Feasibility of automatic marker detection with an a-Si flat-panel imager

A method for robust segmentation of arbitrarily shaped radiopaque structures in cone-beam CT projections

PURPOSE

Implanted markers are commonly used in radiotherapy for x-ray based target localization. The projected marker position in a series of cone-beam CT (CBCT) projections can be used to estimate the three dimensional (3D) target trajectory during the CBCT acquisition. This has important applications in tumor motion management such as motion inclusive, gating, and tumor tracking strategies. However, for irregularly shaped markers, reliable segmentation is challenged by large variations in the marker shape with projection angle. The purpose of this study was to develop a semiautomated method for robust and reliable segmentation of arbitrarily shaped radiopaque markers in CBCT projections.

METHODS

The segmentation method involved the following three steps: (1) Threshold based segmentation of the marker in three to six selected projections with large angular separation, good marker contrast, and uniform background; (2) construction of a 3D marker model by coalignment and backprojection of the threshold-based segmentations; and (3) construction of marker templates at all imaging angles by projection of the 3D model and use of these templates for template-based segmentation. The versatility of the segmentation method was demonstrated by segmentation of the following structures in the projections from two clinical CBCT scans: (1) Three linear fiducial markers (Visicoil) implanted in or near a lung tumor and (2) an artificial cardiac valve in a lung cancer patient.

RESULTS

Automatic marker segmentation was obtained in more than 99.9% of the cases. The segmentation failed in a few cases where the marker was either close to a structure of similar appearance or hidden behind a dense structure (data cable).

CONCLUSIONS

A robust template-based method for segmentation of arbitrarily shaped radiopaque markers in CBCT projections was developed.

Local image enhancement for fiducial marker detection in electronic portal images of prostate radiotherapy

This paper proposes a new method for the automatic contrast enhancement of fiducial markers in low-radiation Electronic Portal Images. It is shown that the proposed approach significantly enhances the contrast of the fiducial markers and produces results where these markers are clearly visible. The main theoretical contribution consists in designing an algorithm that enhances the contrast of small structures in noisy images; the parameters of this algorithm are not empirically selected, but determined via a maximum search over a contrast metric. From a practical standpoint, the proposed method has direct applications in the current clinical workflow involving manual marker detection. It is also able to significantly improve the performances of automatic marker detection reported in literature.

Automatic marker detection and 3D position reconstruction using cine EPID images for SBRT verification

In previous studies, an electronic portal imaging device (EPID) in cine mode was used for validating respiratory gating and stereotactic body radiation therapy (SBRT) by tracking implanted fiducials. The manual marker tracking methods that were used were time and labor intensive, limiting the utility of the validation. The authors have developed an automatic algorithm to quickly and accurately extract the markers in EPID images and reconstruct their 3D positions. Studies have been performed with gold fiducials placed in solid water and dynamic thorax phantoms. In addition, the authors have examined the cases of five patients being treated under an SBRT protocol for hepatic metastases. For each case, a sequence of images was created by collecting the exit radiation using the EPID. The markers were detected and recognized using an image processing algorithm based on the Laplacian of Gaussian function. To reduce false marker detection, a marker registration technique was applied using image intensity as well as the geometric spatial transformations between the reference marker positions produced from the projection of 3D CT images and the estimated marker positions. An average marker position in 3D was reconstructed by backprojecting, towards the source, the position of each marker on the 2D image plane. From the static phantom study, spatial accuracies of <1 mm were achieved in both 2D and 3D marker locations. From the dynamic phantom study, using only the Laplacian of the Gaussian algorithm, the marker detection success rate was 88.8%. However, adding a marker registration technique which utilizes prior CT information, the detection success rate was increased to 100%. From the SBRT patient study, intrafractional tumor motion (3.1-11.3 mm) in the SI direction was measured using the 2D images. The interfractional patient setup errors (0.1-12.7 mm) in the SI, AP, and LR directions were obtained from the average marker locations reconstructed in 3D and compared to the reference planning CT image. The authors have developed an automatic algorithm to extract marker locations from MV images and have evaluated its performance. The measured intrafractional tumor motion and the interfractional daily patient setup error can be used for off-line retrospective verification of SBRT.

Influence of daily setup measurements and corrections on the estimated delivered dose during IMRT treatment of prostate cancer patients

PURPOSE

To evaluate the impact of marker-based position verification, using daily imaging and an off-line correction protocol, by calculating the delivered dose to prostate, rectum and bladder.

METHODS

Prostate cancer patients (n=217) were treated with IMRT, receiving 35 daily fractions. Plans with five beams were optimized taking target coverage (CTV, boost) and organs-at-risk (rectum and bladder) into account. PTV margins were 8mm. Prostate position was verified daily using implanted fiducial gold markers by imaging the first segment of all the five beams on an EPID. Setup deviations were corrected off-line using an adapted shrinking-action-level protocol. The estimated delivered dose, including daily organ movements, was calculated using a version of PLATO's dose engine, enabling batch processing of large numbers of patients. The dose was calculated +/- inclusion of setup corrections, and was evaluated relative to the original static plan. The marker-based measurements were considered representative for all organs.

RESULTS

Daily organ movements would result in an underdosage of 2-3Gy to CTV and boost volume relative to the original plan, which was prevented by daily setup corrections. The dose to rectum and bladder was on average unchanged, but a large spread was introduced by organ movements, which was reduced by including setup corrections.

CONCLUSIONS

Without position verification and setup corrections, margins of 8mm would be insufficient to account for position uncertainties during IMRT of prostate cancer. With the daily off-line correction protocol, the remaining variations are accommodated adequately.

