Optimizing radiotherapy of orbital and paraorbital tumors: intensity-modulated X-ray beams vs. intensity-modulated proton beams

PURPOSE

This study presents a dosimetric optimization effort aiming to compare intensity-modulated (IM) X-rays and IM protons in 4 different orbital and paraorbital tumors. These are most challenging targets for standard radiotherapy due to their close relationship with the eyes and related structures.

METHODS AND MATERIALS

A primary orbital lymphoma, an optic nerve meningioma, a sphenoidal ridge meningioma protruding into the orbit, and a pediatric parameningeal paraorbital rhabdomyosarcoma were selected for the purpose of this study. Planning target volumes (PTVs) and organs at risk (OAR) were defined in each patient CT data set for each tumor site. IM X-ray and IM proton three-dimensional treatment plans were implemented. The following total tumor doses were prescribed: 30 Gy for the orbital lymphoma, 54 Gy for both meningiomas, and 50.4 Gy for the rhabdomyosarcoma case. Dose-volume histograms (DVHs) were obtained for all targets and OAR with both treatment techniques. DVHs were used to predict normal tissue complication probabilities (NTCPs) for the OAR in the vicinity of the tumor.

RESULTS

The PTV coverage was optimal and equally homogeneous with both IM X-rays and IM proton plans in the 4 tumor sites. DVHs for most OAR were better with IM proton beams especially in the low- to mid-dose range region. The integral nontarget dose was lower with IM protons in every case (factor ranging from 1.5 to 1.9). However, predicted NTCPs (for severe late effects) were equally low for both treatment techniques in every tumor site.

CONCLUSION

Although IM proton plans optimally decreased the dose to the OAR in all tumor sites, both optimized X-ray and proton beams equally succeeded to reduce severe-toxicity prediction risks to less than 5% while optimally treating the PTV.

A high-precision system for conformal intracranial radiotherapy

PURPOSE

Currently, optimally precise delivery of intracranial radiotherapy is possible with stereotactic radiosurgery and fractionated stereotactic radiotherapy. We report on an optimally precise optically guided system for three-dimensional (3D) conformal radiotherapy using multiple noncoplanar fixed fields.

METHODS AND MATERIALS

The optically guided system detects infrared light emitting diodes (IRLEDs) attached to a custom bite plate linked to the patient's maxillary dentition. The IRLEDs are monitored by a commercially available stereo camera system, which is interfaced to a personal computer. An IRLED reference is established with the patient at the selected stereotactic isocenter, and the computer reports the patient's current position based on the location of the IRLEDs relative to this reference position. Using this readout from the computer, the patient may be dialed directly to the desired position in stereotactic space. The patient is localized on the first day and a reference file is established for 5 different couch positions. The patient's image data are then imported into a commercial convolution-based 3D radiotherapy planning system. The previously established isocenter and couch positions are then used as a template upon which to design a conformal 3D plan with maximum beam separation.

RESULTS

The use of the optically guided system in conjunction with noncoplanar radiotherapy treatment planning using fixed fields allows the generation of highly conformal treatment plans that exhibit a high degree of dose homogeneity and a steep dose gradient. To date, this approach has been used to treat 28 patients.

CONCLUSION

Because IRLED technology improves the accuracy of patient localization relative to the linac isocenter and allows real-time monitoring of patient position, one can choose treatment-field margins that only account for beam penumbra and image resolution without adding margin to account for larger and poorly defined setup uncertainty. This approach enhances the normal tissue sparing, high degree of conformality, and homogeneity characteristics possible with 3D conformal radiotherapy.

A new verification film system for routine quality control of radiation fields: Kodak EC-L

BACKGROUND

The use of modern irradiation techniques requires better verification films for determining set-up deviations and patient movements during the course of radiation treatment. This is an investigation of the image quality and time requirement of a new verification film system compared to a conventional portal film system.

MATERIAL AND METHODS

For conventional verifications we used Agfa Curix HT 1000 films which were compared to the new Kodak EC-L film system. 344 Agfa Curix HT 1000 and 381 Kodak EC-L portal films of different tumor sites (prostate, rectum, head and neck) were visually judged on a light box by 2 experienced physicians. Subjective judgement of image quality, masking of films and time requirement were checked.

RESULTS

In this investigation 68% of 175 Kodak EC-L ap/pa-films were judged "good", only 18% were classified "moderate" or "poor" 14%, but only 22% of 173 conventional ap/pa verification films (Agfa Curix HT 1000) were judged to be "good".

CONCLUSIONS

The image quality, detail perception and time required for film inspection of the new Kodak EC-L film system was significantly improved when compared with standard portal films. They could be read more accurately and the detection of set-up deviation was facilitated.

