Treatment Planning System Dose Distributions Research Articles

Commissioning Validation of a Brachytherapy Treatment Planning System with EGSnrc Monte Carlo Code and EBT3 GAFChromic Film.

Most brachytherapy treatment planning system (TPS) commissioning requires data input based on the American Association of Physicists in Medicine Task Group-43 formalism. The commissioning accuracy is very important for dose calculation. The aim of this study is the implementation of a brachytherapy TPS into a clinical environment and check the TPS calculated dose accuracy. After introducing data of the different catheters (CIS Bio International, Saclay, France), composed of several Cesium-137 Eckert and Ziegler BEBIG CSM-11 radioactive sources; for XiO (CMS, St. Louis) brachytherapy TPS, the TPS dose calculation accuracy was investigated by comparing between the TPS calculated dose distribution (DD) for all the catheters with (1) the measuring DD using EBT3 GAFChromic film and (2) calculating DD by egs_brachy (Electron Gamma Shower, National Research Council of Canada) Monte Carlo simulation. The phantom used for this study consists of six PTW slabs 30 cm × 30 cm × 1 cm of polymethyl methacrylate with a Delouche MEDpro applicator on the top. The TPS DD was calculated on the computed tomography scan of this phantom. PTW VeriSoft version 6.0.1.7 (PTW-Freiburg, Germany) software was used for analyzing scanned films and to perform the comparison based on the gamma index distribution. For each catheter, the gamma index distribution showed agreement >95% of all pixels in both verification methods, with gamma ≤1. We confirm the commissioning accuracy and that the TPS can be used for clinical purposes.

Technical Note: Three-dimensional QA of simultaneous integrated boost radiotherapy treatments by a dose-volume histogram methodology and its comparison with 3D gamma results.

Intensity-modulated radiotherapy with simultaneous integrated boost (SIB) presents several attractive advantages to be employed in clinical practice. Its secure application demands a rigorous quality assurance (QA) procedure, ideal for three-dimensional (3D) dose distribution measurements. Thus, a gel dosimetry methodology to evaluate the dose delivery of SIB treatments is presented and compared to conventional gamma evaluation. MAGIC-f gel dosimeter with magnetic resonance images for dose reading were used following its standard procedures. Four SIB QA plans created in gel dosimeter phantoms were used. The gel measured and treatment planning system (TPS) calculated doses were compared using 3D gamma analyses (3%/3mm/15% threshold). Two structures were artificially on the TPS dose distribution expected on the phantom by converting the 1.7 and 2.0Gy isodose levels into structures to represent the treatment. The gel and TPS dose-volume histogram (DVH) were compared based on five dose points: D95%, D90%, D50%, D10%, and D5%. Approvals of 93%, 96%, 98%, and 92% were achieved in the 3D gamma analyses for the plans QA 1, 2, 3, and 4. In the DVH analyses, QA plan 1 measured and expected curves showed a good agreement. QA plan 2 showed deviations in the highest doses for both structures with a maximum deviation (Δmáx ) of 8.0%. QA plans 3 and 4 showed the highest dose variation between the gel and TPS in the smaller doses of the DVH (Δmáx of 7.2% and -8.9%, respectively). For QA plan 4, the curves of the 1.7Gy structure presented a good agreement, but deviations in the smaller dose region of the DVH occurred for the 2Gy structure (Δmáx of 7.7%). A methodology for 3D dose evaluation of complex SIB treatments was proposed. It provided an important overview of the dose distributions. Their results significantly complemented the usual gamma analysis results.

Feasibility of patient specific quality assurance for proton therapy based on independent dose calculation and predicted outcomes