A fiducial detection algorithm for real-time image guided IMRT based on simultaneous MV and kV imaging

The advantage of highly conformal dose techniques such as 3DCRT and IMRT is limited by intrafraction organ motion. A new approach to gain near real-time 3D positions of internally implanted fiducial markers is to analyze simultaneous onboard kV beam and treatment MV beam images (from fluoroscopic or electronic portal image devices). Before we can use this real-time image guidance for clinical 3DCRT and IMRT treatments, four outstanding issues need to be addressed. (1) How will fiducial motion blur the image and hinder tracking fiducials? kV and MV images are acquired while the tumor is moving at various speeds. We find that a fiducial can be successfully detected at a maximum linear speed of 1.6 cm/s. (2) How does MV beam scattering affect kV imaging? We investigate this by varying MV field size and kV source to imager distance, and find that common treatment MV beams do not hinder fiducial detection in simultaneous kV images. (3) How can one detect fiducials on images from 3DCRT and IMRT treatment beams when the MV fields are modified by a multileaf collimator (MLC)? The presented analysis is capable of segmenting a MV field from the blocking MLC and detecting visible fiducials. This enables the calculation of nearly real-time 3D positions of markers during a real treatment. (4) Is the analysis fast enough to track fiducials in nearly real time? Multiple methods are adopted to predict marker positions and reduce search regions. The average detection time per frame for three markers in a 1024 x 768 image was reduced to 0.1 s or less. Solving these four issues paves the way to tracking moving fiducial markers throughout a 3DCRT or IMRT treatment. Altogether, these four studies demonstrate that our algorithm can track fiducials in real time, on degraded kV images (MV scatter), in rapidly moving tumors (fiducial blurring), and even provide useful information in the case when some fiducials are blocked from view by the MLC. This technique can provide a gating signal or be used for intra-fractional tumor tracking on a Linac equipped with a kV imaging system. Any motion exceeding a preset threshold can warn the therapist to suspend a treatment session and reposition the patient.

A probabilistic framework based on hidden markov model for fiducial identification in image-guided radiation treatments

Fiducial tracking is a common target tracking method widely used in image-guided procedures such as radiotherapy and radiosurgery. In this paper, we present a multifiducial identification method that incorporates context information in the process. We first convert the problem into a state sequence problem by establishing a probabilistic framework based on a hidden Markov model (HMM), where prior probability represents an individual candidate's resemblance to a fiducial; transition probability quantifies the similarity of a candidate set to the fiducials' geometrical configuration; and the Viterbi algorithm provides an efficient solution. We then discuss the problem of identifying fiducials using stereo projections, and propose a special, higher order HMM, which consists of two parallel HMMs, connected by an association measure that captures the inherent correlation between the two projections. A novel algorithm, the concurrent viterbi with association (CVA) algorithm, is introduced to efficiently identify fiducials in the two projections simultaneously. This probabilistic framework is highly flexible and provides a buffer to accommodate deformations. A simple implementation of the CVA algorithm is presented to evaluate the efficacy of the framework. Experiments were carried out using clinical images acquired during patient treatments, and several examples are presented to illustrate a variety of clinical situations. In the experiments, the algorithm demonstrated a large tracking range, computational efficiency, ease of use, and robustness that meet the requirements for clinical use.

Fast internal marker tracking algorithm for onboard MV and kV imaging systems

Intrafraction organ motion can limit the advantage of highly conformal dose techniques such as intensity modulated radiation therapy (IMRT) due to target position uncertainty. To ensure high accuracy in beam targeting, real-time knowledge of the target location is highly desired throughout the beam delivery process. This knowledge can be gained through imaging of internally implanted radio-opaque markers with fluoroscopic or electronic portal imaging devices (EPID). In the case of MV based images, marker detection can be problematic due to the significantly lower contrast between different materials in comparison to their kV-based counterparts. This work presents a fully automated algorithm capable of detecting implanted metallic markers in both kV and MV images with high consistency. Using prior CT information, the algorithm predefines the volumetric search space without manual region-of-interest (ROI) selection by the user. Depending on the template selected, both spherical and cylindrical markers can be detected. Multiple markers can be simultaneously tracked without indexing confusion. Phantom studies show detection success rates of 100% for both kV and MV image data. In addition, application of the algorithm to real patient image data results in successful detection of all implanted markers for MV images. Near real-time operational speeds of approximately 10 frames/sec for the detection of five markers in a 1024 x 768 image are accomplished using an ordinary PC workstation.

MRI/linac integration

PURPOSE/OBJECTIVES

In radiotherapy the healthy tissue involvement still poses serious dose limitations. This results in sub-optimal tumour dose and complications. Daily image guided radiotherapy (IGRT) is the key development in radiation oncology to solve this problem. MRI yields superb soft-tissue visualization and provides several imaging modalities for identification of movements, function and physiology. Integrating MRI functionality with an accelerator can make these capacities available for high precision, real time IGRT.

DESIGN AND RESULTS

The system being built at the University Medical Center Utrecht is a 1.5T MRI scanner, with diagnostic imaging functionality and quality, integrated with a 6MV radiotherapy accelerator. The realization of a prototype of this hybrid system is a joint effort between the Radiotherapy Department of the University of Utrecht, the Netherlands, Elekta, Crawley, U.K., and Philips Research, Hamburg, Germany. Basically, the design is a 1.5 T Philips Achieva MRI scanner with a Magnex closed bore magnet surrounded by a single energy (6 MV) Elekta accelerator. Monte Carlo simulations are used to investigate the radiation beam properties of the hybrid system, dosimetry equipment and for the construction of patient specific dose deposition kernels in the presence of a magnetic field. The latter are used to evaluate the IMRT capability of the integrated MRI linac.

CONCLUSIONS

A prototype hybrid MRI/linac for on-line MRI guidance of radiotherapy (MRIgRT) is under construction. The aim of the system is to deliver the radiation dose with mm precision based on diagnostic quality MR images.

Feasibility of fully automated detection of fiducial markers implanted into the prostate using electronic portal imaging: a comparison of methods

PURPOSE

To investigate the feasibility of fully automated detection of fiducial markers implanted into the prostate using portal images acquired with an electronic portal imaging device.

