DVH Calculation Research Articles

EyeDose: An Open Source Tool For Calculating The Radiation Dose Delivered To The Tumor And Critical Ocular Structures For 125I COMS Eye Plaques

Radiation side effects and visual outcome for uveal melanoma patients managed with plaque radiotherapy are dependent on the radiation dose administered to the tumor and nearby healthy tissues. The AAPM Task Group Report 129 recommends that the dose to critical normal structures be recorded, preferably using heterogeneity corrected dose distributions. We have developed an open source software tool, EyeDose, to simplify and standardize tumor and critical structure dose reporting for COMS eye plaques. This work builds upon the accomplishments of previous researchers and aims to combine the best features of prior tools, including the accuracy of Monte Carlo, the elimination of manual contouring and the treatment planning system, the standardization of eye geometry, and precise plaque localization using high resolution fundus diagrams. The software may be used for prospective clinical decision making, for example to help with plaque size selection or a comparison of plaque radiotherapy versus proton therapy, or to rapidly process large retrospective data sets, without repetitive manual entry, for reporting on the relationships between dosimetry and outcomes. EyeDose is a Matlab program that calculates point and volume (DVH) dose metrics for the tumor and critical ocular structures. The three-dimensional dose distributions for 125I Collaborative Ocular Melanoma Study (COMS) eye plaques, calculated with Monte Carlo methods, are oriented with respect to the eye using the tumor’s position on a fundus diagram. A standard model for the ocular structures was created using published measurements and patient CT scans. EyeDose reports radiation statistics for the fovea, optic disc, lens, lacrimal gland, retina, and tumor. The dosimetric margin for implant placement uncertainty is also calculated. EyeDose calculations were validated against published Monte Carlo results for nasal, temporal, inferior, and superior tumor positions, including the dose to the fovea, optic disc, lacrimal gland, lens, and along the central axis. The EyeDose program demonstrated close agreement with published Monte Carlo results. The mean, median, and maximum point dose differences between EyeDose and previous Monte Carlo studies were 0.0, 0.0, and 0.5 Gy. The mean difference, reported as %volume, among the DVH calculations was -0.2% ± 0.2% (Std. Dev.) with a range from -1.08% to 0.05%. The execution time averaged <40 sec. per patient on an Intel i-5-6500 CPU @ 3.20 GHz with 32 GB RAM. EyeDose is published as an open source tool for easy adaptation at different institutions. EyeDose calculates radiation statistics for COMS eye plaque patients with Monte Carlo accuracy and without a treatment planning system. EyeDose streamlines data collection for large retrospective studies and can also be used prospectively to assess plaque applicability.

161. A crowd-knowledge-based analysis of DVHs in SBRT: First steps towards a national virtual audit

Purpose Currently, most of the multicenter analyses on treatment planning rely on the extraction of selected data from the DVH of each plan. A grouped analysis can be biased due to different algorithms implemented in different TPSs used to generate the DVH. In this work we used a consistent method to present a preliminary analysis of multiple data coming from a national survey on stereotactic body radiotherapy (SBRT) planning. Methods A single spine case was shared among 36 radiation oncology centers. The dose prescription was 30 Gy in 3 fractions with specific constraints on target coverage and dose to nearby organs at risk. Data were collected in DICOM-RT format. A script was developed in R language using the RadOnc R-Package for recalculating the DVHs using the same algorithm. Specific DVH points (V30Gy, D90%, D2%) collected from the centers were compared with those recalculated with RadOnc. A grouped analysis of recalculated DVHs was performed therefore eliminating the bias due to different DVH calculation algorithms. Results Differences between collected and recalculated DVHs were minimal, however in some cases deviations up to 1.5% were observed. The multiple-DVH analysis showed a notable variability on target dose level (Fig. 1), up to 150% likely related to constraints on target coverage and SBRT technique. This variability was caused mainly by different planning optimization strategies, rather than by the use of a specific treatment technology. Conclusions The observed variability suggests that comparable standards in patient treatment among different centers can be obtained if a consistent high-level data sharing capability is granted. In the strive to harmonize the planning process, this analysis constitutes a first step toward the creation of a platform of crowd-knowledge-based planning guidelines. This platform could represent a high-quality benchmark for those centers that are willing to implement SBRT techniques (concept expressed in Fig. 1). Currently, most of the multicenter analyses on treatment planning rely on the extraction of selected data from the DVH of each plan. A grouped analysis can be biased due to different algorithms implemented in different TPSs used to generate the DVH. In this work we used a consistent method to present a preliminary analysis of multiple data coming from a national survey on stereotactic body radiotherapy (SBRT) planning. A single spine case was shared among 36 radiation oncology centers. The dose prescription was 30 Gy in 3 fractions with specific constraints on target coverage and dose to nearby organs at risk. Data were collected in DICOM-RT format. A script was developed in R language using the RadOnc R-Package for recalculating the DVHs using the same algorithm. Specific DVH points (V30Gy, D90%, D2%) collected from the centers were compared with those recalculated with RadOnc. A grouped analysis of recalculated DVHs was performed therefore eliminating the bias due to different DVH calculation algorithms. Differences between collected and recalculated DVHs were minimal, however in some cases deviations up to 1.5% were observed. The multiple-DVH analysis showed a notable variability on target dose level (Fig. 1), up to 150% likely related to constraints on target coverage and SBRT technique. This variability was caused mainly by different planning optimization strategies, rather than by the use of a specific treatment technology. The observed variability suggests that comparable standards in patient treatment among different centers can be obtained if a consistent high-level data sharing capability is granted. In the strive to harmonize the planning process, this analysis constitutes a first step toward the creation of a platform of crowd-knowledge-based planning guidelines. This platform could represent a high-quality benchmark for those centers that are willing to implement SBRT techniques (concept expressed in Fig. 1).

