the transformation parameters. In the initial version of the algorithm (Bansal et al. 1999), we assumed image pixels to be independently distributed, an assumption not true in general. Thus, to better segment the portal images and to improve the accuracy of the estimated registration parameters, in this initial formulation of the problem, the correlation among pixel intensities is modeled using a one-dimensional Markov random process. Line processes are incorporated into the model to improve the estimation of segmentation of the portal images. In the max step, the principle of maximum entropy is invoked to estimate the probability distribution on the segmentations. The estimated distribution is then incorporated into the min step to estimate the registration parameters. Performance of the proposed framework is evaluated and compared to that of a mutual information-based registration algorithm using both simulated and real patient data. In the proposed registration framework, registration of the 3-D CT image and the portal images is guided by an estimated segmentation of the pelvic bone. However, as the prostate can move with respect to the pelvic structure, further localization of the prostate using ultrasound image data is required, an issue to be further explored in future.

The use of rectal balloon during the delivery of intensity modulated radiotherapy (IMRT) for prostate cancer: more than just a prostate gland immobilization device?

PURPOSE

The purpose of this study was to investigate the role of a rectal balloon for prostate immobilization and rectal toxicity reduction in patients receiving dose-escalated intensity-modulated radiotherapy for prostate cancer.

PATIENTS AND METHODS

Patients with localized prostate cancer who were undergoing intensity-modulated radiotherapy were treated in a prone position, immobilized with a customized Vac-Lok bag (MED-TEC, Orange City, IA). A rectal balloon with 100 cc of air was used to immobilize the prostate. The prostate displacements were measured using computed tomography (CT)-CT fusion on 10 patients who received radioactive seed implant before intensity-modulated radiotherapy. They were scanned twice weekly during 5 weeks of intensity-modulated radiotherapy, and breathing studies were also performed. Rectal toxicity was evaluated by use of Radiation Therapy Oncology Group scoring in 100 patients. They were treated to a mean dose of 76 Gy over 35 fractions (2.17-Gy fraction size). Dose-volume histogram of the rectum was assessed. A film phantom was constructed to simulate the 4-cm diameter air cavity that was created by the rectal balloon. Kodak XV2 films (Rochester NY) were used to measure and compare dose distribution with and without the air cavity. A fraction of 1.25 Gy was delivered to the phantom at isocenter with 15-MV photons by use of the NOMOS Peacock system and the MIMiC treatment delivery system (Sewickley, PA).

RESULTS

The anterior-posterior and lateral prostate displacements were minimal, on the order of measurement uncertainty (approximately 1 mm). The standard deviation of superior-inferior displacement was 1.78 mm. Breathing studies showed no organ displacement during normal breathing when the rectal balloon was in place. The rectal toxicity profile was very favorable: 83% (83/100) patients had no rectal complaint, and 11% and 6% had grade 1 and 2 toxicity, respectively. Dose-volume histogram analysis revealed that in all of the patients, no more than 25% of the rectum received 70 Gy or greater. As visualized by film dosimetry, the dose at air-tissue interface was approximately 15% lower than that without an air cavity. The dose built up rapidly so that at 1 and 2 mm, the differential was approximately 8% and 5%, respectively. The dosimetric coverage at the depth of the posterior prostate wall was essentially equal, with or without the air cavity.

DISCUSSION

The use of a rectal balloon during intensity-modulated radiotherapy significantly reduces prostate motion. Prostate immobilization thus allows a safer and smaller planning target volume margin. It has also helped spare the anterior rectal wall (by its dosimetric effects) and reduced the rectal volume that received high-dose radiation (by rectal wall distension). All these factors may have further contributed to the decreased rectal toxicity achieved by intensity-modulated radiotherapy, despite dose escalation and higher-than-conventional fraction size.

On-line correction of beam portals in the treatment of prostate cancer using an endorectal balloon device

BACKGROUND

Reproducible target volume assessment is required in order to optimize portal field margins in the treatment of prostate cancer. The benefits of an endorectal balloon on target volume assessment remain unclear.

MATERIAL AND METHODS

Nine patients were treated with a daily placed air filled rectal balloon. Portal films and computer-associated tomography during the treatment were used to determine the position of the structures of interest. Comparative planning with or without a balloon was performed in order to determine rectal wall exposure to radiation.