METHODS AND MATERIALS

We have made a direct comparison of 4 different methods (2 template matching-based methods, a method incorporating attenuation and constellation analyses and a cross correlation method) that have been published in the literature for the automatic detection of fiducial markers. The cross-correlation technique requires a-priory information from the portal images, therefore the technique is not fully automated for the first treatment fraction. Images of 7 patients implanted with gold fiducial markers (8 mm in length and 1 mm in diameter) were acquired before treatment (set-up images) and during treatment (movie images) using 1MU and 15MU per image respectively. Images included: 75 anterior (AP) and 69 lateral (LAT) set-up images and 51 AP and 83 LAT movie images. Using the different methods described in the literature, marker positions were automatically identified.

RESULTS

The method based upon cross correlation techniques gave the highest percentage detection success rate of 99% (AP) and 83% (LAT) set-up (1MU) images. The methods gave detection success rates of less than 91% (AP) and 42% (LAT) set-up images. The amount of a-priory information used and how it affects the way the techniques are implemented, is discussed.

CONCLUSIONS

Fully automated marker detection in set-up images for the first treatment fraction is unachievable using these methods and that using cross-correlation is the best technique for automatic detection on subsequent radiotherapy treatment fractions.

Intrafraction motion of the prostate during external-beam radiation therapy: analysis of 427 patients with implanted fiducial markers

PURPOSE

To analyze the intrafraction motion of the prostate during external-beam radiation therapy of patients with prostate cancer.

METHODS AND MATERIALS

Between August 2001-December 2005, 427 patients with Stage T3Nx/0Mx/0 prostate carcinoma received intensity-modulated radiation therapy treatment combined with position verification with fiducial gold markers. For a total of 11,426 treatment fractions (average, 27 per patient), portal images were taken of the first segment of all five beams. The irradiation time of the technique varied between 5-7 min. From these data, the location of gold markers could be established within every treatment beam under the assumption of minimal marker movement.

RESULTS

In 66% of treatment fractions, a motion outside a range of 2 mm was observed, with 28% outside a range of 3 mm. The intrafraction marker movements showed that motion directions were often reversed. However, the effect was small. Even with perfect online position-correction at the start of irradiation, intrafraction motion caused position uncertainty, but systematic errors (Sigma) were limited to <0.6 mm, and random errors (sigma) to <0.9 mm. This would result in a lower limit of 2 mm for margins, in the absence of any other uncertainties.

CONCLUSIONS

Intrafraction motion of the prostate occurs frequently during external-beam irradiation on a time scale of 5-7 min. Margins of 2 mm account for these intrafraction motions. However, larger margins are required in practice to accommodate other uncertainties in the treatment.

Analysis of fiducial marker-based position verification in the external beam radiotherapy of patients with prostate cancer

PURPOSE

Evaluate the fiducial marker-based position verification in the external-beam radiotherapy of patients with prostate cancer.

METHODS

Four hundred and fifty-three patients with prostate cancer received an IMRT treatment combined with fiducial marker-based position verification. Portal images were taken in all 35 treatment fractions. This database was used to study the accuracy of detecting the prostate position as well as the presence of time trends and the effectiveness of commonly used off-line correction protocols.

RESULTS

The variation in inter-marker distance shows that the prostate position can be detected with an accuracy better than 0.6 mm. Significant time trends in prostate position occurred in 35%, 18% and 48% of the patients in the vertical, lateral and longitudinal directions, respectively, with 34%, 9% and 35% deviating more than 3 mm over the course of the treatment. Off-line correction protocols that estimate a deviation only in the first fractions of the treatment (shrinking action level (SAL), no action level (NAL)) are not effective in following these trends. With daily off-line position correction using an adapted SAL protocol we reduced systematic positioning errors in clinical practice to less than 0.8 mm in all directions.

CONCLUSION

Fiducial markers are a reliable tool for prostate position verification. Time trends occur frequently. Correction procedures must take such trends into account.

Intensity-modulated radiation therapy and image-guided radiation therapy: small clinic implementation

In a small clinic with a small patient base, the implementation of IMRT/IGRT should be slow, measured, and meticulous. Most radiation oncologists in the United States have had no formal training in IMRT/IGRT because the modalities are so new. Proper patient selection and a team effort among the clinician, physicist, dosimetrist, and therapist are thus all the more critical. The clinician in the small clinic can take comfort in remembering that the technologies are new, but the principles of good radiation medicine are not. With patient selection, a team approach, and publication of data and maturation of the literature, IMRT/IGRT will become the new standard of care in academic centers, large private clinics, and small clinics alike.

Daily organ tracking in intensity-modulated radiotherapy of prostate cancer using an electronic portal imaging device with a dose saving acquisition mode

BACKGROUND AND PURPOSE

Daily use of conventional electronic portal imaging devices (EPID) for organ tracking is limited due to the relatively high dose required for high quality image acquisition. We studied the use of a novel dose saving acquisition mode (RadMode) allowing to take images with one monitor unit per image in prostate cancer patients undergoing intensity-modulated radiotherapy (IMRT) and tracking of implanted fiducial gold markers.

PATIENTS AND METHODS

Twenty five patients underwent implantation of three fiducial gold markers prior to the planning CT. Before each treatment of a course of 37 fractions, orthogonal localization images from the antero-posterior and from the lateral direction were acquired. Portal images of both the setup procedure and the five IMRT treatment beams were analyzed.

RESULTS

On average, four localization images were needed for a correct patient setup, resulting in four monitor units extra dose per fraction. The mean extra dose delivered to the patient was thereby increased by 1.2%. The procedure was precise enough to reduce the mean displacements prior to treatment to < o =0.3 mm.

CONCLUSIONS

The use of a new dose saving acquisition mode enables to perform daily EPID-based prostate tracking with a cumulative extra dose of below 1 Gy. This concept is efficiently used in IMRT-treated patients, where separation of setup beams from treatment beams is mandatory.