SU‐E‐T‐615: Plan Comparison Between Photon IMRT and Proton Plans Incorporating Uncertainty Analysis

Purpose:In this study, we investigate the effect of setup uncertainty on DVH calculations which may impact plan comparison.Methods:Treatment plans (6 MV VMAT calculated on Pinnacle TPS) were chosen for different disease sites: brain, prostate, H&amp;N and spine in this retrospective study. A proton plan (PP) using double scattering beams was generated for each selected VMAT plan subject to the same set of dose‐volume constraints as in VMAT. An uncertainty analysis was incorporated on the DVH calculations in which isocenter shifts from 1 to 5 mm in each of the ±x, ±y and ±z directions were used to simulate the setup uncertainty and residual positioning errors. A total of 40 different combinations of isocenter shifts were used in the re‐calculation of DVH of the PTV and the various OARs for both the VMAT and the corresponding PT.Results:For the brain case, both VMAT and PP are comparable in PTV coverage and OAR sparing, and VMAT is a clear choice for treatment due to its ease of delivery. However, when incorporating isoshifts in DVH calculations, a significant change in dose‐volume relationship emerges. For example, both VMAT and PT provide adequate coverage, even with ±3mm isoshift. However, +3mm isoshift results in increase of V40(Lcochlea, VMAT) from 7.2% in the original plan to 45% and V40(R cochlea, VMAT) from 75% to 92%. For protons, V40(Lcochlea, PT) increases from 62% in the initial plan to 75%, while V40(Rcochea, PT) increases from 7% to 26%.Conclusion:DVH alone may not be sufficient to allow an unequivocal decision in plan comparison, especially when two rival plans are very similar in both PTV coverage and OAR sparing. It is a good practice to incorporate uncertainty analysis on photon and proton plan comparison studies to test the plan robustness in plan evaluation.

SU‐F‐BRD‐02: Incorporating Uncertainty Analysis in Plan Comparison of VMAT, Double Scattering Proton Plan and IMPT for Lung Irradiation

Purpose:We investigate the effect of residual setup and motion errors in lung irradiation for VMAT, double scattering (DS) proton beams and spot scanning (IMPT) in a case study.Methods:The CT image and contour sets of a lung patient treated with 6 MV VMAT is re‐planned with DS as well as IMPT subject to the same constraints; V20(lung), V10(lung) and V5(lung)&lt; 15%, 20% and 25% respectively, V20(heart)&lt;25% and V100%(PTV)≥95%. In addition, uncertainty analysis in the form of isocenter shifts (±1–3mm) was incorporated in the DVH calculations to assess the plan robustness.Results:Only the IMPT plan satisfies all the specified constraints. The 3D‐conformal DS proton plan is able to achieve better sparing of the lung and heart dose compared to VMAT. For the lung, V20, V10 and V5 are 13%, 19% and 25% respectively for IMPT, 18%, 23% and 30% respectively for DS, and 20%, 30% and 42% respectively for VMAT. For heart: 0.6% for IMPT, 2.4% for DS and 30% for VMAT. When incorporating isocenter shifts in DVH calculations, the maximum changes in V20, V10 and V5 for lung are 14%, 21% and 28% respectively for IMPT. The corresponding max changes are19%, 24% and 32% respectively for DS, and 22%, 32% and 44% respectively for VMAT. The largest change occurs in the PTV coverage. For IMPT, V100%(PTV) varies between 88–96%, while V100%(PTV) for VMAT suffers a larger change compared to DS (Δ=5.5% vs 3.3%).Conclusion:While only IMPT satisfies the stringent dose‐volume constraints for the lung irradiation, it is not as robust as the 3D conformal DS plan. DS also has better sparing in lung and heart compared to VMAT and similar PTV coverage. By including isocenter shifts in dose‐volume calculations in treatment planning of lung, DS appears to be more robust than VMAT.