PurposePatient specific quality assurance (PSQA) is required to verify the treatment delivery and the dose calculation by the treatment planning system (TPS). The objective of this work is to demonstrate the feasibility to substitute resource consuming measurement based PSQA (PSQAM) by independent dose recalculations (PSQAIDC), and that PSQAIDC results may be interpreted in a clinically relevant manner using normal tissue complication probability (NTCP) and tumor control probability (TCP) models. Methods and materialsA platform for the automatic execution of the two following PSQAIDC workflows was implemented: (i) using the TPS generated plan and (ii) using treatment delivery log files (log-plan). 30 head and neck cancer (HNC) patients were retrospectively investigated. PSQAM results were compared with those from the two PSQAIDC workflows. TCP/NTCP variations between PSQAIDC and the initial TPS dose distributions were investigated. Additionally, for two example patients that showed low passing PSQAM results, eight error scenarios were simulated and verified via measurements and log-plan based calculations. For all error scenarios ΔTCP/NTCP values between the nominal and the log-plan dose were assessed. ResultsResults of PSQAM and PSQAIDC from both implemented workflows agree within 2.7% in terms of gamma pass ratios. The verification of simulated error scenarios shows comparable trends between PSQAM and PSQAIDC. Based on the 30 investigated HNC patients, PSQAIDC observed dose deviations translate into a minor variation in NTCP values. As expected, TCP is critically related to observed dose deviations. ConclusionsWe demonstrated a feasibility to substitute PSQAM with PSQAIDC. In addition, we showed that PSQAIDC results can be interpreted in clinically more relevant manner, for instance using TCP/NTCP.

Artifact-free CT images for electron beam therapy using a patient-specific non metallic shield

Patient’s CT images taken with metallic shields for radiotherapy suffer from artifacts. Furthermore, the treatment planning system (TPS) has a limitation on accurate dose calculations for high density materials. In this study, a Monte Carlo (MC)-based method was developed to accurately evaluate the dosimetric effect of the metallic shield. Two patients with a commercial tungsten shield of lens and two patients with a custom-made lead shield of lip were chosen to produce their non-metallic dummy shields using 3D scanner and printer. With these dummy shields, we generated artifact-free CT images. The maximum CT number allowed in TPS was assigned to metallic shields. MC simulations with real material information were carried out. In addition, clinically relevant dose-volumetric parameters were calculated for the comparison between MC and TPS. Relative dosimetry was performed using radiochromic films. The dose reductions below metallic structures were shown on MC dose distributions, but not evident on TPS dose distributions. The differences in dose-volumetric parameters of PTV between TPS and MC for eye shield cases were not clearly shown. However, the mean dose of lens from TPS and MC was different. The MC results were in superior agreement with measured data in relative dosimetry. The lens dose could be overestimated by TPS. The differences in dose-volumetric parameters of PTV between TPS and MC were generally larger in lip cases than in eye cases. The developed method is useful in predicting the realistic dose distributions around the organs blocked by the metallic shields.

A Monte Carlo model for organ dose reconstruction of patients in pencil beam scanning (PBS) proton therapy for epidemiologic studies of late effects

Significant efforts such as the Pediatric Proton/Photon Consortium Registry (PPCR) involving multiple proton therapy centers have been made to conduct collaborative studies evaluating outcomes following proton therapy. As a groundwork dosimetry effort for the late effect investigation, we developed a Monte Carlo (MC) model of proton pencil beam scanning (PBS) to estimate organ/tissue doses of pediatric patients at the Maryland Proton Treatment Center (MPTC), one of the proton centers involved in the PPCR. The MC beam modeling was performed using the TOPAS (TOol for PArticle Simulation) MC code and commissioned to match measurement data within 1% for range, and 0.3 mm for spot sizes. The established MC model was then tested by calculating organ/tissue doses for sample intracranial and craniospinal irradiations on whole-body pediatric computational human phantoms. The simulated dose distributions were compared with the treatment planning system dose distributions, showing the 3 mm/3% gamma index passing rates of 94%–99%, validating our simulations with the MC model. The calculated organ/tissue doses per prescribed doses for the craniospinal irradiations (1 mGy Gy−1 to 1 Gy Gy−1) were generally much higher than those for the intracranial irradiations (2.1 μGy Gy−1 to 0.1 Gy Gy−1), which is due to the larger field coverage of the craniospinal irradiations. The largest difference was observed at the adrenal dose, i.e. ∼3000 times. In addition, the calculated organ/tissue doses were compared with those calculated with a simplified MC model, showing that the beam properties (i.e. spot size, spot divergence, mean energy, and energy spread) do not significantly influence dose calculations despite the limited irradiation cases. This implies that the use of the MC model commissioned to the MPTC measurement data might be dosimetrically acceptable for patient dose reconstructions at other proton centers particularly when their measurement data are unavailable. The developed MC model will be used to reconstruct organ/tissue doses for MPTC pediatric patients collected in the PPCR.