RESULTS

The range of movements during treatment predicting the position of the prostate in relation to the symphysis was 0.05-0.59 cm in the lateral direction, 0.27-2.2 cm in the antero-posterior direction, and 0.33-1.8 cm in the crano-caudal direction, as compared to the position of the prostate predicted by the balloon ranging from 0.18 to 0.76 cm in the lateral direction, 0.22-1.68 cm in the antero-posterior direction, and 0.58-2.99 cm in the crano-caudal direction. Planning target volumes (PTV) margins as defined by the position of the balloon were 10 mm in the antero-posterior direction, 6 mm in the lateral direction, and 16 mm in the crano-caudal direction. The volume of rectal wall exposed to radiation was reduced from 40 (+/- 12%) to 25% (+/- 19%) with an endorectal balloon (P < 0.05).

CONCLUSIONS

Daily online correction with portal vision for external beam set-up is improved by an endorectal balloon device, leading to improved PTV margins and reduced radiation exposure of the rectal wall.

Measurement of intrafractional prostate motion using magnetic resonance imaging

PURPOSE

To quantify the three-dimensional intrafractional prostate motion over typical treatment time intervals with cine-magnetic resonance imaging (cine MRI) studies.

METHODS AND MATERIALS

Forty-two patients with prostate cancer were scanned supine in an alpha cradle cast using cine MRI. Twenty sequential slices were acquired in the sagittal and axial planes through the center of the prostate. Each scan took approximately 9 min. The posterior, lateral, and superior edges of the prostate were tracked on each frame relative to the initial prostate position, and the size and duration of each displacement was recorded.

RESULTS

The prostate displacements were (mean +/- SD): 0.2 +/- 2.9 mm, 0.0 +/- 3.4 mm, and 0.0 +/- 1.5 mm in the anterior-posterior, superior-inferior, and medial-lateral dimensions respectively. The prostate motion appeared to have been driven by peristalsis in the rectum. Large displacements of the prostate (up to 1.2 cm) moved the prostate both anteriorly and superiorly and in some cases compressed the organ. For such motions, the prostate did not stay displaced, but moved back to its original position. To account for the dosimetric consequences of the motion, we also calculated the time-averaged displacement to be approximately 1 mm.

CONCLUSIONS

Cine MRI can be used to measure intrafractional prostate motion. Although intrafractional prostate motions occur, their effects are negligible compared to interfractional motion and setup error. No adjustment in margin is necessary for three-dimensional conformal or intensity-modulated radiation therapy.

Implementation and utility of a daily ultrasound-based localization system with intensity-modulated radiotherapy for prostate cancer

PURPOSE

To evaluate the clinical feasibility of daily computer-assisted transabdominal ultrasonography for target position verification in the setting of intensity-modulated radiotherapy (IMRT) for prostate cancer.

METHODS AND MATERIALS

Twenty-three patients with clinically localized prostate cancer were treated using a sequential tomotherapy IMRT technique (Peacock) and daily computer-assisted transabdominal ultrasonography (BAT) for target localization. Patients were instructed to maintain a full bladder and were placed in the supine position using triangulation tattoos and a leg immobilizer to minimize pelvic rotation. The BAT ultrasound system is docked to the treatment collimator and electronically imports the CT simulation target contours and isocenter. The system is able to use the machine isocenter as a reference point to overlay the corresponding CT contours onto the ultrasound images captured in the transverse and sagittal planes. A touch screen menu is used to maneuver the CT contours in three dimensions such that they match the ultrasound images. The system then displays the three-dimensional couch shifts required to produce field alignment. Data were prospectively collected to measure the frequency by which useful ultrasound images were obtained, the amount of time required for localization/setup, and the direction/magnitude of the positional adjustments.

RESULTS

Of the 23 patients, the BAT ultrasound system produced images of sufficient quality to perform the overlay of the CT contours in 19 patients such that positional verification could be reliably performed. Poor image quality was associated with patient inability to maintain a full bladder, large body habitus, or other anatomic constraints. Of the 19 assessable patients, a total of 185 treatment alignments were performed (mean 8.8/patient). For all cases, the average time required for the daily ultrasound imaging and positional adjustments was 11.9 min. After the initial 5 cases, the user experience skills improved such that the time required for image verification/positional adjustments decreased to a mean of 5.6 min. The average right-left, AP, and cranial-caudal adjustment was 2.6 +/- 2.1 mm, 4.7 +/- 2.7 mm, and 4.2 +/- 2.8 mm, respectively. Positional adjustments >10 mm were infrequent and related primarily to misidentification of the target structures on the ultrasound image, patient movement, or improper registration of the triangulation tattoos.