RADIANCE—A planning software for intra-operative radiation therapy

In the last decades accumulated clinical evidence has proven that intra-operative radiation therapy (IORT) is a very valuable technique. In spite of that, planning technology has not evolved since its conception, being outdated in comparison to current state of the art in other radiotherapy techniques and therefore slowing down the adoption of IORT. RADIANCE is an IORT planning system, CE and FDA certified, developed by a consortium of companies, hospitals and universities to overcome such technological backwardness. RADIANCE provides all basic radiotherapy planning tools which are specifically adapted to IORT. These include, but are not limited to image visualization, contouring, dose calculation algorithms-Pencil Beam (PB) and Monte Carlo (MC), DVH calculation and reporting. Other new tools, such as surgical simulation tools have been developed to deal with specific conditions of the technique. Planning with preoperative images (preplanning) has been evaluated and the validity of the system being proven in terms of documentation, treatment preparation, learning as well as improvement of surgeons/radiation oncologists (ROs) communication process. Preliminary studies on Navigation systems envisage benefits on how the specialist to accurately/safely apply the pre-plan into the treatment, updating the plan as needed. Improvements on the usability of this kind of systems and workflow are needed to make them more practical. Preliminary studies on Intraoperative imaging could provide an improved anatomy for the dose computation, comparing it with the previous pre-plan, although not all devices in the market provide good characteristics to do so. DICOM.RT standard, for radiotherapy information exchange, has been updated to cover IORT particularities and enabling the possibility of dose summation with external radiotherapy. The effect of this planning technology on the global risk of the IORT technique has been assessed and documented as part of a failure mode and effect analysis (FMEA). Having these technological innovations and their clinical evaluation (including risk analysis) we consider that RADIANCE is a very valuable tool to the specialist covering the demands from professional societies (AAPM, ICRU, EURATOM) for current radiotherapy procedures.

RADIANCE—A planning software for intra-operative radiation therapy

In the last decades accumulated clinical evidence has proven that intra-operative radiation therapy (IORT) is a very valuable technique. In spite of that, planning technology has not evolved since its conception, being outdated in comparison to current state of the art in other radiotherapy techniques and therefore slowing down the adoption of IORT. RADIANCE is an IORT planning system, CE and FDA certified, developed by a consortium of companies, hospitals and universities to overcome such technological backwardness. RADIANCE provides all basic radiotherapy planning tools which are specifically adapted to IORT. These include, but are not limited to image visualization, contouring, dose calculation algorithms—Pencil Beam (PB) and Monte Carlo (MC), DVH calculation and reporting. Other new tools, such as surgical simulation tools have been developed to deal with specific conditions of the technique. Planning with preoperative images (preplanning) has been evaluated and the validity of the system being proven in terms of documentation, treatment preparation, learning as well as improvement of surgeons/radiation oncologists (ROs) communication process. Preliminary studies on Navigation systems envisage benefits on how the specialist to accurately/safely apply the pre-plan into the treatment, updating the plan as needed. Improvements on the usability of this kind of systems and workflow are needed to make them more practical. Preliminary studies on Intraoperative imaging could provide an improved anatomy for the dose computation, comparing it with the previous pre-plan, although not all devices in the market provide good characteristics to do so. DICOM.RT standard, for radiotherapy information exchange, has been updated to cover IORT particularities and enabling the possibility of dose summation with external radiotherapy. The effect of this planning technology on the global risk of the IORT technique has been assessed and documented as part of a failure mode and effect analysis (FMEA). Having these technological innovations and their clinical evaluation (including risk analysis) we consider that RADIANCE is a very valuable tool to the specialist covering the demands from professional societies (AAPM, ICRU, EURATOM) for current radiotherapy procedures.