18 Dosimetric study of the couch top influence in VMAT

Introduction During these last years, treatment planning system (TPS) algorithms that compute the absorbed dose have improved. A high precision between delivered absorbed dose and prescribed dose is required, limiting the uncertainties up to 5% between delivered dose and prescribed dose according to the ICRU recommendations [1] . One of the parameters contributing to these uncertainties is the couch top, which attenuates the beam from 2% to 15% when it passes through [2] . Last generation’s carbon fiber couch tops act as a bolus material, increasing skin dose while decreasing depth-dose. Therefore, the consideration of the couch top by the TPS is important for avoiding bad tumor control, overdose of organs at risk and skin radiotoxicity effects [2] . Methods The reference technique for the dose distribution calculations is based on Monte Carlo method (MC) [3] . Among the most well-known codes, BEAMnrc/EGSnrc allows to model the accelerator treatment head and to use CT images (DICOM format) in order to simulate the dose distribution to voxelized phantoms. Furthermore, the CT data have to be converted to densities and to the corresponding materials, taking into account the couch top [4] . A software developed by the team using a conversion ramp between Hounsfield Units (HU) and densities/materials, specific to the couch top, will be used to create the voxelized phantoms. Results MC method takes into consideration not only voxel densities, but also materials, if a wrong material assignment is made, it could lead to dosimetric errors in the dose distribution calculations. Different treatment sites will be considered in Volumetric Modulated Arc Therapy (VMAT). Phantoms with the couch top will be created and dose distributions will be compared to TPS dose distributions. Conclusions The results of this study contributed to evaluate different methods of taking into account the couch top, using MC.

An automated Monte Carlo QC system for volumetric modulated arc therapy: Possibilities and challenges

PurposeTo develop and implement an automated Monte Carlo (MC) system for patient specific VMAT quality control in a patient geometry that generates treatment planning system (TPS) compliant DICOM objects and includes a module for 3D analysis of dose deviations. Also, the aims were to recommend diagnose specific tolerance criteria and an evaluation procedure. MethodsThe EGSnrc code package formed the basis for development of the MC system. The workflow consists of a number of modules connected to a TPS by means of manual DICOM exports and imports which were executed sequentially without user interaction. DVH comparison was performed in the TPS. In addition, MC- and TPS dose distributions were analysed by applying the normalized dose difference (NDD) formalism. NDD failure maps and a pass rate for a certain threshold were obtained. 170 clinical plans (prostate, thorax, head-and-neck and gynecological) were selected for analysis. ResultsAgreement within 1.5% was found between clinical- and MC data for the mean dose to the target volumes and within 3% for parameters more sensitive to the shape of the DVH e.g. D98% PTV. Regarding the NDD analysis, tolerance criteria 2%/3 mm were established for prostate plans and 3%/3 mm for the rest of the cases. ConclusionsAn automated MC system was developed and implemented. Evaluation procedure is recommended with NDD-analysis as a first step. For pass rate < 95%, the evaluation continues with comparison of DVH parameters. For deviations larger than 2%, a visual inspection of the clinical- and MC dose distributions is performed.

Neural network dose models for knowledge-based planning in pancreatic SBRT.

Stereotactic body radiation therapy (SBRT) for pancreatic cancer requires a skillful approach to deliver ablative doses to the tumor while limiting dose to the highly sensitive duodenum, stomach, and small bowel. Here, we develop knowledge-based artificial neural network dose models (ANN-DMs) to predict dose distributions that would be approved by experienced physicians. Arc-based SBRT treatment plans for 43 pancreatic cancer patients were planned, delivering 30-33 Gy in five fractions. Treatments were overseen by one of two physicians with individual treatment approaches, with variations in prescribed dose, target volume delineation, and primary organs at risk. Using dose distributions calculated by a commercial treatment planning system (TPS), physician-approved treatment plans were used to train ANN-DMs that could predict physician-approved dose distributions based on a set of geometric parameters (vary from voxel to voxel) and plan parameters (constant across all voxels for a given patient). Patient datasets were randomly allocated, with two-thirds used for training, and one-third used for validation. Differences between TPS and ANN-DM dose distributions were used to evaluate model performance. ANN-DM design, including neural network structure and parameter choices, was evaluated to optimize dose model performance. Remarkable improvements in ANN-DM accuracy (i.e., from > 30% to < 5% mean absolute dose error, relative to the prescribed dose) were achieved by training separate dose models for the treatment style of each physician. Increased neural network complexity (i.e., more layers, more neurons per layer) did not improve dose model accuracy. Mean dose errors were less than 5% at all distances from the PTV, and mean absolute dose errors were on the order of 5%, but no more than 10%. Dose-volume histogram errors (in cm3 ) demonstrated good model performance above 25 Gy, but much larger errors were seen at lower doses. ANN-DM dose distributions showed excellent overall agreement with TPS dose distributions, and accuracy was substantially improved when each physician's treatment approach was taken into account by training their own dedicated models. In this manner, one could feasibly train ANN-DMs that could predict the dose distribution desired by a given physician for a given treatment site.

