MC Dose Distributions Research Articles

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.

TU-EF-304-12: Proton Radiation Therapy for Left-Sided Breast Cancer: LET and RBE Considerations for Cardiac Toxicity

Purpose: Increased risk of coronary artery disease has been documented for patients treated with radiation for left-sided breast cancer. Proton therapy (PRT) has been shown to significantly decrease cardiac irradiation, however variations in relative biological effectiveness (RBE) have been ignored so far. In this study we evaluate the impact of accounting for RBE variations on sensitive structures located within high linear energy transfer (LET) areas (distal end) of the proton treatment fields, for this treatment site. Methods: Three patients treated in our institution with PRT for left-sided breast cancer were selected. All patients underwent reconstructive surgery after mastectomy and treated to a total dose of 50.4Gy with beam(s) vertical to the chest wall. Dose and LET distributions were calculated using Monte Carlo (MC-TOPAS - TOol for PArticle Simulation). The LET-based, variable-RBE-weighted dose was compared to the analytical calculation algorithm (ACA) and MC dose distributions for a constant RBE of 1.1, based on volume histograms and mean values for the target, heart and left anterior descending coronary artery (LAD). Results: Assuming a constant RBE and compared to the ACA dose, MC predicted lower mean target and heart doses by 0.5% to 2.7% of the prescription dose. For variable RBE, plan evaluation showed increased mean target dose by up to 5%. Mean variable-RBE-weighted doses for the LAD ranged from 2.7 to 5.9Gy(RBE) among patients increased by 41%–64.2% compared to constant RBE ACA calculation (absolute dose: 1.7–3.9Gy(RBE)). Smaller increase in mean heart doses was noticed. Conclusion: ACA overestimates the target mean dose by up to 2.7%. However, disregarding variations in RBE may lead to significant underestimation of the dose to sensitive structures at the distal end of the proton treatment field and could thus impact outcome modeling for cardiac toxicities after proton therapy. These results are subject to RBE model and parameter uncertainties.

Sci-Thur AM: Planning - 07: A fast and accurate source model for energy and intensity modulated electron beams.

The purpose of this study is to develop a highly accurate and fast method for calculating electron beam dose distributions in Modulated Electron Radiation Therapy (MERT). An algorithm has been developed for creating phase-space files at the exit of a linear accelerator for any arbitrary intensity and energy electron beam without the need of full Monte Carlo simulations. The model assigns each particle to one of the 3 following sources: primary, secondary collimator and electron collimator scatter. The primary component is derived by fast MC transport in air. The scatter components are derived by the use of MC pre-calculated leaf kernels. Each kernel includes the fluence distribution, energy distribution and scatter probability of generating an electron from a leaf. The original position is sampled from tunable Gaussian or uniform distributions. The direction is estimated by geometrical means. According to the projection of the direction a particle is rejected if it is expected to suffer a leaf-hit. A leaf-hit counter is used to calculate the output of scatter particles based on the pre-calculated scatter probabilities. To account for multiple coulomb scattering in air a MC-corrected version of the Fermi-Eyges scattering theory was implemented. Depth and profile dose distributions were derived for the largest and smallest square field sizes, as well as for irregular and off-axis fields. The model agreed with full MC dose distributions within 3 % in all cases. Output at the depth of maximum dose exhibited discrepancies less than 2.6 % in all cases. The model was 16-22 times faster in generating a phase-space file than a full MC simulation with the BEAMnrc code.

WE‐C‐BRB‐04: Fast and Accurate Hybrid Source Model for Modulated Electron Radiotherapy