The potential for systematic field placement errors when using asymmetric collimation for photon field junctions

During radiotherapy of breast or head and neck cancer it is often necessary to junction photon radiation fields to cover primary disease as well as the draining lymphatics. One increasingly popular technique to achieve this is a mono-isocentric technique for all abutting fields employing independent jaws. While this technique has many advantages it was found in a study of quality assurance requirements that the mono-isocentric technique was prone to some systematic errors in field placement due to incorrect calibration of the collimator read-outs and the isocentre definition of linear accelerator. It is not only important that operators are aware of these potential problems but also that quality assurance activities specific to the use of asymmetric collimation are implemented.

Clinical experience with intensity modulated radiation therapy (IMRT) in prostate cancer

PURPOSE

To compare acute and late toxicities of high-dose radiation for prostate cancer delivered by either conventional three-dimensional conformal radiation therapy (3D-CRT) or intensity modulated radiation therapy (IMRT).

MATERIALS AND METHODS

Between September 1992 and February 1998, 61 patients with clinical stage T1c- T3 prostate cancer were treated with 3D-CRT and 171 with IMRT to a prescribed dose of 81 Gy. To quantitatively evaluate the differences between conventional 3D-CRT and IMRT, 20 randomly selected patients were planned concomitantly by both techniques and the resulting treatment plans were compared. Acute and late radiation-induced morbidity was evaluated in all patients and graded according to the Radiation Therapy Oncology Group toxicity scale.

RESULTS

Compared with conventional 3D-CRT, IMRT improved the coverage of the clinical target volume (CTV) by the prescription dose and reduced the volumes of the rectal and bladder walls carried to high dose levels (P<0.01), indicating improved conformality with IMRT. Acute and late urinary toxicities were not significantly different for the two methods. However, the combined rates of acute grade 1 and 2 rectal toxicities and the risk of late grade 2 rectal bleeding were significantly lower in the IMRT patients. The 2-year actuarial risk of grade 2 bleeding was 2% for IMRT and 10% for conventional 3D-CRT (P<0.001).

CONCLUSIONS

The data demonstrate the feasibility and safety of high-dose IMRT for patients with localized prostate cancer and provide a proof-of-principle that this method improves dose conformality relative to tumor coverage and exposure to normal tissues.

The impact of fluctuations in intensity patterns on the number of monitor units and the quality and accuracy of intensity modulated radiotherapy

The purpose of this work is to examine the potential impact of the frequency and amplitude of fluctuations ("complexity") in intensity distributions on intensity-modulated radiotherapy (IMRT) dose distributions. The intensity-modulated beams are efficiently delivered using a multileaf collimator (MLC). Radiation may be delivered through a continuous (dynamic mode) or discrete (step-and-shoot) sequence of windows formed by the leaves. Algorithms and software that convert optimized intensity distributions into leaf trajectories apply approximate empirical corrections to account for the various effects associated with MLC characteristics, such as the rounded leaf tips, tongue-and-groove leaf design, leaf transmission, leaf scatter, and collimator scatter upstream from the MLC. Typically, the difference between inter- and intraleaf transmissions is ignored. In this paper, using a schematic example of IMRT for head and neck carcinomas, we demonstrate that complex anatomy and severe optimization constraints produce complex intensity patterns. Using idealized intensity patterns we also demonstrate that, for complex intensity patterns, the average window width tends to be smaller and, for the same dose received by the tumor, the number of MUs is larger. We found that as the complexity increases, so does the contribution of radiation transmitted through and scattered from the leaves ("indirect radiation") to the total delivered dose. As a consequence, the lowest deliverable intensity in complex intensity patterns may be significantly greater than that required to provide adequate protection for some normal tissues. Furthermore, since corrections for leaf transmission and scatter effects are approximate and the difference between inter- and intraleaf transmission is ignored, the accuracy of the delivered dose may be affected. Using the results of a simple experiment and a typical intensity-modulated beam for a head and neck case as examples, we show the effect of window width and complexity on the accuracy and deliverability of intensity patterns. Some possible strategies for improving the accuracy and for relaxing the lower limit on deliverable intensity are discussed.

Intensity modulation delivery techniques: "step & shoot" MLC auto-sequence versus the use of a modulator