Abstract ID: 16 Automated Monte Carlo QA system for volumetric modulated arc therapy: Possibilities and challenges

The objective of the work is to develop and implement an automated MC system for patient specific VMAT QA generating treatment planning system (TPS) compliant DICOM objects and including a stand-alone module for 3D analysis of dose deviations based on the normalized dose difference (NDD) method. The MC system developed is based on the EGSnrc code package with modifications [1] . The workflow consists of a number of modules connected to the TPS by means of DICOM exports and imports which are executed sequentially without user interaction. DVH comparison is performed in the TPS. In addition, MC- and TPS dose distributions are imported to the stand-alone analysis module based on the NDD formalism [2] . NDD failure maps and a pass rate for a certain threshold are obtained. 70 clinical plans are selected for analysis; 21 thorax plans, 26 prostate plans, 13 H&N plans and 10 gynecological plans. Agreement within 1.5% has been found between clinical- and MC data for the mean dose to the target volumes. The agreement is within 3% for parameters more sensitive to the shape of the DVH, e.g. D95% PTV or minimum dose to CTV. Tolerance criteria of 2%/3 mm are recommended for NDD analysis of prostate plans and 3%/3 mm for rest of the cases. Evaluation procedure is suggested where NDD analysis is the first step. For pass rate lower than 95% the evaluation continues with comparison of DVH parameters. For deviations larger than 2%, a visual inspection of the clinical- and MC dose distributions is performed. A fully automated evaluation is hindered by artefacts in the CT images, presence of contrast in the bladder, dose to air included in the target volume, interpretation of HU in rectum etc.

SU-F-T-364: Monte Carlo-Dose Verification of Volumetric Modulated Arc Therapy Plans Using AAPM TG-119 Test Patterns

Purpose: To investigate the Monte Carlo (MC)-based dose verification for VMAT plans by a treatment planning system (TPS). Methods: The AAPM TG-119 test structure set was used for VMAT plans by the Pinnacle3 (convolution/superposition), using a Synergy radiation head of a 6 MV beam with the Agility MLC. The Synergy was simulated with the EGSnrc/BEAMnrc code, and VMAT dose distributions were calculated with the EGSnrc/DOSXYZnrc code by the same irradiation conditions as TPS. VMAT dose distributions of TPS and MC were compared with those of EBT3 film, by 2-D gamma analysis of ±3%/3 mm criteria with a threshold of 30% of prescribed doses. VMAT dose distributions between TPS and MC were also compared by DVHs and 3-D gamma analysis of ±3%/3 mm criteria with a threshold of 10%, and 3-D passing rates for PTVs and OARs were analyzed. Results: TPS dose distributions differed from those of film, especially for Head & neck. The dose difference between TPS and film results from calculation accuracy for complex motion of MLCs like tongue and groove effect. In contrast, MC dose distributions were in good agreement with those of film. This is because MC can model fully the MLC configuration and accurately reproduce the MLC motion between control points in VMAT plans. D95 of PTV for Prostate, Head & neck, C-shaped, and Multi Target was 97.2%, 98.1%, 101.6%, and 99.7% for TPS and 95.7%, 96.0%, 100.6%, and 99.1% for MC, respectively. Similarly, 3-D gamma passing rates of each PTV for TPS vs. MC were 100%, 89.5%, 99.7%, and 100%, respectively. 3-D passing rates of TPS reduced for complex VMAT fields like Head & neck because MLCs are not modeled completely for TPS. Conclusion: MC-calculated VMAT dose distributions is useful for the 3-D dose verification of VMAT plans by TPS.