An assessment of clinically optimal gold marker length and diameter for pelvic radiotherapy verification using an amorphous silicon flat panel electronic portal imaging device

Verification of target organ position is essential for the accurate delivery of conformal radiotherapy. Megavoltage electronic portal imaging with flat panel amorphous silicon detectors delivers high quality images that can be used for verification of bony landmark position. Gold markers implanted into the target organ can be visualized and used as a surrogate of actual organ position. On-line compensation for marker displacement, by adjusting patient position, can reduce geometric errors associated with radiation delivery. This study assesses the optimal marker length and diameter to be used with an amorphous silicon (a-Si) flat panel detector and electronic portal images (EPIs), prior to implementation of a clinical programme of gold marker insertion in prostate cancer patients. Seven marker sizes varying from 3 mm to 8 mm in length and 0.8 mm to 1.1 mm in diameter were investigated in a group of patients undergoing pelvic radiotherapy using an 8 MV Elekta SL20 linear accelerator. Markers were placed on the skin entry and exit sites of the treatment beam and EPIs in both lateral and anterior pelvic views were acquired. Three observers independently assessed visibility success and failure using a subjective scoring system. Markers less than 5 mm in length or 0.9 mm in diameter were poorly visualized (<70% visualization success in lateral EPIs). The marker measuring 0.9 mm x 5 mm appears to be clinically optimal in pelvic radiotherapy patients (80% visualization success in lateral EPIs) and will be used for actual organ implantation.

New Radiation Therapy Techniques for the Treatment of Head and Neck Cancer

This article reviews the most recent technology used in the treatment of head and neck cancer. It discusses brachytherapy, new ways to mix radionuclides for enhanced radiobiologic effects, and different fractionation schemes that have grown in clinical importance. Intensity-modulated radiotherapy has become a mainstay in head and neck cancer treatment, and the authors discuss several popular and emerging approaches. Patient immobilization and imaging are also discussed.

Integrating a MRI scanner with a 6 MV radiotherapy accelerator: dose increase at tissue–air interfaces in a lateral magnetic field due to returning electrons

In the framework of the development of the integration of a MRI-scanner with a linear accelerator, the influence of a lateral, magnetic field on the dose distribution has to be determined. Dose increase is expected at tissue-air boundaries, due to the electron return effect (ERE): electrons entering air will describe a circular path and return into the phantom causing extra dose deposition. Using IMRT with many beam directions, this exit dose will not constitute a problem. Dose levels behind air cavities will decrease because of the absence of electrons crossing the cavity. The ERE has been demonstrated both by simulation and experiment. Monte Carlo simulations are performed with GEANT4, irradiating a water-air-water phantom in a lateral magnetic field. Also an air tube in water has been simulated, resulting in slightly twisted regions of dose increase and decrease. Experimental demonstration is achieved by film measurement in a perspex-air-perspex phantom in an electromagnet. Although the ERE causes dose increase before air cavities, relatively flat dose profiles can be obtained for the investigated cases using opposite beam configurations. More research will be necessary whether this holds for more realistic geometries with the use of IMRT and whether the ERE can be turned to our advantage when treating small tumour sites at air cavities.

Adequate margins for random setup uncertainties in head-and-neck IMRT

PURPOSE

To investigate the effect of random setup uncertainties on the highly conformal dose distributions produced by intensity-modulated radiotherapy (IMRT) for clinical head-and-neck cancer patients and to determine adequate margins to account for those uncertainties.

METHODS AND MATERIALS

We have implemented in our clinical treatment planning system the possibility of simulating normally distributed patient setup displacements, translations, and rotations. The planning CT data of 8 patients with Stage T1-T3N0M0 oropharyngeal cancer were used. The clinical target volumes of the primary tumor (CTV(primary)) and of the lymph nodes (CTV(elective)) were expanded by 0.0, 1.5, 3.0, and 5.0 mm in all directions, creating the planning target volumes (PTVs). We performed IMRT dose calculation using our class solution for each PTV margin, resulting in the conventional static plans. Then, the system recalculated the plan for each positioning displacement derived from a normal distribution with sigma = 2 mm and sigma = 4 mm (standard deviation) for translational deviations and sigma = 1 degrees for rotational deviations. The dose distributions of the 30 fractions were summed, resulting in the actual plan. The CTV dose coverage of the actual plans was compared with that of the static plans.

RESULTS

Random translational deviations of sigma = 2 mm and rotational deviations of sigma = 1 degrees did not affect the CTV(primary) volume receiving 95% of the prescribed dose (V(95)) regardless of the PTV margin used. A V(95) reduction of 3% and 1% for a 0.0-mm and 1.5-mm PTV margin, respectively, was observed for sigma = 4 mm. The V(95) of the CTV(elective) contralateral was approximately 1% and 5% lower than that of the static plan for sigma = 2 mm and sigma = 4 mm, respectively, and for PTV margins <5.0 mm. An additional reduction of 1% was observed when rotational deviations were included. The same effect was observed for the CTV(elective) ipsilateral but with smaller dose differences than those for the contralateral side. The effect of the random uncertainties on the mean dose to the parotid glands was not significant. The maximal dose to the spinal cord increased by a maximum of 3 Gy.

CONCLUSIONS

The margins to account for random setup uncertainties, in our clinical IMRT solution, should be 1.5 mm and 3.0 mm in the case of sigma = 2 mm and sigma = 4 mm, respectively, for the CTV(primary). Larger margins (5.0 mm), however, should be applied to the CTV(elective), if the goal of treatment is a V(95) value of at least 99%.