Two intensity modulation radiotherapy (IMRT) delivery systems, the "step & shoot" multileaf collimator (MLC) auto-sequence and the use of an intensity modulator, are compared with emphasis on the dose optimization quality and the treatment irradiation time. The intensity modulation (IM) was created by a dose gradient optimization algorithm which maximizes the target dose uniformity while maintaining dose to critical structures below a set tolerance defined by the user in terms of either a single dose value or a dose volume histogram curve for each critical structure. Two clinical cases were studied with and without dose optimization: a three-field sinus treatment and a six-field nasopharyngeal treatment. The optimization goal of the latter case included the sparing of several nearby normal structures in addition to the target dose uniformity. In both cases, the target dose uniformity initially improved quickly as the IM level increased to 5, then started to approach saturation when the MLC technique was used. In the absence of the both space and intensity discreteness intrinsic to the MLC technique, the modulator technique produced greater tumor dose uniformity and normal structure sparing. The latter showed no systematic improvement with increasing IM level using the MLC technique. For the sinus tumor treatment of 2 Gy the treatment irradiation time of the modulator technique is no more than that of the conventional treatment. For the MLC technique the irradiation time increased rapidly from 4.4 min to 12.4 min as the IM level increased from 2 to 10. Both clinical cases suggested that an IM level of 5 offered a good compromise between the dose optimization quality and treatment irradiation time. We showed that a realistic photon source model is necessary for dose computation accuracy in the MLC-IM treatments.

Intensity modulated radiation therapy: a clinical review

Intensity modulated radiotherapy represents a significant advance in conformal radiotherapy. In particular, it allows the delivery of dose distributions with concave isodose profiles such that radiosensitive normal tissue close to, or even within a concavity of, a tumour may be spared from radiation injury. This article reviews the clinical application of this technique to date, and discusses the practical issues of treatment planning and delivery from the clinician's perspective.

Optimization of beam weights in conformal radiotherapy planning of stage III non-small cell lung cancer: effects on therapeutic ratio

PURPOSE

To evaluate the effects of beam weight optimization for 3D conformal radiotherapy plans, with or without beam intensity modulation, in Stage III non-small cell lung cancer (NSCLC).

METHODS AND MATERIALS

Ten patients with Stage III NSCLC were planned using a conventional 3D technique and a technique involving noncoplanar beam intensity modulation (BIM). Two planning target volumes (PTVs) were defined: PTV1 included macroscopic tumor volume and PTV2 included macroscopic and microscopic tumor volume. Virtual simulation defined the beam shapes and incidences as well as the wedge orientations (3D) and segment outlines (BIM). Weights of wedged beams, unwedged beams, and segments were determined by human trial and error for the 3D-plans (3D-manual), by a standard weight table (SWT) for the BIM-plans (BIM-SWT) and by optimization (3D-optimized and BIM-optimized) using an objective function with a biological and a physical component. The resulting non-optimized and optimized dose distributions were compared, using physical endpoints, after normalizing the median dose of PTV1 to 80 Gy.

RESULTS

Optimization improved dose homogeneity at the target for 3D- and BIM-plans and the minimum dose at PTV1. The minimum dose at PTV2 was decreased by optimization especially in 3D-plans. After optimization, the dose-volume histograms (DVHs) of lung and heart were shifted to lower doses for 80-90% of the organ volume. Since lung is the dose-limiting organ in Stage III NSCLC, an increased minimum dose at PTV1 together with a decreased dose at the main lung volume suggests an improved therapeutic ratio. Optimization allows 10% dose escalation for 3D-plans and 20% for BIM-plans at isotoxicity levels of lung and spinal cord. Upon dose escalation, esophagus may become the dose-limiting structure when PTV1 extends close to the esophagus.

CONCLUSIONS

Optimization using a biophysical objective function allowed an increase of the therapeutic ratio of radiotherapy planning for Stage III NSCLC.

A modified method of planning and delivery for dynamic multileaf collimator intensity-modulated radiation therapy

PURPOSE

To develop a modified planning and delivery technique that reduces dose nonuniformity for tomographic delivery of intensity-modulated radiation therapy (IMRT).

METHODS AND MATERIALS

The NOMOS-CORVUS system delivers IMRT in a tomographic paradigm. This type of delivery is prone to create multiple dose nonuniformity regions at the arc abutment regions. The modified technique was based on the cyclical behavior of arc positions as a function of a target length. With the modified technique, two plans are developed for the same patient, one with the original target and the second with a slightly increased target length and the abutment regions shifted by approximately 5 mm compared to the first plan. Each plan is designed to deliver half of the target prescription dose delivered on alternate days, resulting in periodic shifts of abutment regions. This method was experimentally tested in phantoms with and without intentionally introduced errors in couch indexing.

RESULTS

With the modified technique, the degree of dose nonuniformity was reduced. For example, with 1 mm error in couch indexing, the degree of dose nonuniformity changed from approximately 25% to approximately 12%.

CONCLUSION

Use of the modified technique reduces dose nonuniformity due to periodic shifts of abutment regions during treatment delivery.

3-D portal image analysis in clinical practice: an evaluation of 2-D and 3-D analysis techniques as applied to 30 prostate cancer patients

PURPOSE

To investigate the clinical importance and feasibility of a 3-D portal image analysis method in comparison with a standard 2-D portal image analysis method for pelvic irradiation techniques.