Feasibility of portal dosimetry for flattening filter-free radiotherapy.

The feasibility of using portal dosimetry (PD) to verify 6 MV flattening filter‐free (FFF) IMRT treatments was investigated. An Elekta Synergy linear accelerator with an Agility collimator capable of delivering FFF beams and a standard iViewGT amorphous silicon (aSi) EPID panel (RID 1640 AL5P) at a fixed SSD of 160 cm were used. Dose rates for FFF beams are up to four times higher than for conventional flattened beams, meaning images taken at maximum FFF dose rate can saturate the EPID. A dose rate of 800 MU/min was found not to saturate the EPID for open fields. This dose rate was subsequently used to characterize the EPID for FFF portal dosimetry. A range of open and phantom fields were measured with both an ion chamber and the EPID, to allow comparison between the two. The measured data were then used to create a model within The Nederlands Kanker Instituut's (NKI's) portal dosimetry software. The model was verified using simple square fields with a range of field sizes and phantom thicknesses. These were compared to calculations performed with the Monaco treatment planning system (TPS) and isocentric ion chamber measurements. It was found that the results for the FFF verification were similar to those for flattened beams with testing on square fields, indicating a difference in dose between the TPS and portal dosimetry of approximately 1%. Two FFF IMRT plans (prostate and lung SABR) were delivered to a homogeneous phantom and showed an overall dose difference at isocenter of ∼0.5% and good agreement between the TPS and PD dose distributions. The feasibility of using the NKI software without any modifications for high‐dose‐rate FFF beams and using a standard EPID detector has been investigated and some initial limitations highlighted.PACS number: 87.55.Qr

Impact of daily anatomical changes on EPID-based in vivo dosimetry of VMAT treatments of head-and-neck cancer

Background and purposeTarget dose verification for VMAT treatments of head-and-neck (H&N) cancer using 3D in vivo EPID dosimetry is expected to be affected by daily anatomical changes. By including these anatomical changes through cone-beam CT (CBCT) information, the magnitude of this effect is investigated. Materials and methodsFor 20 VMAT-treated H&N cancer patients, all plan-CTs (pCTs), 633 CBCTs and 1266 EPID movies were used to compare four dose distributions per fraction: treatment planning system (TPS) calculated dose and EPID reconstructed in vivo dose, both determined using the pCT and using the CBCT. D2, D50 and D98 of the planning target volume (PTV) were determined per dose distribution. ResultsWhen including daily anatomical information, D2, D50 and D98 of the PTV change on average by 0.0±0.4% according to TPS calculations; the standard deviation of the difference between EPID and TPS target dose changes from 2.5% (pCT) to 2.1% (CBCT). Small time trends are seen for both TPS and EPID dose distributions when using the pCT, which disappear when including CBCT information. ConclusionsDaily anatomical changes hardly influence the target dose distribution for H&N VMAT treatments according to TPS recalculations. Including CBCT information in EPID dose reconstructions slightly improves the agreement with TPS calculations.

TH-C-17A-03: Dynamic Visualization and Dosimetry of IMRT and VMAT Treatment Plans by Video-Rate Imaging of Cherenkov Radiation in Pure Water

Purpose: A novel optical dosimetry technique for the QA and verification of intensity-modulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT) radiotherapy plans was investigated for the first time by capturing images of the induced Cherenkov radiation in water. Methods: An intensified CCD camera (ICCD) was used to acquire a two-dimensional (2D) projection image of the Cherenkov radiation induced by IMRT and VMAT plans, based on the Task Group 119 C-Shape geometry. Plans were generated using the Varian Eclipse treatment planning system (TPS) and delivered using 6 MV x-rays from a Varian TrueBeam Linear Accelerator (Linac) incident on a water tank. The ICCD acquisition was gated to the Linac, operated for single pulse imaging, and binned to a resolution of 512×512 pixels. The resulting videos were analyzed temporally for regions of interest (ROI) covering the planning target volume (PTV) and organ at risk (OAR) and summed to obtain an overall light distribution, which was compared to the expected dose distribution from the TPS using a gammaindex analysis. Results: The chosen camera settings resulted in data at 23.5 frames per second. Temporal intensity plots of the PTV and OAR ROIs confirmed the preferential delivery of dose to the PTV versus the OAR, and the gamma analysis yielded 95.2% and 95.6% agreement between the light distribution and expected TPS dose distribution based upon a 3% / 3 mm dose difference and distance-to-agreement criterion for the IMRT and VMAT plans respectively. Conclusion: The results from this initial study demonstrate the first documented use of Cherenkov radiation for optical dosimetry of dynamic IMRT and VMAT treatment plans. The proposed modality has several potential advantages over alternative methods including the real-time nature of the acquisition, and upon future refinement may prove to be a robust and novel dosimetry method with both research and clinical applications. NIH R01CA109558 and R21EB017559.