Purpose: To develop a highly accurate and fast method for calculating electron beam dose distributions in Modulated Electron Radiation Therapy (MERT). Method: An algorithm has been developed for creating phase‐ space files at the exit of a linear accelerator for any arbitrary intensity and energy electron beam without the need of full Monte Carlo simulations. The model assigns each particle to one of the 3 following sources: primary, secondary collimator and electron collimator scatter. The primary component is derived by fast MC transport in air. The scatter components are derived by the use of MC pre‐calculated leaf kernels. Each kernel includes the fluence distribution, energy distribution and scatter probability of generating an electron from a leaf. The original position is sampled from Gaussian or uniform distributions. The direction is estimated by geometrical means. According to the projection of the direction, a particle is rejected if it is expected to suffer a leaf‐hit. A leaf‐hit counter is used to calculate the output of scatter particles based on the pre‐calculated scatter probabilities. To account for multiple coulomb scattering in air a MC‐corrected version of the Fermi‐Eyges scattering theory was implemented. Results: Depth and profile dose distributions were derived for the largest and smallest square field sizes, as well as for irregular and off‐axis fields. The model agreed with full MC dose distributions within 3% in all cases. Output at the depth of maximum dose exhibited discrepancies less than 2.6% in all cases. The model was 16–22 times faster in generating a phase‐space file than a full MC simulation with the BEAMnrc code. Conclusions: Fast, dynamic electron beam calculations open up the possibility for real time delivery of MERT in the clinic and renew interest in electron beam therapy.

MO-D-BRB-08: Optimization of the Collimator Design for a Novel Stereotactic Radiotherapy Device for Breast Cancer

Purpose: A dedicated stereotactic gamma irradiation device, GammaPod from Xcision Medical Systems, has been developed to treat breast cancer. An optimization of the collimator design is presented for the hemi-spherical source carrier housing the Co60 irradiators. Methods: The GammaPod unit contains 36 Co-60 sources modulated by 36 tungsten collimators positioned along 6 columns in a non-coplanar and isocentric arrangement. The rotating collimator structure accommodates two different sizes of circular holes in addition to a position where sources are fully blocked. 9 dual-collimator arrangements with diameter combinations of 5–15, 5–25, 5–30, 10–20, 10–25, 10–30, 15–25, 15–30 and 20–30 millimeters were investigated. Treatment plans were generated for five patients using MC dose distributions following a pre-operative SBRT strategy and dose fractionation of 3×20Gy. The collimator arrangements are evaluated in terms of treatment plan quality and treatment efficiency. Results: Of the nine different arrangements, four (5–15, 5–25, 5–30 and 10–20 milllimeters) resulted in treatment lengths in excess of 30 minutes (range 40.6–59.7). In the remainder, treatments required 16.4, 21.9, 22.6, 27.4, 28.7 minutes for 20–30, 15–25, 15–30, 10–25 and 10–30 millimeter collimators respectively. All arrangements achieved target dose coverage goals (D95=DRx). Low dose spillage to ipsilateral breast, defined as V50% and V25% were least for the 15–25mm collimator, 8.3% and 15.6% respectively, while 20–30mm was only slightly higher, 8.4% and 16.3% (V50%=[8.3–9.0%] and V25%=[15.6–19.3%]). Among all five patients, the differences in all dosimetric parameters were statistically insignificant for these two collimators (p=0.47, 0.50 and 0.47 for V25%, V50% and V100%). The only significant parameter was treatment duration with population averages of 14.6 minutes for 20–30mm versus 26.7 for 15–25mm (p < 0.01). Conclusions: Among all collimator arrangements, no significant dosimetric advantage was observed with any collimator size. However, the 20–30mm collimator set was deemed most practical due to shortest treatment time required. Some co-authors declare employment and shareholder interests with Xcision Medical Systems.

Intensity- and energy-modulated electron radiotherapy by means of an xMLC for head and neck shallow tumors