METHODS AND MATERIALS

In this study, images of 30 patients who were treated for prostate cancer were used. A total of 837 imaged fields were analyzed by a single technologist, using automatic 2-D and 3-D techniques independently. Standard deviations (SDs) of the random, systematic, and overall variations, and the overall mean were calculated for the resulting data sets (2-D and 3-D), in the three principal directions (left-right [L-R], cranial-caudal [C-C], anterior-posterior [A-P]). The 3-D analysis included rotations as well. For the translational differences between the three data sets, the overall SD and overall mean were computed. The influence of out-of-plane rotations on the 2-D registration accuracy was determined by analyzing the difference between the 2-D and 3-D translation data as function of rotations. To assess the reliability of the 2-D and 3-D methods, the number of times the automatic match was manually adjusted was counted. Finally, an estimate of the workload was made.

RESULTS

The SDs of the random and systematic components of the rotations around the three orthogonal axes were 1. 1 (L-R), 0.6 (C-C), 0.5 (A-P) and 0.9 (L-R), 0.6 (C-C), 0.8 (A-P) degrees, respectively. The overall mean rotation around the L-R axis was 0.7 degrees, which deviated significantly from zero. Translational setup errors were comparable for 2-D and 3-D analysis (ranging from 1.4 to 2.2 mm SD and from 1.5 to 2.5 mm SD, respectively). The variation of the difference between the 2-D and 3-D translation data increased from 1.1 mm (SD) for zero rotations to 2.7 mm (SD) for out-of-plane rotations of 3 degrees, due to a reduced 2-D registration accuracy for large rotations. The number of times the analysis was not considered acceptable and was manually adjusted was 44% for the 2-D analysis, and 6% for the 3-D analysis.

CONCLUSION

True 3-D analysis of setup errors for a group of 30 patients with prostate cancer demonstrated that setup rotations are rather small. The deformation of the projected anatomy in portal images caused by out-of-plane rotations leads to a reduced 2-D registration accuracy. For rotations larger than 3 degrees this effect can be quite pronounced, making 3-D registration the preferred method. Furthermore, the automatic 3-D registration has a higher success rate, most likely because this technique uses more information compared to the 2-D method.

Effect of distortions and asymmetry in MR images on radiotherapeutic treatment planning

Before using MR-images for radiotherapeutic treatment planning, it is necessary to check whether geometrical distortions influence the calculated dose distribution. In this study, 27 MRI units from different manufacturers and with different magnetic field strengths were investigated. First, all geometrical distortions and asymmetry were determined by specially defined parameters. Virtual planning target volume and organ of risk were drawn into MR images from different units and optimized dose distributions were calculated. The resulting field parameters were then transferred to the reference geometry. Geometric distortions caused small variations of the dose maximum (+/-0.5%). Twenty-five percent of the investigated calculated dose distributions showed an increase of the 95% isodose volume, 60% a decrease. The dose deviations in the organ of risk were comparable with the dose deviations in the planning target volume (PTV). This study has shown that measurements of geometrical distortions and asymmetry are necessary for MR-assisted treatment planning. For evaluation, we recommend dose calculations with hypothetical PTV's and organs of risk. Int. J. Cancer (Radiat. Oncol. Invest.) 90, 46-50 (2000).

The dosimetric consequences of inter-fractional patient movement on conventional and intensity-modulated breast radiotherapy treatments

BACKGROUND AND PURPOSE

A method has been developed to enable a comparison to be made between the effects of movement on conventional tangential breast treatments and intensity-modulated treatments delivered using compensators.

MATERIALS AND METHODS

The effects of set-up error and organ motion were studied for a set of six patients. Images were taken of these patients over the course of their treatment and conventional wedged and compensated treatment plans were designed for each. Dose-volume statistics were used to evaluate each of the treatment plans by examining the volume outside the dose range 95-105%. To assess the effects of movement alone, the volume change from day 1 was also calculated.

RESULTS

Thirty-six estimated CT-sets were available for evaluation. Measurements of breast volume showed the volume to increase to a peak between fraction 4 and 8 and then decrease back below the initial volume. The standard treatment was found to yield 29/36 plans (81%) with greater than 5% volume outside the dose range 95-105%. For the compensated plans this dropped to 11/36 plans (31%). The analysis of the volume changes from day 1 showed that for both standard and compensated treatments 7/30 plans (23%) had an increase in volume outside the dose range 95-105% of greater than 5% of the total planning target volume.

CONCLUSIONS

The compensated treatment is more susceptible to patient movement. However, the actual volume of tissue outside 95-105% dose is less when compared to standard treatment implying the compensated treatment is still superior.