CONCLUSION

Daily computer-assisted BAT ultrasound positional verification of the prostate can be successfully performed through the acquisition of high-quality images in most patients with only a modest increase in treatment setup time. Positional data obtained with this system resulted in clinically meaningful adjustments in daily setup for sequential IMRT that would not be otherwise apparent from other verification modalities.

Intrafraction prostate motion during IMRT for prostate cancer

PURPOSE

Although the interfraction motion of the prostate has been previously studied through the use of fiducial markers, CT scans, and ultrasound-based systems, intrafraction motion is not well documented. In this report, the B-mode, Acquisition, and Targeting (BAT) ultrasound system was used to measure intrafraction prostate motion during 200 intensity-modulated radiotherapy (IMRT) sessions for prostate cancer.

METHODS AND MATERIALS

Twenty men receiving treatment with IMRT for clinically localized prostate cancer were selected for the study. Pre- and posttreatment BAT ultrasound alignment images were collected immediately before and after IMRT on 10 treatment days for a total of 400 BAT alignment procedures. Any ultrasound shifts of the prostate borders in relation to the planning CT scan were recorded in 3 dimensions: right-left (RL), anteroposterior (AP), and superior-inferior (SI). Every ultrasound procedure was evaluated for image quality and alignment according to a 3-point grading scale.

RESULTS

All the BAT images were judged to be of acceptable quality and alignment. The dominant directions of intrafraction prostate motion were anteriorly and superiorly. The mean magnitude of shifts (+/-SD) was 0.01 +/- 0.4 mm, 0.2 +/- 1.3 mm, and 0.1 +/- 1.0 mm in the left, anterior, and superior directions, respectively. The maximal range of motion occurred in the AP dimension, from 6.8 mm anteriorly to 4.6 mm posteriorly. The percentage of treatments during which prostate motion was judged to be <or=5 mm was 100%, 99%, and 99.5% in the RL, AP, and SI directions, respectively. Three of the measurements were >5 mm. The extent of intrafraction motion was much smaller than that of interfraction motion. Linear regression analysis showed very little correlation between the two types of motion (r = 0.014, 0.029, and 0.191, respectively) in the RL, AP, and SI directions.

CONCLUSION

Using an ultrasound-based system, intrafraction prostate motion occurred predominantly in the anterior and superior directions, but was clinically insignificant. Intrafraction motion was much smaller than interfraction motion, and the two types of movement did not correlate.

Automatic MR volume registration and its evaluation for the pelvis and prostate

A three-dimensional (3D) mutual information registration method was created and used to register MRI volumes of the pelvis and prostate. It had special features to improve robustness. First, it used a multi-resolution approach and performed registration from low to high resolution. Second, it used two similarity measures, correlation coefficient at lower resolutions and mutual information at full resolution, because of their particular advantages. Third, we created a method to avoid local minima by restarting the registration with randomly perturbed parameters. The criterion for restarting was a correlation coefficient below an empirically determined threshold. Experiments determined the accuracy of registration under conditions found in potential applications in prostate cancer diagnosis, staging, treatment and interventional MRI (iMRI) guided therapies. Images were acquired in the diagnostic (supine) and treatment position (supine with legs raised). Images were also acquired as a function of bladder filling and the time interval between imaging sessions. Overall studies on three patients and three healthy volunteers, when both volumes in a pair were obtained in the diagnostic position under comparable conditions, bony landmarks and prostate 3D centroids were aligned within 1.6 +/- 0.2 mm and 1.4 +/- 0.2 mm, respectively, values only slightly larger than a voxel. Analysis suggests that actual errors are smaller because of the uncertainty in landmark localization and prostate segmentation. Between the diagnostic and treatment positions, bony landmarks continued to register well, but prostate centroids moved towards the posterior 2.8-3.4 mm. Manual cropping to remove voxels in the legs was necessary to register these images. In conclusion, automatic, rigid body registration is probably sufficiently accurate for many applications in prostate cancer. For potential iMRI-guided treatments, the small prostate displacement between the diagnostic and treatment positions can probably be avoided by acquiring volumes in similar positions and by reducing bladder and rectal volumes.