On the use of EPID-based implanted marker tracking for 4D radiotherapy

Four-dimensional (4D) radiotherapy delivery to dynamically moving tumors requires a real-time signal of the tumor position as a function of time so that the radiation beam can continuously track the tumor during the respiration cycle. The aim of this study was to develop and evaluate an electronic portal imaging device (EPID)-based marker-tracking system that can be used for real-time tumor targeting, or 4D radiotherapy. Three gold cylinders, 3 mm in length and 1 mm in diameter, were implanted in a dynamic lung phantom. The phantom range of motion was 4 cm with a 3-s "breathing" period. EPID image acquisition parameters were modified, allowing image acquisition in 0.1 s. Images of the stationary and moving phantom were acquired. Software was developed to segment automatically the marker positions from the EPID images. Images acquired in 0.1 s displayed higher noise and a lower signal-noise ratio than those obtained using regular (> 1 s) acquisition settings. However, the markers were still clearly visible on the 0.1-s images. The motion of the phantom blurred the images of the markers and further reduced the signal-noise ratio, though they could still be successfully segmented from the images in 10-30 ms of computation time. The positions of gold markers placed in the lung phantom were detected successfully, even for phantom velocities substantially higher than those observed for typical lung tumors. This study shows that using EPID-based marker tracking for 4D radiotherapy is feasible, however, changes in linear accelerator technology and EPID-based image acquisition as well as patient studies are required before this method can be implemented clinically.

Performing daily prostate targeting with a standard V-EPID and an automated radio-opaque marker detection algorithm

Online prostate positioning using gold markers and a standard video-based electronic portal imaging device is reported. The average systematic (random) errors have been reduced from 2.1 mm (2.7 mm) to 0.5 mm (1.5 mm) in AP direction, 1.1 mm (1.7 mm) to 0.7 mm (1.2 mm) SI and 1.2 mm (1.7 mm) to 0.6 mm (1.3 mm) LR.

Measurements of intrafraction motion and interfraction and intrafraction rotation of prostate by three-dimensional analysis of daily portal imaging with radiopaque markers

PURPOSE

To measure the interfraction and intrafraction motion of the prostate during the course of external beam radiotherapy using a video electronic portal imaging device and three-dimensional analysis.

METHODS AND MATERIALS

Eighteen patients underwent implantation with two or three gold markers in the prostate before five-angle/11-field conformal radiotherapy. Using CT data as the positional reference, multiple daily sets of portal images, and a three-dimensional reconstruction algorithm, intrafraction translations, as well as interfraction and intrafraction rotations, were analyzed along the three principal axes (left-right [LR], superoinferior [SI], and AP). The overall mean values and standard deviations (SDs), along with random and systematic SDs, were computed for these translations and rotations.

RESULTS

For 282 intrafraction translational displacements, the random SD was 0.8 mm (systematic SD, 0.2) in the LR, 1.0 mm (systematic SD, 0.4) in the SI, and 1.4 mm (systematic SD, 0.7) in the AP axes. The analysis of 348 interfraction rotations revealed random SDs of 6.1 degrees (systematic SD, 5.6 degrees ) around the LR axis, 2.8 degrees (systematic SD, 2.4 degrees ) around the SI axis, and 2.0 degrees (systematic SD, 2.2 degrees ) around the AP axis. The intrafraction rotational motion observed during 44 fractions had a random SD of 1.8 degrees (systematic SD, 1.0 degrees ) around the LR, 1.1 degrees (systematic SD, 0.8 degrees ) around the SI, and 0.6 degrees (systematic SD, 0.3 degrees ) around the AP axis.

CONCLUSION

The interfraction rotations observed were more important than those reported in previous studies. Intrafraction motion was generally smaller in magnitude than interfraction motion. However, the intrafraction rotations and translations of the prostate should be taken into account when designing planning target volume margins because their magnitudes are not negligible.

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.

Integrating a MRI scanner with a 6 MV radiotherapy accelerator: dose deposition in a transverse magnetic field

Integrating magnetic resonance imaging (MRI) functionality with a radiotherapy accelerator can facilitate on-line, soft-tissue based, position verification. A technical feasibility study, in collaboration with Elekta Oncology Systems and Philips Medical Systems, led to the preliminary design specifications of a MRI accelerator. Basically the design is a 6 MV accelerator rotating around a 1.5 T MRI system. Several technical issues and the clinical rational are currently under investigation. The aim of this paper is to determine the impact of the transverse 1.5 T magnetic field on the dose deposition. Monte Carlo simulations were used to calculate the dose deposition kernel in the presence of 1.5 T. This kernel in turn was used to determine the dose deposition for larger fields. Also simulations and measurements were done in the presence of 1.1 T. The pencil beam dose deposition is asymmetric. For larger fields the asymmetry persists but decreases. For the latter the distance to dose maximum is reduced by approximately 5 mm, the penumbra is increased by approximately 1 mm, and the 50% isodose line is shifted approximately 1 mm. The dose deposition in the presence of 1.5 T is affected, but the effect can be taken into account in a conventional treatment planning procedure. The impact of the altered dose deposition for clinical IMRT treatments is the topic of further research.

Implanted gold markers for position verification during irradiation of head-and-neck cancers: a feasibility study

PURPOSE

To assess the toxicity and reliability of the use of implanted gold markers for position verification during irradiation of head-and-neck cancer.

METHODS AND MATERIALS

Ten patients with localized head-and-neck tumors received two gold markers in the parapharyngeal region. The acute and late radiation-related toxicity were scored prospectively using the Common Toxicity Criteria. The patients were immobilized during irradiation using a five-point mask. The marker location was detected in portal images taken with an a-Si flat panel imager. The intermarker distance, as well as the interfraction motion of the markers, was determined for all patients.

RESULTS

No acute major complications were observed. The acute toxicity grade was not greater than normally detected. The markers were visible in all images. On average, the projected intermarker distance varied 0.8 mm (1 standard deviation). A small time trend was observed in the intermarker distance for 3 patients. For these patients, at least one marker was located in the mucosa or pharyngeal constrictor muscle. Deeper-seated gold markers did not show a time trend in the intermarker distance. The random positioning uncertainty determined using the markers was on average 1.1 and 1.4 mm (1 SD) in the craniocaudal and AP direction, respectively.