Comparison of proton therapy and conformal X-ray therapy in non-small cell lung cancer (NSCLC)

This study compares the performance of one proton and four conformal X-ray planning techniques in treating non-small cell lung cancer (NSCLC). The treatment volumes for 13 NSCLC patients undergoing radical radiotherapy were planned using the five different techniques and dose-volume histograms (DVH) were used extensively in the comparative analysis. The minimum dose to the phase 2 target volume was escalated to 90 Gy, or until the point at which pre-set tolerance limits of spinal cord or lung were exceeded. The proton plan could treat nine of the 13 patients up to a dose of 90 Gy. Among the four X-ray techniques, performance varied enormously. One of them could not treat any of the patients, even to the conventional 60 Gy level, without failing to meet one or more of the criteria, whilst another one could treat 10 out of the 13 patients, although with this technique only four were permitted to have the dose escalated to 90 Gy. It was also found that two of the 13 patients could not be treated by any of the proton or X-ray plans to the conventional level, and were therefore considered unsuitable for radical radiotherapy. Various issues in conformal NSCLC radiotherapy including organ movement, tumour control, other possible organs at risk etc., are also discussed.

Dose quality assurance for stereotactic radiotherapy treatments

A simple Perspex phantom is presented for routine dosimetry checks of stereotactic radiotherapy procedures, along with a procedure for checking the planning process. The phantom is hemispherical and water filled. It can be fitted into the Brown-Robert-Wells fiducial system for CT scanning and attached to the stereotactic frame for treatment. Both arcing and fixed field plans can be carried out on the phantom. We present our results, performed on a Varian 600C linac, which were found to be within 2% of the expected dose.

The performance of a fluoroscopic electronic portal imaging device modified for portability

Advances in external beam therapy technology have made routine, efficient conformal therapy a reality. With it comes the increasing need for online treatment verification, which is only achievable at present through the use of electronic portal imaging devices (EPIDs). For a large radiotherapy centre, the provision of one EPID per treatment machine proves extremely expensive. This paper details modifications to the design of a commercial fluoroscopic EPID (the SRI-100) to produce a portable system, capable of providing quick, high quality imaging on more than one treatment machine. We describe the necessary hardware and software changes made to the system, as well as the variety of mechanical and quality control checks performed for testing the stability and quality of the imaging. The modified system has been found to be both electronically and mechanically robust, with associated image quality, scaling, distortion and movement similar to other EPIDs in the department. Although the modification was designed specifically to allow for the acquisition of images from multiple treatment machines, it may also enable the operation of the EPID for other uses such as total body irradiation (TBI) treatment verification and a further range of quality control procedures on the linear accelerator itself.

Dose-volume analysis for quality assurance of interstitial brachytherapy for breast cancer

PURPOSE/OBJECTIVE

The use of brachytherapy in the management of breast cancer has increased significantly over the past several years. Unfortunately, few techniques have been developed to compare dosimetric quality and target volume coverage concurrently. We present a new method of implant evaluation that incorporates computed tomography-based three-dimensional (3D) dose-volume analysis with traditional measures of brachytherapy quality. Analyses performed in this fashion will be needed to ultimately assist in determining the efficacy of breast implants.

METHODS AND MATERIALS

Since March of 1993, brachytherapy has been used as the sole radiation modality after lumpectomy in selected protocol patients with early-stage breast cancer treated with breast-conserving therapy. Eight patients treated with high-dose-rate (HDR) brachytherapy who had surgical clips outlining the lumpectomy cavity and underwent computed tomography (CT) scanning after implant placement were selected for this study. For each patient, the postimplant CT dataset was transferred to a 3D treatment planning system. The lumpectomy cavity, target volume (lumpectomy cavity plus a 1-cm margin), and entire breast were outlined on each axial slice. Once all volumes were entered, the programmed HDR brachytherapy source positions and dwell times were imported into the 3D planning system. Using the tools provided by the 3D planning system, the implant dataset was then registered to the visible implant template in the CT dataset. The distribution of the implant dose was analyzed with respect to defined volumes via dose-volume histograms (DVH). Isodose surfaces, the dose homogeneity index, and dosimetric coverage of the defined volumes were calculated and contrasted. All patients received 32 Gy to the entire implanted volume in 8 fractions of 4 Gy over 4 days.

RESULTS

Three-plane implants were used for 7 patients and a two-plane implant for 1 patient. The median number of needles per implant was 16.5 (range 11-18). Despite visual verification by the treating physician that surgical clips (with an appropriate margin) were within the boundaries of the implant needles, the median proportion of the lumpectomy cavity that received the prescribed dose was only 87% (range 73-98%). With respect to the target volume, a median of only 68% (range 56-81%) of this volume received 100% of the prescribed dose. On average, the minimum dose received by at least 90% of the target volume was 22 Gy (range 17.3-26.9), which corresponds to 69% of the prescribed dose.