The purpose of this paper is to assess the feasibility of delivering intensity- and energy-modulated electron radiation treatment (MERT) by a photon multileaf collimator (xMLC) and to evaluate the improvements obtained in shallow head and neck (HN) tumors. Four HN patient cases covering different clinical situations were planned by MERT, which used an in-house treatment planning system that utilized Monte Carlo dose calculation. The cases included one oronasal, two parotid and one middle ear tumors. The resulting dose-volume histograms were compared with those obtained from conventional photon and electron treatment techniques in our clinic, which included IMRT, electron beam and mixed beams, most of them using fixed-thickness bolus. Experimental verification was performed with plane-parallel ionization chambers for absolute dose verification, and a PTW ionization chamber array and radiochromic film for relative dosimetry. A MC-based treatment planning system for target with compromised volumes in depth and laterally has been validated. A quality assurance protocol for individual MERT plans was launched. Relative MC dose distributions showed a high agreement with film measurements and absolute ion chamber dose measurements performed at a reference point agreed with MC calculations within 2% in all cases. Clinically acceptable PTV coverage and organ-at-risk sparing were achieved by using the proposed MERT approach. MERT treatment plans, based on delivery of intensity-modulated electron beam using the xMLC, for superficial head and neck tumors, demonstrated comparable or improved PTV dose homogeneity with significantly lower dose to normal tissues. The clinical implementation of this technique will be able to offer a viable alternative for the treatment of shallow head and neck tumors.

Sci—Wed PM: Delivery—08: Monte Carlo Based RapidArc QA Using LINAC Log Files

Purpose/Objective(s): To present our Monte Carlo based RapidArc quality assurance (QA) process to validate both the dose calculation and dynamic beam delivery accuracy using the planning MLC control files and the post‐delivery MLC diagnostic files. Materials/Methods: Ten clinically acceptable RapidArc treatment plans were generated with a clinical version of the planning system for various tumor sites. Monte Carlo dose calculations were performed in a water equivalent phantom for each plan using both DynaLog files and the planning control (DVA) files. Results were compared to measurements using a calibrated Farmer ionization chamber with an active volume of . Comparison of RapidArc and Monte Carlo 3D doses was performed using a 3 dimensional Gamma‐factor analysis with a 3%/3mm DTA criteria. A thorough analysis of the DynaLog files was performed to evaluate the treatment delivery accuracy. Results: Good agreement was observed between chamber measurements and MC dose calculations and between RapidArc and MC dose distributions with Gamma values below 1 in over 90% of the points considered for all plans. The analysis of the MLC DynaLog files indicated that the leaf position errors were lower than 1 mm in more than 94% of the time with none above 2.5 mm and that few beam hold‐offs occurred Conclusions The accuracy and flexibility of our Monte Carlo based RapidArc QA system was demonstrated. Good machine performance and accurate dose distributions delivery of RapidArc plans was observed.

TH‐E‐224C‐04: MMCTP, a Radiotherapy Research Environment for Monte Carlo and Patient‐Specific Treatment Planning

Purpose: To develop a flexible software package, on low cost hardware with the aim of integrating new patient specific treatment planning with Monte Carlo dose calculation suitable for large‐scale prospective and retrospective treatment planning studies.Programming Philosophy: The McGill Monte Carlo Treatment Planning system (MMCTP) is designed as a software environment for the research development of patient specific treatment planning. The design includes a workstation GUI for treatment planning tools, and anonymous access to standard low cost hardware for MC dose calculation. Results: Before using MMCTP, treatment plans are converted into the so‐called McGill RT format. This new file structure was designed for saving patient plans on the workstation. The current MMCTP features are: (a) DICOM and RTOG imports; (b) transverse/sagittal/coronal slice viewing for contours, CT scans, dose distributions; (c) contouring tools; (d) colour‐wash and isodose line display; (e) DVH analysis, and dose matrix comparison tools; (f) external beam editing; (g) thumbnail CT navigation tool; (h) EGS/Beam calculation and XVMC patient transport for photon and electron beams. MMCTP uses a two‐step process to generate MC dose distributions. The MC module controls egs/Beam and XVMC calculations. Input files, prepared from the beam geometry, are uploaded and run on the cluster using shell commands. Upon completion of XVMC, the GUI downloads individual dose files. Conclusion: The MMCTP GUI provides a flexible research platform for the development of patient specific MC treatment planning for photon and electron external beam radiation therapy. MMCTP uses an internal storage format that is flexible in that it allows for multi‐instance multi‐modality image information useful in the planning process. The visualization, dose matrix operation and DVH tools offer extensive possibility for plan analysis and comparison to plans imported from commercial treatment planning systems through well‐documented image storage protocols such as DICOM.