SU‐E‐T‐319: The Effect of Slice Thickness On IMRT Planning

Purpose:The accuracy of volume estimated of a treatment planning system is investigated in this study. In addition, the effect of slice thickness on IMRT planning is also studied.Methods:The accuracy in volume determination was investigated using a water phantom containing various objects with known volumes ranging from 1–100cm3. The phantom was scanned with different slice thickness (1–10 mm). The CT data sets were sent to Eclipse TPS for contour delineation and volume calculation. The effect of slice thickness on IMRT planning was studied using a commercial phantom containing four different shaped objects. The phantom was scanned with different slice thickness (1–5 mm). IMRT plans were generated for the different CT datasets to calculate TCP, homogeneity (HI) and conformity indices (CI).Results:The variability of volumes with CT slice thickness was significant especially for small volume structures. The minimum and maximum error in the volume estimation is in the range of −2.3% to 92%. On the other hand, with increasing slice thickness, the PTV mean dose and TCP values decreases. Maximum variation of ∼5% was observed in mean dose and ∼2% in TCP with slice thickness change from 1–5 mm. The relative decrease in target volume receiving 95% of prescribed dose is ∼5% slice thickness change from 1–5 mm. HI increases up to 163% and CI decreases by 4% between 1–5 mm slice thickness change, producing highly inhomogeneous and least conformal plan.Conclusion:Accuracy of volume estimation is dependent on CT slice thickness and the contouring algorithm in a TPS. During TPS commissioning and for all clinical protocols, evaluation of volume should be included to provide the limit of accuracy in DVH calculation. A smaller slice thickness provides superior dosimetry with improved TCP values. Thus, the smallest possible slice thickness should be used for IMRT planning

SU‐E‐T‐29: A Web Application for GPU‐Based Monte Carlo IMRT/VMAT QA with Delivered Dose Verification

Purpose:To enable an existing web application for GPU‐based Monte Carlo (MC) 3D dosimetry quality assurance (QA) to compute “delivered dose” from linac logfile data.Methods:We added significant features to an IMRT/VMAT QA web application which is based on existing technologies (HTML5, Python, and Django). This tool interfaces with python, c‐code libraries, and command line‐based GPU applications to perform a MC‐based IMRT/VMAT QA. The web app automates many complicated aspects of interfacing clinical DICOM and logfile data with cutting‐edge GPU software to run a MC dose calculation. The resultant web app is powerful, easy to use, and is able to re‐compute both plan dose (from DICOM data) and delivered dose (from logfile data). Both dynalog and trajectorylog file formats are supported. Users upload zipped DICOM RP, CT, and RD data and set the expected statistic uncertainty for the MC dose calculation. A 3D gamma index map, 3D dose distribution, gamma histogram, dosimetric statistics, and DVH curves are displayed to the user. Additional the user may upload the delivery logfile data from the linac to compute a “delivered dose” calculation and corresponding gamma tests. A comprehensive PDF QA report summarizing the results can also be downloaded.Results:We successfully improved a web app for a GPU‐based QA tool that consists of logfile parcing, fluence map generation, CT image processing, GPU based MC dose calculation, gamma index calculation, and DVH calculation. The result is an IMRT and VMAT QA tool that conducts an independent dose calculation for a given treatment plan and delivery log file. The system takes both DICOM data and logfile data to compute plan dose and delivered dose respectively.Conclusion:We sucessfully improved a GPU‐based MC QA tool to allow for logfile dose calculation. The high efficiency and accessibility will greatly facilitate IMRT and VMAT QA.

Monte Carlo dosimetry of high dose rate gynecologic interstitial brachytherapy

PurposeTo assess the dosimetric effects of the presence of the applicator, air pockets in clinical target volume (CTV) and OARs along with tissue heterogeneities using the Monte Carlo (MC) method in high dose rate (HDR) gynecologic interstitial brachytherapy with a Syed-Neblett template. Methods and materialsThe CT based dosimetry has been achieved with the Geant4 MC toolkit version 9.2. DICOM-RT files of 38 patients were imported into our own platform for MC simulations. The dose distributions were then compared to those obtained with a conventional TG-43 calculation. ResultsTaking account of heterogeneities has effects of the order of 1% on the HDR gynecological dose distributions. However, the exclusion of air pockets and applicator from the DVH calculation can lower the CTV D90 and V100 by as much as 8.7% and 5.0% in comparison with TG-43. Rectum dosimetric indices can also be lowered by approximately 3% compared with TG-43 for most cases. Differences for urethra and bladder are for most cases below 1%. ConclusionsExclusion of non-biological material such as air pockets and applicator volume from the CTV is important for both TG-43 and MC calculations. It could be easily implemented and automated in treatment planning systems without affecting computation times.