Video-rate optical dosimetry and dynamic visualization of IMRT and VMAT treatment plans in water using Cherenkov radiation.

A novel technique for optical dosimetry of dynamic intensity-modulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT) plans was investigated for the first time by capturing images of the induced Cherenkov radiation in water. A high-sensitivity, intensified CCD camera (ICCD) was configured to acquire a two-dimensional (2D) projection image of the Cherenkov radiation induced by IMRT and VMAT plans, based on the Task Group 119 (TG-119) C-Shape geometry. Plans were generated using the Varian Eclipse treatment planning system (TPS) and delivered using 6 MV x-rays from a Varian TrueBeam Linear Accelerator (Linac) incident on a water tank doped with the fluorophore quinine sulfate. The ICCD acquisition was gated to the Linac target trigger pulse to reduce background light artifacts, read out for a single radiation pulse, and binned to a resolution of 512 × 512 pixels. The resulting videos were analyzed temporally for various regions of interest (ROI) covering the planning target volume (PTV) and organ at risk (OAR), and summed to obtain an overall light intensity distribution, which was compared to the expected dose distribution from the TPS using a gamma-index analysis. The chosen camera settings resulted in 23.5 frames per second dosimetry videos. Temporal intensity plots of the PTV and OAR ROIs confirmed the preferential delivery of dose to the PTV versus the OAR, and the gamma analysis yielded 95.9% and 96.2% agreement between the experimentally captured Cherenkov light distribution and expected TPS dose distribution based upon a 3%/3 mm dose difference and distance-to-agreement criterion for the IMRT and VMAT plans, respectively. The results from this initial study demonstrate the first documented use of Cherenkov radiation for video-rate optical dosimetry of dynamic IMRT and VMAT treatment plans. The proposed modality has several potential advantages over alternative methods including the real-time nature of the acquisition, and upon future refinement may prove to be a robust and novel dosimetry method with both research and clinical applications.

Three-dimensional gamma analysis of dose distributions in individual structures for IMRT dose verification.

Our purpose in this study was to implement three-dimensional (3D) gamma analysis for structures of interest such as the planning target volume (PTV) or clinical target volume (CTV), and organs at risk (OARs) for intensity-modulated radiation therapy (IMRT) dose verification. IMRT dose distributions for prostate and head and neck (HN) cancer patients were calculated with an analytical anisotropic algorithm in an Eclipse (Varian Medical Systems) treatment planning system (TPS) and by Monte Carlo (MC) simulation. The MC dose distributions were calculated with EGSnrc/BEAMnrc and DOSXYZnrc user codes under conditions identical to those for the TPS. The prescribed doses were 76 Gy/38 fractions with five-field IMRT for the prostate and 33 Gy/17 fractions with seven-field IMRT for the HN. TPS dose distributions were verified by the gamma passing rates for the whole calculated volume, PTV or CTV, and OARs by use of 3D gamma analysis with reference to MC dose distributions. The acceptance criteria for the 3D gamma analysis were 3/3 and 2 %/2 mm for a dose difference and a distance to agreement. The gamma passing rates in PTV and OARs for the prostate IMRT plan were close to 100 %. For the HN IMRT plan, the passing rates of 2 %/2 mm in CTV and OARs were substantially lower because inhomogeneous tissues such as bone and air in the HN are included in the calculation area. 3D gamma analysis for individual structures is useful for IMRT dose verification.