Prostate immobilization using a rectal balloon

We use a rectal balloon for prostate immobilization during intensity modulated radiotherapy (IMRT) prostate treatment. To improve the accuracy of our prostate planning target volume, we have measured prostate displacements using computed tomography (CT)-CT fusion on patients that previously received gold seed implants. The study consists of ten patients that were scanned twice per week during the course of IMRT treatment. In addition to biweekly scans, breathing studies were performed on each patient to estimate organ motion during treatment. The prostate displacement in the anterior-posterior and the lateral direction is minimal, on the order of measurement uncertainty (~1 mm). The standard deviation of the superior-inferior (SI) displacements is 1.78 mm. The breathing studies show that no organ displacement was detected during normal breathing conditions with a rectal balloon.

The influence of a rectal balloon tube as internal immobilization device on variations of volumes and dose-volume histograms during treatment course of conformal radiotherapy for prostate cancer

PURPOSE

A prospective comparative study of a subset of 10 consecutive patients was performed, to describe the effects of an air-inflated rectal balloon tube that has been used for prostate immobilization in 360 patients since 1994. In particular, influences on prostate motion, rectum filling variations, and dose-volume histograms (DVHs) of the rectum during a course of conformal radiotherapy were investigated.

METHODS AND MATERIALS

Computed tomographic (CT) examinations without and with rectal balloon (filled with 40 mL air) were performed at the start (t(0)), middle (t(mi)), and end of treatment (t(e)), resulting in 6 CT scans for each patient. Prostate displacement was measured from a lateral beam's-eye-view. DVHs of rectum as a solid organ, and anterior, posterior, and whole rectum wall were calculated at t(0), t(mi), and t(e), and variations during treatment were analyzed for both examinations, with and without balloon.

RESULTS

By use of the balloon, rectum filling variations (p = 0.04) and maximum anterior-posterior displacements of the prostate (p = 0.008) were reduced significantly, leading to a reduction in DVH variations during treatment. Maximum displacements of posterior prostate border (>5 mm) were found in 8/10 patients without a rectum balloon and in only 2/10 patients with the balloon. The balloon led to a significant reduction in partial posterior rectal wall volumes included in the high-dose regions, without significant changes at the anterior rectum wall in cases of irradiation of the prostate only. However, when entirely irradiating the whole seminal vesicles, this advantage was lost.

CONCLUSIONS

The rectal balloon catheter represents a simple technique to immobilize the prostate and to determine the position of the anterior rectal wall at daily treatment. This allows a reduction of margins, because of reduced prostate movement during treatment course.

Correlation between lung fibrosis and radiation therapy dose after concurrent radiation therapy and chemotherapy for limited small cell lung cancer

PURPOSE

To evaluate the relationship between physician-identified radiographic fibrosis, lung tissue physical density change, and radiation dose after concurrent radiation therapy and chemotherapy for limited small cell lung cancer.

MATERIALS AND METHODS

Fibrosis volumes of different severity levels were delineated on computed tomography (CT) images obtained at 1-year follow-up of 21 patients with complete response to concurrent radiation therapy and chemotherapy for limited small cell lung carcinoma. Delivered treatments were reconstructed with a three-dimensional treatment planning system and geometrically registered to the follow-up CT images. Tissue physical density change and radiation dose were computed for each voxel within each fibrosis volume and within normal lung. Patient responses were grouped per radiation and chemotherapy protocol.

RESULTS

A significant correlation was noted between fibrosis grade and tissue physical density change and fibrosis grade. For doses less than 30 Gy, the probability of observing fibrosis was less than 2% with conventional fractionation and less than 4% with accelerated fractionation. Physical lung density change also showed a threshold of 30-35 Gy. For doses of 30-55 Gy and cisplatin and etoposide (PE) chemotherapy, fibrosis probability was 2.0 times greater for accelerated fractionation compared with conventional fractionation (P < .005) and was correlated to increasing dose for both fractionation schedules.

CONCLUSION

Lung tissue physical density changes correlated well with fibrosis incidence, and both increased with increasing dose greater than a threshold of 30-35 Gy. With concurrent PE chemotherapy, fibrosis probability was twice as great with accelerated fractionation as with once-daily fractionation.