CONCLUSION

Preliminary results using our new technique to evaluate implant quality with CT-based 3D dose-volume analysis appear promising. Dosimetric quality and target volume coverage can be concurrently analyzed, allowing the possibility of evaluating implants prospectively. Considering that target volume coverage may be suboptimal even after radiographically verifying accurate implant placement, techniques similar to this need to be developed to ultimately determine the true efficacy of brachytherapy in the management of breast cancer.

[Conformal radiotherapy of non-small-cell lung cancer]

About one third of lung cancers initially present with a localised disease, without any curative surgery potential, because of local spread or comorbidity. Definitive radiation, alone or combined with chemotherapy, then represents the treatment of choice for these patients. The results, however, are disappointing, with a biopsy-proven local control of 10% at two years and a 5-10% five-year survival rate. These poor results may be partially explained by the difficulties in delineating the tumour volume as well as the dose limitations due to poor tolerance of surrounding normal organs. Lung parenchyma sequelae remain daily worrying events for the oncologist. The advent of 3D conformal radiation therapy (3DRT) allows progress and innovations, including the use of modern imaging techniques, sophisticated dosimetry and treatment planning, efficient immobilisation devices and on-line verification procedures. With more precise (and time-consuming) procedures, 3DRT will allow a better tumour volume delineation, an increased tumour dose and a dose limitation in normal tissues. These improvements may help increase local control and survival results. 3DRT, which has been used for several years for prostate cancer and benefits from recent imaging improvements, will now allow treatment of other locations, such as lung cancer, with conformal therapy. The few preliminary results are encouraging. This work reviews the current data and remaining questions regarding lung cancer treated with 3DRT, and presents and discusses the literature before discussing future trends in this area.

[Conformal radiotherapy in cancer of the upper aerodigestive tract]

The outcome of head and neck malignancies is closely related to the achievement of local tumor control. The occurrence of severe late complications is the consequence of the anatomic vicinity between dose-limiting normal tissues and the tumor, thus reducing the curative potential of radiotherapy. Conformal radiotherapy is presently actively investigated as a way to improve the dose distribution in head and neck malignancies (especially when originating from the nasopharynx or the sinuses) while protecting healthy organs. Such an approach requires a chain of sophisticated procedures, including efficient quality assurance programs, in order to achieve the proper degree of safety. The relatively limited clinical studies published to-date have already confirmed the advantage of conformal radiotherapy in terms of dose delivery. The development of beam intensity modulation is likely to further contribute to the improvement of the therapeutic ratio.

Abutment region dosimetry for serial tomotherapy

PURPOSE

A commercial intensity modulated radiation therapy system (Corvus, NOMOS Corp.) is presently used in our clinic to generate optimized dose distributions delivered using a proprietary dynamic multileaf collimator (DMLC) (MIMiC) composed of 20 opposed leaf pairs. On our accelerator (Clinac 600C/D, Varian Associates, Inc.) each MIMiC leaf projects to either 1.00 x 0.84 or 1.00 x 1.70 cm2 (depending on the treatment plan and termed 1 cm or 2 cm mode, respectively). The MIMiC is used to deliver serial (axial) tomotherapy treatment plans, in which the beam is delivered to a nearly cylindrical volume as the DMLC is rotated about the patient. For longer targets, the patient is moved (indexed) between treatments a distance corresponding to the projected leaf width. The treatment relies on precise indexing and a method was developed to measure the precision of indexing devices. A treatment planning study of the dosimetric effects of incorrect patient indexing and concluded that a dose heterogeneity of 10% mm(-1) resulted. Because the results may be sensitive to the dose model accuracy, we conducted a measurement-based investigation of the consequences of incorrect indexing using our accelerator. Although the indexing provides an accurate field abutment along the isocenter, due to beam divergence, hot and cold spots will be produced below and above isocenter, respectively, when less than 300 degree arcs were used. A preliminary study recently determined that for a 290 degree rotation in 1 cm mode, 15% cold and 7% hot spots were delivered to 7 cm above and below isocenter, respectively. This study completes the earlier work by investigating the dose heterogeneity as a function of position relative to the axis of rotation, arc length, and leaf width. The influence of random daily patient positioning errors is also investigated.

METHODS AND MATERIALS

Treatment plans were generated using 8.0 cm diameter cylindrical target volumes within a homogeneous rectilinear film phantom. The plans included both 1 and 2 cm mode, optimized for 300 degrees, 240 degrees, and 180 degrees gantry rotations. Coronal-oriented films were irradiated throughout the target volumes and scanned using a laser film digitizer. The central target irradiated in 1 cm mode was also used to investigate the effects of incorrect couch indexing.