CONCLUSION

The use of implanted gold markers for position verification during radiotherapy for head-and-neck cancer patients seems safe and feasible. To avoid any chance of migration, markers should be placed in deep muscular compartments.

Application of video imaging for improvement of patient set-up

BACKGROUND AND PURPOSE

For radiotherapy of prostate cancer, the patient is usually positioned in the left-right (LR) direction by aligning a single marker on the skin with the projection of a room laser. The aim of this study is to investigate the feasibility of a room-mounted video camera in combination with previously acquired CT data to improve patient set-up along the LR axis.

MATERIAL AND METHODS

The camera was mounted in the treatment room at the caudal side of the patient. For 22 patients with prostate cancer 127 video and portal images were acquired. The set-up error determined by video imaging was found by matching video images with rendered CT images using various techniques. This set-up error was retrospectively compared with the set-up error derived from portal images. It was investigated whether the number of corrections based on portal imaging would decrease if the information obtained from the video images had been used prior to irradiation. Movement of the skin with respect to bone was quantified using an analysis of variance method.

RESULTS

The measurement of the set-up error was most accurate for a technique where outlines and groins on the left and right side of the patient were delineated and aligned individually to the corresponding features extracted from the rendered CT image. The standard deviations (SD) of the systematic and random components of the set-up errors derived from the portal images in the LR direction were 1.5 and 2.1 mm, respectively. When the set-up of the patients was retrospectively adjusted based on the video images, the SD of the systematic and random errors decreased to 1.1 and 1.3 mm, respectively. From retrospective analysis, a reduction of the number of set-up corrections (from nine to six corrections) is expected when the set-up would have been adjusted using the video images. The SD of the magnitude of motion of the skin of the patient with respect to the bony anatomy was estimated to be 1.1 mm.

CONCLUSION

Video imaging is an accurate technique for measuring the set-up of prostate cancer patients in the LR direction. The outline of the patient is a more accurate estimate of the set-up of the bony anatomy than the marker on the patient's abdomen.

Comparison of megavoltage position verification for prostate irradiation based on bony anatomy and implanted fiducials

PURPOSE

The patient position during radiotherapy treatment of prostate cancer can be verified with the help of portal images acquired during treatment. In this study we quantify the clinical consequences of the use of image-based verification based on the bony anatomy and the prostate target itself.

PATIENTS AND METHODS

We analysed 2025 portal images and 23 computed tomography (CT) scans from 23 patients with prostate cancer. In all patients gold markers were implanted prior to CT scanning. Statistical data for both random and systematic errors were calculated for displacements of bones and markers and we investigated the effectiveness of an off-line correction protocol.

RESULTS

Standard deviations for systematic marker displacement are 2.4 mm in the lateral (LR) direction, 4.4 mm in the anterior-posterior (AP) direction and 3.7 mm in the caudal-cranial direction (CC). Application of off-line position verification based on the marker positions results in a shrinkage of the systematic error to well below 1 mm. Position verification based on the bony anatomy reduces the systematic target uncertainty to 50% in the AP direction and in the LR direction. No reduction was observed in the CC direction. For six out of 23 patients we found an increase of the systematic error after application of bony anatomy-based position verification.

CONCLUSIONS

We show that even if correction based on the bony anatomy is applied, considerable margins have to be set to account for organ motion. Our study highlights that for individual patients the systematic error can increase after application of bony anatomy-based position verification, whereas the population standard deviation will decrease. Off-line target-based position verification effectively reduces the systematic error to well below 1 mm, thus enabling significant margin reduction.

Robustness and precision of an automatic marker detection algorithm for online prostate daily targeting using a standard V-EPID

An algorithm for the daily localization of the prostate using implanted markers and a standard video-based electronic portal imaging device (V-EPID) has been tested. Prior to planning, three gold markers were implanted in the prostate of seven patients. The clinical images were acquired with a BeamViewPlus 2.1 V-EPID for each field during the normal course radiotherapy treatment and are used off-line to determine the ability of the automatic marker detection algorithm to adequately and consistently detect the markers. Clinical images were obtained with various dose levels from ranging 2.5 to 75 MU. The algorithm is based on marker attenuation characterization in the portal image and spatial distribution. A total of 1182 clinical images were taken. The results show an average efficiency of 93% for the markers detected individually and 85% for the group of markers. This algorithm accomplishes the detection and validation in 0.20-0.40 s. When the center of mass of the group of implanted markers is used, then all displacements can be corrected to within 1.0 mm in 84% of the cases and within 1.5 mm in 97% of cases. The standard video-based EPID tested provides excellent marker detection capability even with low dose levels. The V-EPID can be used successfully with radiopaque markers and the automatic detection algorithm to track and correct the daily setup deviations due to organ motions.

Clinical feasibility study for the use of implanted gold seeds in the prostate as reliable positioning markers during megavoltage irradiation

BACKGROUND AND PURPOSE

The aim of this study was to assess the feasibility of using gold seed implants in the prostate for position verification, using an a-Si flat panel imager as a detector during megavoltage irradiation of prostate carcinoma. This is a study to guarantee positioning accuracy in intensity-modulated radiotherapy.

METHODS AND MATERIALS

Ten patients with localized prostate carcinoma (T2-3) received between one and three fiducial gold markers in the prostate. All patients were treated with 3-D conformal radiotherapy with an anterior-posterior (AP) and two lateral wedge fields. The acute gastrointestinal (GI) and genitourinary (GU) toxicities were scored using common toxicity criteria scales (CTC). Using three consecutive CT scans and portal images obtained during the treatment we have studied the occurrence of any change in prostate shape (deformation), seed migration and the magnitude of translations and rotations of the prostate.