Toward Customizable Radiation Therapy Enhancement (CuRE) With Gold Nanoparticles Released, In Situ, From Gold-Loaded Brachytherapy Spacers

DVHs were generated and compared. Monte Carlo calculations were performed to simulate ISA and to verify plan robustness. Results: On average the total number of seeds compared to the standard plan is 32% and 70% less for hemi and ultra focal plans respectively, whereas the average needles were 38% and 58% less for hemi and ultra focal respectively. Target volume coverage was adequately achieved for all plans with V100 above 97.8%. D90 was increased by 19% and 69% for hemi and ultra focal plans respectively. Meanwhile, D10 of urethral dose was reduced by 7% and 55% for hemi and ultra focal plans respectively. D2cc of rectum was reduced by 28% and 60% for hemi and ultra focal respectively. Plan robustness shows that a random shift in seed position of 4 mm would result in V100 and D90 dropping by 7.2% and 14%, respectively, for standard plans, with a corresponding reduction of 7.3% and 20% for hemi plans and 5% and 32% for ultra focal plans. Despite the fact that seed density is higher for the hemi and ultra focal approaches, ISA has similar effect on DHV parameters. For example, V100 and D90 are reduced by less than 1% and 3%, respectively, for all types of plans. Conclusions: Treatment planning for hemi and ultra focal options is feasible. Dose constraints are easily met with a notable reduction to organs at risk such as the bladder, urethra and rectum. Treating smaller targets, such as for ultra focal, makes seed positioning more critical. This should be taken into consideration during treatment planning. ISA has the same effect on DVH calculations for standard, hemi and ultra focal plans. Author Disclosure: B. Al-Qaisieh: G. Consultant; GE Oncura Ltd. J. Mason: None. A. Henry: None. P. Bownes: None. L. Dickinson: None. A. Hashim: None. M. Emberton: None. S. Langley: None.

SU‐E‐T‐193: A Custom Web Application for a GPU‐Based Monte Carlo IMRT/VMAT QA Tool

Purpose: To develop a user friendly web application for a GPU‐based Monte Carlo (MC) 3D dosimetry quality assurance (QA) tool which runs in modern web browsers and interacts remotely with specialized hardware/software resources. Methods: We developed a new QA web application based on existing technologies (HTML5, Python, and Django) to interface with a command line‐based MC QA tool. It eliminates the need for users of this cutting‐edge GPU software to purchase expensive/specialized hardware and use a command‐line based environment. The Results is a web app with a user‐friendly interface enabling control of GPU accelerated software for MC‐based QA. Users upload zipped DICOM RP, CT, and RD data and set the expected statistic uncertainty for the MC dose calculation. After the files are uploaded, the remote software is automatically launched to generate intermediate data necessary for the GPU‐based MC dose calculation. The MC dose calculation is then executed on the remote GPU server and the resultant dose and original plan dose (from uploaded RD file) are displayed in the web GUI for the user to review. A 3D gamma index map and DVH curves are also displayed to the user. Finally, a PDF QA report that summarizes the results can be downloaded. Results: We successfully developed a web app for a GPU‐based QA tool that consists of fluence map generation, CT image processing, GPU based MC dose calculation, gamma index calculation, and DVH calculation. The Results is an IMRT and VMAT QA tool that conducts an independent dose calculation for a given treatment plan. The computation time from data uploading to viewing results and downloading report is less than 2 min. Conclusion: We developed a GPU‐based MC QA tool with a user‐friendly web‐based interface. The high efficiency and accessibility greatly facilitate IMRT and VMAT QA.

SU-E-T-387: Evaluation of VMAT Dosimetric Effect Caused by Slowing MLC Leaves Using MapCHECK

Purpose: This study is to report the effects on the quality of volumetric modulated arc therapy (VMAT) dose delivery by simulating multileaf collimator (MLC) leaf speed errors that do not trigger any MLC motion interlocks (i.e. <2 mm leaf position error). Method and Materials: Four VMAT plans (two one-arc brain cases and two four-arc anal cases) were modified using MATLAB. Within each arc, five consecutive MLC leafs on the same bank were selected for modification. Leaf speed at each control point was calculated. Only those control points with highest speed (average ∼4.6% of total control points) were rise to the Varian MLC speed limit (2.5 cm/s), which gave a leaf position difference of less than 2 mm. No changes in MU per control point and gantry speed were introduced. The modified plans were now considered as “standard” plans and the original plans were treated as “wrong” plans with leafs moving at a lower speed. Modified DICOM plans were imported back into Eclipse for dose calculations and DVH comparisons. The recalculated planar dose was compared with original MapCHECK measurement using the standard patient-specific VMAT quality assurance (QA) approach, with a gamma-index of 3%/3mm, 3%/2mm, and 2%/2mm. Results: All cases passed the >85% criterion for a gamma-index of 3%/3mm. There was only a small passing rate difference (<1%) between the original and the modified plans. The DVH calculations of the modified plans demonstrated very minimal changes in PTV and OAR doses. Conclusions: By introducing the leaf speed error without triggering the MLC motion interlock, the dose changes cannot be detected by MapCHECK. However, the consequential effects on target and OAR doses are also negligible, which supports the reliability of current patient IMRT QA method for VMAT plan verification.