WE‐E‐141‐11: Optical Cone Beam 3D Tomography of Radiation Beams Using Cerenkov‐Excited Fluorescence

Purpose: Here, we explore a novel medical physics application of the Cerenkov effect for the dosimetric profiling of megavoltage beams used in radiation therapy. The technology to achieve these images has recently progressed from capturing single 2D projections with an intensified charge‐coupled device, to full 3D optical tomography using parallel beam back projection and a telecentric lens. In the current study, the method is extended to 3D imaging with a standard CMOS commercial camera and lens, using cone beam tomography algorithms for the reconstruction. Methods: Projection images of the induced Cerenkov light from a 20×20 cm2 6 MV beam operating at a dose rate of 600 MU/min incident on a water tank with a 1.0 g/L concentration of Quinine Sulfate were captured from 0–360° with a 3° angular resolution and 5.0 second exposure time, resulting in a total scan time of 10 minutes. The resulting projection images were used in a Feldkamp‐Davis‐Kress (FDK) cone beam tomography algorithm to reconstruct the imparted 3D dose distribution. An accuracy analysis was performed by comparing the reconstructed lateral profile at a depth of 10 cm and a percent depth dose curve to a reference dose distribution from the treatment planning system (TPS). Results: A 3D dose distribution was successfully reconstructed from the captured projections yielding results which were within +/− 8% of the TPS dose distribution. Conclusion: Optical cone beam 3D tomography of radiation beams using Cerenkov‐excited fluorescence has been demonstrated for the first time. The proposed technique has several advantages over alternative methods (e.g., ionization chambers, scintillation, and gel dosimetry) including speed, flexibility, and necessitates only water, which serves as a cheap, abundant, and easily standardized tissue equivalent medium. Upon future refinement and improved accuracy the proposed modality may prove to be an important dosimetric tool with both clinical and research applications.

GATE as a GEANT4-based Monte Carlo platform for the evaluation of proton pencil beam scanning treatment plans

Active scanning delivery systems take full advantage of ion beams to best conform to the tumor and to spare surrounding healthy tissues; however, it is also a challenging technique for quality assurance. In this perspective, we upgraded the GATE/GEANT4 Monte Carlo platform in order to recalculate the treatment planning system (TPS) dose distributions for active scanning systems. A method that allows evaluating the TPS dose distributions with the GATE Monte Carlo platform has been developed and applied to the XiO TPS (Elekta), for the IBA proton pencil beam scanning (PBS) system. First, we evaluated the specificities of each dose engine. A dose-conversion scheme that allows one to convert dose to medium into dose to water was implemented within GATE. Specific test cases in homogeneous and heterogeneous configurations allowed for the estimation of the differences between the beam models implemented in XiO and GATE. Finally, dose distributions of a prostate treatment plan were compared. In homogeneous media, a satisfactory agreement was generally obtained between XiO and GATE. The maximum stopping power difference of 3% occurred in a human tissue of 0.9 g cm−3 density and led to a significant range shift. Comparisons in heterogeneous configurations pointed out the limits of the TPS dose calculation accuracy and the superiority of Monte Carlo simulations. The necessity of computing dose to water in our Monte Carlo code for comparisons with TPSs is also presented. Finally, the new capabilities of the platform are applied to a prostate treatment plan and dose differences between both dose engines are analyzed in detail. This work presents a generic method to compare TPS dose distributions with the GATE Monte Carlo platform. It is noteworthy that GATE is also a convenient tool for imaging applications, therefore opening new research possibilities for the PBS modality.

WE‐A‐BRB‐07: Characterization and Use of a 2D‐Array of Ion Chambers for Brachytherapy Dosimetric Quality Assurance

Purpose: The 2D ionization chamber array MatriXX Evolution is one of the 2D ionization chamber arrays developed for megavoltage real‐time absolute 2D dosimetry and verification of IMRT. The purpose of this study is to evaluate the performance of ion chamber array for sub‐megavoltage range brachytherapy beam dose verification and quality assurance. Methods: The dose linearity and energy dependence of the 2D ion chamber array was studied using kilovoltage X‐ray beams (100 to 300 kVp). For the Ir‐192 brachytherapy HDR source dosimetric evaluation the detector response (kuser) factor was determined using 300 kVp beam. The phantom used for this measurement consists of multiple catheters, the IBA MatriXX detector and a slab of RW3 to provide full scattering conditions. The treatment planning system dose distribution was calculated on the CT scan of this phantom. The measured and TPS calculated distributions were compared in IBA Dosimetry OmniPro‐IˈmRT software. The quality of agreement was quantified by the gamma index for nine sets of plans. Using a dedicated phantom capable of receiving 5 brachytherapy intralumenal catheters a QA procedure was developed for end‐to‐end dosimetric evaluation for routine quality assurance checks. Results: The 2D ion chamber array dose dependence was found to be linear for 100 to 300 kVp and kuser showed strong energy dependence for 100 to 300 kVp energy ranges. The maximum mean difference between ion chamber array measured and TPS calculated was 3.7%. Comparisons of dose distribution for different test patterns have shown agreement with greater than 94.5 % for &amp;#947; &amp;#8804; 1 (with 3% delta dose and distance criterion of 2 mm). Dosimetric QA can be performed with the 2D ion chamber array. Conclusions: The MatriXX Evolution ionization chamber array has been found to be reliable for measurement of both absolute dose and relative dose distributions for the Ir‐192 brachytherapy HDR source.