RESULTS

We observed no acute major complications for prostate irradiation regarding the seed implantation. The maximum acute GU toxicity grade 2 (dysuria and frequency) was observed in seven patients during the treatment. The maximum grade 2 (diarrhoea) was scored in two patients regarding the acute GI toxicities. No significant prostate deformation could be detected in the consecutive CT scans. It appeared that the distances between the markers only slightly changed during treatment (S.D. 0.5 mm). Random prostate translations were (1 S.D.) 2.1, 3.2 and 2.2 mm in the lateral (LR), AP and cranial-caudal (CC) directions, respectively, whereas systematic translations were 3.3, 4.8 and 3.5 mm in the LR, AP and CC directions, respectively. Random prostate rotations were (1 S.D.) 3.6, 1.7 and 1.9 degrees around the LR, AP and CC axis, respectively, whereas systematic rotations were 4.7, 2.0 and 2.7 degrees around the LR, AP and CC axis, respectively.

CONCLUSIONS

We found that the fiducial gold seeds are a safe and appropriate device to verify and correct the position of prostate during megavoltage irradiation. The amount of seed migration and prostate deformation is far below our present tumour delineation accuracy.

Focal spot motion of linear accelerators and its effect on portal image analysis

The focal spot of a linear accelerator is often considered to have a fully stable position. In practice, however, the beam control loop of a linear accelerator needs to stabilize after the beam is turned on. As a result, some motion of the focal spot might occur during the start-up phase of irradiation. When acquiring portal images, this motion will affect the projected position of anatomy and field edges, especially when low exposures are used. In this paper, the motion of the focal spot and the effect of this motion on portal image analysis are quantified. A slightly tilted narrow slit phantom was placed at the isocenter of several linear accelerators and images were acquired (3.5 frames per second) by means of an amorphous silicon flat panel imager positioned approximately 0.7 m below the isocenter. The motion of the focal spot was determined by converting the tilted slit images to subpixel accurate line spread functions. The error in portal image analysis due to focal spot motionwas estimated by a subtraction of the relative displacement of the projected slit from the relative displacement of the field edges. It was found that the motion of the focal spot depends on the control system and design of the accelerator. The shift of the focal spot at the start of irradiation ranges between 0.05-0.7 mm in the gun-target (GT) direction. In the left-right (AB) direction the shift is generally smaller. The resulting error in portal image analysis due to focal spotmotion ranges between 0.05-1.1 mm for a dose corresponding to two monitor units (MUs). For 20 MUs, the effect of the focal spot motion reduces to 0.01-0.3 mm. The error in portal image analysis due to focal spot motion can be reduced by reducing the applied dose rate.

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.

Potential and limitations of the automatic detection of fiducial markers using an amorphous silicon flat-panel imager

Amorphous silicon electronic portal imaging devices (a-Si EPIDs) allow fast acquisition of high resolution portal images (PI). A visualization of organ movement for adaptive image-guided radiotherapy (IGRT) can be reached by implantation and automatic detection of fiducial markers. A method of automatic detection has been developed for fiducial spherical tungsten markers on PIs, acquired with an a-Si flat-panel imager. The detection method consists of a 2D Mexican hat filter (MHF), whose parameters are tuned to the particular marker signal. The high selectivity of this filter allows a reliable and precise detection of tungsten markers down to a diameter of 1.5 mm. The presented method allows fast, automatic and unsupervised detection of markers. Inevitably, the detection is hampered by image background (bone structures, etc) and noise. A detection success rate higher than 95% was reached, analysing PIs of patients with markers fixed on their skin. Furthermore, this approach to automatic marker detection can also be generalized to elliptic MHFs for the detection of cylindrical markers. The accuracy and detection probability of this method may allow accurate and fast online localization of the considered organ.

Feasibility of geometrical verification of patient set-up using body contours and computed tomography data

BACKGROUND AND PURPOSE

Body contours can potentially be used for patient set-up verification in external-beam radiotherapy and might enable more accurate set-up of patients prior to irradiation. The aim of this study is to test the feasibility of patient set-up verification using a body contour scanner.

MATERIAL AND METHODS

Body contour scans of 33 lung cancer and 21 head-and-neck cancer patients were acquired on a simulator. We assume that this dataset is representative for the patient set-up on an accelerator. Shortly before acquisition of the body contour scan, a pair of orthogonal simulator images was taken as a reference. Both the body contour scan and the simulator images were matched in 3D to the planning computed tomography scan. Movement of skin with respect to bone was quantified based on an analysis of variance method.

RESULTS

Set-up errors determined with body-contours agreed reasonably well with those determined with simulator images. For the lung cancer patients, the average set-up errors (mm)+/-1 standard deviation (SD) for the left-right, cranio-caudal and anterior-posterior directions were 1.2+/-2.9, -0.8+/-5.0 and -2.3+/-3.1 using body contours, compared to -0.8+/-3.2, -1.0+/-4.1 and -1.2+/-2.4 using simulator images. For the head-and-neck cancer patients, the set-up errors were 0.5+/-1.8, 0.5+/-2.7 and -2.2+/-1.8 using body contours compared to -0.4+/-1.2, 0.1+/-2.1, -0.1+/-1.8 using simulator images. The SD of the set-up errors obtained from analysis of the body contours were not significantly different from those obtained from analysis of the simulator images. Movement of the skin with respect to bone (1 SD) was estimated at 2.3 mm for lung cancer patients and 1.7 mm for head-and-neck cancer patients.

CONCLUSION

Measurement of patient set-up using a body-contouring device is possible. The accuracy, however, is limited by the movement of the skin with respect to the bone. In situations where the error in the patient set-up is relatively large, it is possible to reduce these errors using a computer-aided set-up technique based on contour information.

A review of intensity modulated radiation therapy: incorporating a report on the seventh education workshop of the ACPSEM--ACT/NSW branch. Australasian College of Physical Scientists and Engineers in Medicine

Intensity modulated radiation therapy (IMRT) is an evolving treatment technique that has become a clinical treatment option in several radiotherapy centres around the world. In August 2001 the ACT/NSW branch of the ACPSEM held its seventh education workshop, the subject was IMRT. This review considers the current use of IMRT and reports on the proceedings of the workshop. The workshop provided some of the theory behind IMRT, discussion of the practical issues associated with IMRT, and also involved presentations from Australian centres that had clinically implemented IMRT. The main topics of discussion were patient selection, plan assessment, multi-disciplinary approach, quality assurance and delivery of IMRT. Key points that were emphasised were the need for a balanced multi-disciplinary approach to IMRT, in both the establishment and maintenance of an IMRT program; the importance of the accuracy of the final dose distribution as compared to the minor in-field fluctuations of individual beams; and that IMRT is an emerging treatment technique, undergoing continuing development and refinement.