Assessment of Acute Cervical Skin Toxicity Due to Fractionated Proton Beam Therapy for Head-and-Neck Cancer

By our actual clinical experience, moderate to severe cervical radiation dermatitis occurs in the majority of patients treated with proton beam therapy (PBT) for head and neck cancer (HNC) patients. The aim of this study is to assess the factors affecting the onset of acute cervical dermatitis in HNC patients with nodal metastasis treated with PBT. We retrospectively investigated the clinical and dosimetric predictors of planning system for skin toxicity. Twenty-seven patients with HNC treated with fractionated PBT to nodal metastasis with or without chemotherapy were reviewed. Acute cervical skin toxicity of irradiated area was estimated based on the CTCAE version 4.0 grading. Clinical factors thought to be influencing skin toxicity, such as the prescribed dose and number of fractions, physical beam characteristics, size and location of the target volume, concomitant chemotherapy and DVH parameters of skin surface were analyzed for their relation with risk of acute dermatitis. In this study, the skin surface region was defined the area contoured divided into the thickness in every 2mm structure inside away from the skin surface at the irradiated portions such as surface-2mm, 2-4mm, 4-6mm, 6-8mm, 8-10mm. Typical dosimetric parameters, including the size of the irradiated volume, value of V40-80GyE, and the mean and maximal point dose delivered to skin surface region were obtained from DVH calculation. To evaluate steepness of the entrance dose to skin surface, gradient dose index (surface-2mm volume-dose to 8-10mm volume-dose) was defined. Median biological effective dose (BED) at an α/β ratio of 10 as the prescribed dose was 84.5GyE10 (range, 60-96.8GyE10). Eleven (40%) patients developed acute Grade 1, 12 (45%) patients Grade 2, and 4(15%) patients Grade 3 acute dermatitis. In uni-variate analysis, factors associated with onset of Grade 2 or higher acute dermatitis included BED (GyE10) as prescription dose (p = 0.030), skin dose parameter (V45-70GyE10) (p = 0.044, 0.039, 0.031, 0.015, 0.010, and 0.022, respectively), and gradient dose index (GDI65GyE and 70GyE) (p = 0.031, and 0.048, respectively). When plotting the p value of the logistic regression analysis between the variables, V65 GyE10 of skin surface area show the strongest statistical significance (p ≦ 0.01). High incidence of severe (Grade 2 or higher) radiation dermatitis (60% of entire cohort) was observed during clinical use of PBT in HNC patients. The entrance portal area to skin irradiated volume with 45-70GyE10 was possible predictor for acute skin toxicity and higher dose volume of superficial regions over 65GyE10 should be considered in the treatment planning as sensitive organ volume in PBT.

Commissioning and Comprehensive Quality Assurance of commercial 3D Treatment Planning System using IAEA Technical Report Series — 430

The purpose of this work is to report the results of commissioning and to establish a quality assurance (QA) program for commercial 3D treatment planning system (TPS) based on IAEA Technical Report Series 430. Eclipse v 7.3.10, (Varian Medical Systems, Palo Alto, CA, U.S.A.) TPS was commissioned for a Clinac 6EX (Varian Medical Systems, Palo Alto, CA, USA) linear accelerator. CT images of a phantom with various known in-homogeneities were acquired. The images were transferred to TPS and tested for various parameters related to patient data acquisition, anatomical modeling, plan evaluation and dose calculation. Dosimetric parameters including open, asymmetric and wedged shaped fields, oblique incidence, buildup region behavior and SSD dependence were evaluated. Representative clinical cases were tested for MU calculation and point doses. The maximum variation between the measured and the known CT numbers was 20 +/- 11.7 HU (1 SD). The results of all non-dosimetric tests were found within tolerance, however expansion at the sharp corners was found distorted. The accuracy of the DVH calculations depends on the grid size. TPS calculations of all the dosimetric parameters were in good agreement with the measured values, however for asymmetric open and wedged fields, few points were found out of tolerance. Smaller grid size calculation showed better agreement of dose calculation in the build-up region. Independent tests for MU calculation showed a variation within +/-2% (relative to planning system), meanwhile variation of 3.0% was observed when the central axis was blocked. The test results were in agreement with the tolerance specified by IAEA TRS 430. A subset of the commissioning tests has been identified as a baseline data for an ongoing QA program.