RESULTS

The dose error as a function of couch index error was 25% mm(-1), significantly greater than previously reported. The clinically provided indexing system yielded 0.10 mm indexing precision. The intrinsic dose distributions indicated that more heterogeneous dose distributions resulted from the use of smaller gantry angle ranges and larger leaf projections. Using 300 degrees gantry angle and 1 cm mode yielded 7% hot and 15% cold spots 7 cm below and above isocenter, respectively. When a 180 degree gantry angle was used, the values changed to 22% hot and 27% cold spots for the same locations. The heterogeneities for the 2 cm mode were 70% greater than the corresponding 1 cm values.

CONCLUSIONS

While serial tomotherapy is used to deliver highly conformal dose distributions, significant dosimetric factors must be considered before treatment. The patient must be immobilized during treatment to avoid dose heterogeneities caused by incorrect indexing due to patient movement. Even under ideal conditions, beam divergence can cause significant abutment-region dose heterogeneities. The use of larger gantry angle ranges, smaller leaf widths, and appropriate locations of the gantry rotation axis can minimize these effects.

Portal imaging based definition of the planning target volume during pelvic irradiation for gynecological malignancies

PURPOSE

Geometrical accuracy in patient positioning can vary substantially during external radiotherapy. This study estimated the set-up accuracy during pelvic irradiation for gynecological malignancies for determination of safety margins (planning target volume, PTV).

METHODS AND MATERIALS

Based on electronic portal imaging devices (EPID), 25 patients undergoing 4-field pelvic irradiation for gynecological malignancies were analyzed with regard to set-up accuracy during the treatment course. Regularly performed EPID images were used in order to systematically assess the systematic and random component of set-up displacements. Anatomical matching of verification and simulation images was followed by measuring corresponding distances between the central axis and anatomical features. Data analysis of set-up errors referred to the x-, y-,and z-axes. Additionally, cumulative frequencies were evaluated.

RESULTS

A total of 50 simulation films and 313 verification images were analyzed. For the anterior-posterior (AP) beam direction mean deviations along the x- and z-axes were 1.5 mm and -1.9 mm, respectively. Moreover, random errors of 4.8 mm (x-axis) and 3.0 mm (z-axis) were determined. Concerning the latero-lateral treatment fields, the systematic errors along the two axes were calculated to 2.9 mm (y-axis) and -2.0 mm (z-axis) and random errors of 3.8 mm and 3.5 mm were found, respectively. The cumulative frequency of misalignments < or =5 mm showed values of 75% (AP fields) and 72% (latero-lateral fields). With regard to cumulative frequencies < or =10 mm quantification revealed values of 97% for both beam directions.

CONCLUSION

During external pelvic irradiation therapy for gynecological malignancies, EPID images on a regular basis revealed acceptable set-up inaccuracies. Safety margins (PTV) of 1 cm appear to be sufficient, accounting for more than 95% of all deviations.

A quality assurance phantom for three-dimensional radiation treatment planning

PURPOSE

Three-dimensional (3D) radiation treatment planning is facilitated through the use of computerized radiation treatment planning systems (RTPSs) and CT simulators (CT-sims). Quality assurance (QA) of these systems is necessary for ensuring that they fulfill their potential. However, comprehensive tools for the systematic QA of these systems have not been developed. We present a phantom that facilitates the evaluation of a large number of nondosimetric functions. These include CT image acquisition and transfer, graphical displays of 3D radiation beams, multiplanar CT image reconstructions, digitally reconstructed radiographs, the representation and manipulation of contoured patient anatomy, dose volume histograms, and the conversion of CT numbers to relative electron densities.

METHODS AND MATERIALS

A phantom was constructed which contains materials and geometries that are appropriate for the routine QA of the features described above. The anatomy of the phantom is used as a standard against which the performance of the 3D-RTPS or CT-sim is evaluated. The phantom was used to evaluate three different 3D-RTPSs and a CT-sim at four institutions.

RESULTS

Using this phantom, clinically significant errors and limitations in commercially available 3D treatment planning software were discovered. No errors were discovered in the beam display or image reconstructions in the systems examined. Problems were found in the anatomy display, automatic tools, and the CT number to relative electron density conversion data used in some of the systems.

CONCLUSION

This phantom is a unique tool designed explicitly for the QA of 3D treatment planning software. Errors and limitations discovered through its use indicate that the QA of commercial treatment planning software is necessary, and that this phantom is an effective device for this task.

In vivo dosimetry during conformal radiotherapy: requirements for and findings of a routine procedure

PURPOSE

Conformal radiotherapy requires accurate knowledge of the actual dose delivered to a patient. The impact of routine in vivo dosimetry, including its special requirements, clinical findings and resources, has been analysed for three conformal treatment techniques to evaluate its usefulness in daily clinical practice.