Kilovision: thermal modeling of a kilovoltage x-ray source integrated into a medical linear accelerator

The thermal and thermo-mechanical (fatigue) properties of a stationary-anode kilovoltage x-ray source that can be integrated into the head of a medical linear accelerator have been modeled. A finite element program has been used to model two new target designs. The first design makes minor modifications to the existing target assembly of a Varian medical linear accelerator, while the second design adds an additional cooling tube, changes the target angle, and uses a tungsten-rhenium alloy rather than tungsten as the kilovoltage target material. The thermal calculations have been used to generate cyclic stress/strain values from which estimates of fatigue in the target designs have been made. Both kilovoltage and megavoltage operation have been studied. Analysis of the megavoltage operation shows that there are only small differences in the thermal and fatigue characteristics after the target assembly is modified to include a kilovoltage target. Thus, megavoltage operation should not be compromised. The first kilovoltage target design can handle a 900 W heat load (e.g., 120 kVp, 7.5 mA, 2 x 2 mm2 source size); the heat load being limited by the temperature at the surface of the cooling tubes and mechanical fatigue at the surface of the target. The second design can handle a 1250 W heat load (e.g., 120 kVp, approximately 10.4 mA, 2 x 2 mm2 source size). Our calculations show that installation of a kilovoltage x-ray target is practical from thermal and thermo-mechanical perspectives.

The dose to the parotid glands with IMRT for oropharyngeal tumors: the effect of reduction of positioning margins

PURPOSE

The aim of this paper is to quantify the importance of the reduction of positioning margins applied to the clinical target volume (CTV) on the dose distribution of the parotid gland for different intensity-modulated radiotherapy (IMRT) strategies for the treatment of oropharyngeal cancer.

METHODS AND MATERIALS

CTVs and organs at risk were delineated in the planning computed tomographic (CT) scans of three patients. Margins of 0, 3, 6 and 9mm were applied to the CTVs in order to obtain the planning target volumes (PTVs). Three IMRT strategies were used to optimize the dose distribution.

RESULTS

The analysis of the three IMRT strategies resulted in: (1) an optimal dose distribution in the PTV; (2) optimal dose distribution in the PTV while sparing the parotid gland and (3) more parotid gland sparing but at expense of the dose homogeneity in the PTV. The mean parotid dose increased linearly with increasing margin by approximately 1.3Gy per mm. As a result, the normal complication probability (NTCP) for xerostomia decreased when smaller margins were applied. Reducing the margin from 6 to 3mm resulted in an NTCP reduction of approximately 20%.

CONCLUSION

Reducing the CTV-PTV margin by improving the patient position accuracy may lead to a significant reduction of NTCP for the IMRT treatment of the oropharyngeal tumors and lymph nodes level II.

Measurements and clinical consequences of prostate motion during a radiotherapy fraction

PURPOSE

Here we study the magnitude of prostate motion during the delivery of a radiotherapy fraction. These motions have clinical consequences for on-line position verification and the choice of margins around the target volume.

METHODS AND MATERIALS

We studied the motion of the prostate for 10 patients during 251 radiotherapy treatment fractions by assessing the position of implanted gold markers. Gold markers of 1 mm diameter and 5 mm length were implanted in the prostate before the start of the radiotherapy. We obtained movies during each fraction using an a-Si flat-panel imager. The markers could be detected in separate frames using a marker extraction kernel.

RESULTS

Marker displacements as large as 9.5 mm were detected in one fraction. The motion of the prostate is greatest in the caudal-cranial and the anterior-posterior directions. Within a time window of 2 to 3 min, deviations from the initial marker position, averaged over all patients, are 0.3 +/- 0.5 mm and -0.4 +/- 0.7 mm in the anterior-posterior and caudal-cranial directions, respectively.

CONCLUSIONS

It appeared that on average, the intrafraction prostate motions did not result in margins larger than 1 mm, provided that the position verification is performed at time intervals of 2 to 3 min. Only for some patients performing more frequent position verification or adding extra margins of 2 to 3 mm is required to account for intrafraction prostate motions.

Partial boosting of prostate tumours

BACKGROUND AND PURPOSE

In this planning study we propose a class solution for partial boosting of prostate tumours. Treatment margins and rectum dose are similar to that of the conventional treatment and are supposed to have no direct link to the level of dose escalation. We also study the robustness of our class solution in the presence of geometrical deviations.

METHODS AND MATERIALS

To study the specifications of the class solution ten patients with histologically confirmed prostate cancer were replanned. Besides a conventional plan for each patient, different partial boost plans were produced with an inverse treatment-planning tool. We also simulated treatment geometrical deviations to estimate their effect on partial boost plans.

RESULTS

In our class solution we use three contours in our inverse treatment planning, which are based on the classical CTV. A three beam arrangement appeared to produce a dose distribution, which is comparable to that of a five or seven beam geometry. Comparison of partial boost plans and conventional plans indicated that all conditions for a partial boost plan could be satisfied with the proposed class solution. Simulation of treatment geometrical deviations showed that large random deviations have a minor effect on the overall dose distributions, while systematic deviations may decrease the boost dose and increase the rectal dose.

CONCLUSIONS

We presented a class solution for partial boosting of prostate tumours in which the level of dose escalation is dealt with separately from the margin size and the nominal rectum dose. The framework put forward in this study allows practical introduction of intensity modulated radiotherapy in routine clinical practice using current standards of imaging and position verification.