SU‐E‐T‐681: Patient Dose Distribution Comparison between Monte Carlo Simulation and Treatment Planning System

Purpose: Compare the dose distributions from our Monte Carlo Dose Distribution Verification System (MC‐DDVS) to the Varian Eclipse treatment planning system (TPS) for our pediatric brain‐tumor patients treated with double scattering proton therapy. Method and Materials:Data from 12 pediatric patients, all under 3 years of age, with brain tumors treated with double‐scattering proton therapy were used in this study. Our MC‐DDVS was previously verified for accuracy. Non‐coplanar beam arrangement, patient geometry (including materials and densities), patient‐specific apertures and compensators were modeled in GEANT4 from DICOM data. The dose distribution from MC‐DDVS was scored by cubical mesh, imported to the TPS, and then compared to the TPS dose distribution. The comparisons included field width for each individual field, the distal edge on central axis for each field, and the dose to the CTV for the full plan. Results: Our preliminary results indicate that the distal edge and field width of the MC‐DDVS and TPS matched well, with the largest discrepancy being &lt;3mm. Examining the volumetric data of the full treatment plan for each patient, the mean doses to the CTVs based on the MC‐DDVS were within 1% of the TPS. However, on average the dose to 95% of the CTV in MC‐DDVS was ∼5% lower than the TPS. In general, the CTV dose was lower for the MC‐DDVS near target/bone interface. Conclusion: The preliminary comparison between the MC‐DDVS and the TPS showed reasonable agreement. The main difference appears to be the dose to bone and tissues proximal to bone. This may be due to the fact that the TPS reports dose to water and the MC‐DDVS reports dose to material, further investigation is needed.

WE‐A‐BRB‐08: Comparison of Brachytherapy Dose Distribution Using the Gamma‐Index Method

Purpose: The gamma‐index method was developed to quantitatively compare treatment planning system (TPS)‐derived dose distributions for external‐beam radiotherapy with measured dose distributions. However, the clinical medical physicist does not measure brachytherapy dose distributions and the use of this method for brachytherapy appears novel. The purpose of this study was to evaluate the accuracy of several TPS‐derived brachytherapy dose distributions in comparison to reference brachytherapy dose distributions, and suggest dose and distance criteria such as those recommended in the AAPM Task Group 56 report. Methods: Reference brachytherapy dose distributions from Monte Carlo (MC) simulations were commissioned into the Pinnacle TPS. MC and TPS dose distributions for several HDR and LDR sources were imported into the OmniPro IˈmRT software and compared. Sensitivity of distance‐to‐agreement for 1 mm =&lt; delta(d) =&lt; 5 mm and dose difference for 1% =&lt; delta(D) =&lt; 10% were investigated. TPS dose characterization was considered successful when at least 98% of all pixels examined satisfied a pass tolerance r =&lt;1.00. Results: At least 98% pixels passed for delta(d) = 2 mm and delta(D) = 2%. For delta(d) &gt;= 3 mm and delta(D) &gt;= 3% criteria, r =&lt; 1.00 for all pixels. Failure regions were located within the brachytherapy source, near the brachytherapy source surface, or in high dose‐gradient regions (&gt;50%/mm). A potentially useful standard is delta(d) = 2 mm and delta(D) = 2%. Conclusions: The gamma‐index method was used to evaluate brachytherapy dose distribution comparisons, and agreement criteria were proposed. The medical physics community may further investigate this method and consider standardized criteria for brachytherapy dosimetry.