SU‐GG‐T‐315: McGill Monte Carlo Research Platform (MMCTP) for Dose Comparison Studies

Purpose: To demonstrate recent improvements in the functionality of MMCTP, a radiotherapy research platform GUI, with integrated Monte Carlo (MC) dose calculation submission and dose analysis tools. MMCTP allows for quick MC dose calculations from standard radiotherapy formats. Clinical and MC dose distributions are analyzed under the same platform, which eliminates inherent planning system tool dependent effects. History: MMCTP has been internally distributed within the last year. External distribution, which has so far been limited to close contacts, will be open to the public. DICOM‐RT plans, which include: images, structures, dose distributions and beam information, have been successfully imported from TPS such as: Eclipse, Corvus and Tomotherapy. MC calculations use the images structures, and beam information to generate a beam and patient model. The GUI is capable of handling the following beam parameters: dynamic wedge, static/dynamic MLC, and static wedge. MC dose distributions are normalized to absolute dose with the beam monitor units and directly compared to the imported clinical dose distributions. The GUI includes a set of built in comparison tools: isodose lines and colour‐wash displays in transverse/sagittal/coronal view, DVH calculations, dose difference maps and dose profile graphs. Validation has been extensive and includes profile comparisons between MC and measured data. Results: Patient recalculations include a set of: conformal lung and breast patients, gated high dose lung patients, IMRT head and neck patients. Recalculations times vary depending on the number and complexity of beams, size and resolution of patient model. Heterogeneous recalculations reveal differences between planned and delivered dose distributions. Conclusion: MMCTP is a flexible research platform for the development of patient specific MC treatment planning for photon and electron external beam radiation therapy. The visualization, dose analysis tools offer extensive possibility for plan analysis and comparison to plans imported from commercial TPS through well‐documented protocols such as DICOM‐RT.

SU-FF-T-204: Effect of Voxel Size On Monte Carlo Dose Calculation for Intensity Modulated Radiotherapy Treatment Planning

Purpose: In radiotherapytreatment planning, the voxel size plays a role in each step, from the structure contouring, beamlet calculation, treatment optimization to plan analysis. Large voxels may affect the planning accuracy while small voxels will increase computation time significantly. The choice of voxel size is a compromise between planning accuracy and computation speed. This work investigates the effect of voxel size on the accuracy and computational speed of Monte Carlo based treatment planning for IMRT.Method and Materials: Selected patients with various treatment sites were scanned on a CT simulator and the CT data with RT structures were converted to patient geometry files with different voxel sizes for Monte Carlo simulations. Comparisons of the structure volume, beamlet dose distribution, plan optimization, DVHs and dose distributions were made with different voxel sizes. Target and critical structures with small volumes, such as the optical nerves in head and neck cases, were specially considered in the comparison. Results: In Monte Carlo IMRT planning, differences in the open‐field dose calculations prior to the optimization process using different voxel sizes (4 × 4 mm2 and 2 × 2 mm2) were usually small. However, the results from the optimization showed that different voxel sizes could generate different beamlet weights resulting in different treatment plans. The effects could be clinically significant. For the same IMRT plans, the effects of voxel sizes in the dose and DVH calculations vary depending on the treatment sites, anatomy and target/organ volume. Small voxel sizes (1–2mm) are necessary for small structures in and near heterogeneous geometries such as in the head and neck regions. Conclusions: The effect of voxel size on dose calculation and plan optimization can be clinically significant in IMRTtreatment planning. Small voxel (1–2mm) should be used for head and neck cases.

Quality assurance of the 22961 EORTC trial. A phase III study of the optimal combination of hormonal adjuvant treatment by LHRH analogue and radiation therapy for the management of locally advanced prostate cancer: the dummy run