MATERIALS AND METHODS

Based on pilot studies, routine in vivo dosimetry quality control (QC) protocols were implemented in the clinic. Entrance and exit diode dose measurements have been performed during two treatment sessions for 378 patients having prostate, bladder and parotid gland tumours. Dose calculations were performed with a CT-based three-dimensional treatment planning system. In our QC-protocol we applied action levels of 2.5% for the prostate and bladder tumour group and 4.0% for the parotid gland patients. When the difference between the measured dose at the dose specification point and the prescribed dose exceeded the action level the deviation was investigated and the number of monitor units (MUs) adjusted. Since an accurate dose measurement was necessary, some properties of the on-line high-precision diode measurement system and the long-term change in sensitivity of the diodes were investigated in detail.

RESULTS

The sensitivity of all diodes decreased by approximately 7% after receiving an integrated dose of 10 kGy, for 4 and 8 MV beams. For 34 (9%) patients the difference between the measured and calculated dose was larger than the action level. Systematic errors in the use of a new software release of the monitor unit calculation program, limitations of the dose calculation algorithms, errors in the planning procedure and instability in the performance of the accelerator have been detected.

CONCLUSIONS

Accurate in vivo dosimetry, using a diode measurement system, is a powerful tool to trace dosimetric errors during conformal radiotherapy in the range of 2.5-10%, provided that the system is carefully calibrated. The implementation of an intensive in vivo dosimetry programme requires additional staff for measurements and evaluation. The patient measurements add only a few minutes to the total treatment time per patient and guarantee an accurate dose delivery, which is a prerequisite for conformal radiotherapy.

Potential role of various dosimetric quality indicators in prostate brachytherapy

PURPOSE

Postoperative CT-based dosimetric analysis provides detailed information regarding the coverage and uniformity of an implant, but the assessment of implant quality remains an unanswered and controversial issue. There is no disagreement that a good implant should cover the target volume with an adequate dose, but there is no consensus as to what represents an adequate dose.

MATERIALS AND METHODS

The American Brachytherapy Society has recently proposed that prostate brachytherapy quality be measured in terms of the following parameters: D90, V100, and V150 where D90 is defined as the minimal dose covering 90% of the prostate volume and V100 and V150 are defined as the percent volume of the prostate receiving at least 100% or 150% of the prescribed minimal peripheral dose (mPD), respectively. We report detailed day 0 dosimetric evaluation for 60 consecutive prostate brachytherapy patients implanted via a standard transperineal ultrasound guided approach in terms of D90, D100, V90, V100, and V150 and also the maximal and average rectal and urethral dose.

RESULTS

Dosimetric evaluation resulted in a V100 greater than 80% of the prostate volume and a D90 greater than 90% of the mPD in the entire patient population. There was a statistically significant difference between the quality scores of 125I implants and 103Pd implants with the 125I mean V100 and D90 at 95.3% volume and 109.9% mPD, respectively, vs. 103Pd at 91.8% volume and 103.7% mPD. Likewise, the rectal and urethral doses as a fraction of mPD were significantly lower in 103Pd than in 125I implants. This occurred despite the fact that palladium implants were typically preplanned with significantly better coverage and hotter V150 than iodine implants. We consider V150 to be an important parameter for determining dose homogeneity although the clinical utility of dose homogeneity remains unknown. The mean V150 was 45.6 +/- 9.6% volume. There was no additional dosimetric utility from a determination of V90 while D100 was found to be overly sensitive to steep dose gradients at the periphery of the prostate.

CONCLUSIONS

This report represents the first detailed postimplant day 0 dosimetric evaluation comparing ABS recommended quality parameters used to evaluate prostate brachytherapy. At the present time, no long-term clinical outcomes are available because of short follow-up. As PSA based follow-up data becomes available, however, this report may help define what represents an adequate implant.

Assessment of accuracy of daily set-ups in prostate radiotherapy using electronic imaging

The purpose of the study was to assess the accuracy, using electronic portal imaging, of daily set-ups, in patients undergoing radiotherapy for prostate carcinoma. We used a scanning liquid ion chamber to assess the accuracy of set-ups in 25 consecutive patients undergoing a 6 1/2 week course of radiotherapy to the prostate. Electronic images (EPIs) were collected during 33 treatments to each of four ports. The positions of anatomical structures on the EPIs were compared with the same structures seen on digitally reconstructed radiographs (DRRs) made after CT simulation before beginning radiotherapy. Displacements of the EPIs compared with the DRRs were computer-calculated in millimetres in lateral, longitudinal and rotational directions for each port. 11 patients had ports moved because of discrepancies between the EPIs and the DRRs; eight required moves in the first five treatments to correct systematic (simulator) errors. In the right-left and anterior-posterior directions, nearly 95% of the EPIs were within 5 mm of the simulated port position. In the superior-inferior direction, 98% of the ports were within 5 mm of the simulated port position. Two patients had in-plane rotational errors on the lateral ports (8 degrees and 10 degrees respectively). It was concluded that daily electronic imaging is an effective technique for assessing the accuracy of set-ups in prostate radiotherapy.