Purpose The EORTC trial 22961, opened in 1997, was designed to investigate the optimal combination of hormonal adjuvant treatment by LHRH analogue and radiation therapy for the management of locally advanced prostate cancer. A dummy run was established to assess centre compliance to the radiotherapy protocol. Materials and Methods Medical and anatomical data obtained from 37 CT slices (5 mm thickness) of an eligible patient were sent to 19 participating centres, which were asked to complete a questionnaire according to their practice and plan a theoretical radiotherapy treatment. The Planning Target Volume 1 (PTV1) should include prostate, seminal vesicles, internal iliac lymph nodes and inferior part of common iliac lymph nodes (extended pelvic fields). Centres which usually irradiate with small pelvic fields (N0 patients), were allowed to include the prostate, seminal vesicles and internal iliac lymph nodes plus a safety margin of 2 cm. For the Planning Target Volume 2 (PTV2), a safety margin of 1.5 to 2 cm should be around the prostate and seminal vesicles. Checks included patient positioning, treatment simulation, target volume definition, treatment set-up and clinical controls during treatment. Results Eleven institutions with actual 81% of patients' accrual in the protocol have responded. All centres used a supine treatment position and positioning lasers for the set-up, while 73 and 45% of the centres performed cystograms and used rectal contrast, respectively. Among the participating centres, 45% and 55% used blocks and MLC, respectively, to treat patients. Extended pelvic fields in terms of PTV1 were used by 63% of the centres. The remaining centres treated a small PTV1 with a 10–20 mm margin around to CTV1. All centres defined PTV2 according to protocol guidelines. Doses to PTV1 and PTV2 were correctly prescribed. It was difficult to assess the treated volumes due to a lack of standardisation in DVH calculations. Conclusion In general, centres participating in the dummy run adhered to the guidelines. The dummy run enhances the reliability of the conclusions of the trial.

1095 Poster Comparing rectal DVH calculation with four commercial treatment planning systems by means of the AAPM TG 23 18 MV unit

1095 Poster Comparing rectal DVH calculation with four commercial treatment planning systems by means of the AAPM TG 23 18 MV unit

Analysis of tumor control probability (TCP) and normal tissue complication probability (NTCP) in 40 prostate cancer patients treated with 2 different techniques of 3D-conformal radiation therapy (3D-CRT)

Purpose: To compare 2 regimens of 3D-CRT (3-field vs 6-field technique) in the treatment of prostate cancer, using TCP and NTCP data. Materials & Methods: Between October and December 1997, 20 consecutive patients with clinically localized adenocarcinoma of the prostate (4 T1, 9 T2, 7 T3) were treated with 3D-CRT 3-field technique. For the purpose of this study the records of 20 patients matched for tumor stage (5 T1, 9 T2, 6 T3) and treated with 3D-CRT 6-field technique, were analyzed and served as a basis for comparison. With the 3-field technique, the patients were simulated with rectum and bladder empty in a supine position using custom-made polyurethane foam or thermoplastic casts as immobilization devices. An anterior field (0) and two wedged posterior oblique fields (110 and 250 °) were devised. In order to minimize the dose delivered to the anterior rectal wall and despite a less homogenous dose distribution within the target, the wedges were inserted with the thick end toward the midline. Non-uniform margins were applied around the outline of the target volume, that is, 1 cm towards the rectal wall and 1.5 cm towards the bladder anteriorly and also for the upper and lower portal margins. The shaping of the beam apertures was realized by conventional cerrobend blocks. The total target dose was prescribed at the ICRU point and was 76 Gy for the prostate delivered by a 15 MV linear accelerator in 38 fractions and 56 days (mean). The seminal vesicles were excluded at 70 Gy. The 6-field technique was different only in positioning of the patient (prone) and arrangement of portals (45 °, 90, 135 °, 225*, 270*, 315*). For the calculation of the NTCP values regarding the bladder and rectum we have employed the Lyman's empirical model. The cumulative dose-volume histograms were reduced according to the effective volume model introduced by Kutcher and Burman. Radiobiological data published by Emami was used in this analysis. The TCP calculation was made according to Goitein's model. The mean TCP and NTCP values over the two groups were compared using a pooled two sample t-test. The log percentages were used to ensure that the normality assumptions were valid. Results: TCP values obtained with the two techniques were similar: mean value equals 98.7 for the 3-field technique (range: 97.6-99.4), 98.6 for the 6-field technique (range: 97.5-99.5), meaning a good target coverage with both techniques; p= 0.27, that means no evidence of difference between the two techniques for target coverage. NTCP values were: bladder NTCP 0.96 for 3-field (range: 0.02-4.99) and 0.91 for 6-field (range: 0.02-4.48) (p=0.67), rectum NTCP 7.6 for 3field (range: 2.7-16.9) and 5.7 for 6-field (range: 1.6-11.9) (p=0.051). There is no evidence of difference between the two techniques for bladder complication, but there may be some evidence of difference between the two groups for rectum complications. Conclusions: The two techniques yielded similar results in terms of TCP and bladder NTCP. In terms of rectum NTCP, there was a slight difference in favour of the 6-field technique. Acute toxicity was similar in either groups, but longer follow-up is needed to evaluate the relationship between the mathematical modelling and the rate of late toxicity. The difficulty in extrapolating data from other institutions when calculating TCP and NTCP prompted us to create our own database to evaluate the validity and the predictibility of these mathematical models, considering that different DVH calculation algorithms are usually applied in each center.