Monte Carlo Dose Distributions Research Articles

SU‐E‐T‐686: Monte Carlo Study of Material Approximation Introduced Dose Difference in Electron Conformal Therapy Planning

Purpose: When Monte Carlo (MC) is used in dose calculation with CT data, materials are often assigned based on the local Houndsfield units if the density threshold method is applied. When bolus is used to compensate for body surface irregularities and/or to modulate the therapeutic depth in bolus electron conformal therapy (bolusECT), the bolus material, which is machineable wax, is treated as soft tissue, since the density of bolus overlaps with soft tissue. This study investigates the dose errors introduced by this material misassignment.Methods: BEAMnrc was used to simulate electron beams from a Trilogy accelerator. SPRRZnrc was used to calculate stopping power ratios (SPR) of tissue to wax, SPR(tissue/wax), and tissue to water, SPR(tissue/water), for 12 and 15 MeV electron beams which are the most commonly used energies in bolusECT. DOSXYZnrc was applied in dose distribution calculation in tissue with wax slabs of various thicknesses and a wedge‐bolus on top respectively. The MC dose distributions calculated using different bolus materials were compared.Results: With the same density, the SPR(tissue/wax) for 12 and 15 MeV beams ranges from 0.934 to 0.944, while the SPR(tissue/water) values are between 0.990 and 0.991, indicating higher stopping powers of wax. The 12 MeV beam range is 6.5 cm in wax and 6.7 cm in tissue. For a 12 MeV beam, the dose in tissue immediate under the bolus is overestimated by about 2.5% of maximum dose at 3 cm bolus thickness if wax bolus is treated as tissue. For 15 MeV beams, the error is about 1.4% Conclusions: Due to stopping power differences, dose differences are observed if the bolus material is misassigned in bolusECT dose calculations. Large dose overestimations occur at thick bolus region. However, the error is mostly within clinical tolerance, especially for thin bolus regions.

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 <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.

SU‐E‐T‐694: Evaluating Pencil Beam Dose Algorithm in Lung with Monte Carlo

Purpose: To validate the Massachusetts General Hospitalˈs (MGHˈs) proton pencil beam algorithm (PBA) dose calculations in lung cancer patients against GEANT4 Monte Carlo simulations. Methods: Three lung cancer patients with variations in tumor size and location, were considered in this study. Treatment planning was first performed for each patient following specific protocol guidelines and dose distributions were calculated with MGH PBA. The same plans were subsequently simulated with the GEANT4 Monte Carlo toolkit using a complete implementation of the MGHˈs proton delivery system with demonstrated accuracy in other tumor sites such as head and neck, prostate, etc. 20 million particles were simulated for each beam to ensure sufficient statistical accuracy. The dose distributions for each plan, as well as for each individual beams, were then compared 1) visually using an in‐house analysis tool Dose Comparison Application (DCA), and 2) systematically using a 2mm/2% and 3mm/3% 3D gamma index analysis. Results: GEANT4 simulations and PBA calculations show excellent visual agreement in lung cancer patients. Every plan successfully passes a full 3D gamma index test (95% γ values ≤ 1). No significant discrepancy were seen for any of each individual beams, hence showing agreement between the two fully independent models. These results highlight the accuracy of MGHˈs proton dosimetry in lung, and the ability to safely plan treatments for lung tumor patients at MGH. Conclusions: MGHˈs PBA dose calculations in lung were successfully compared with simulated Monte Carlo dose distributions. Monte Carlo is considered the gold standard for proton dose calculations, and these results therefore stress the high quality of the pencil beam algorithm calculations at MGH. This work further emphasizes the excellence of radiotherapeutic treatments at MGHˈs Francis H. Burr Proton Center.

Dosimetric accuracy of a deterministic radiation transport based brachytherapy treatment planning system. Part II: Monte Carlo and experimental verification of a multiple source dwell position plan employing a shielded applicator

The aim of this work is the dosimetric validation of a deterministic radiation transport based treatment planning system (BRACHYVISION v. 8.8, referred to as TPS in the following) for multiple 192Ir source dwell position brachytherapy applications employing a shielded applicator in homogeneous water geometries. TPS calculations for an irradiation plan employing seven VS2000 192Ir high dose rate (HDR) source dwell positions and a partially shielded applicator (GM11004380) were compared to corresponding Monte Carlo (MC) simulation results, as well as experimental results obtained using the VIP polymer gel-magnetic resonance imaging three-dimensional dosimetry method with a custom made phantom. TPS and MC dose distributions were found in agreement which is mainly within +/- 2%. Considerable differences between TPS and MC results (greater than 2%) were observed at points in the penumbra of the shields (i.e., close to the edges of the "shielded" segment of the geometries). These differences were experimentally verified and therefore attributed to the TPS. Apart from these regions, experimental and TPS dose distributions were found in agreement within 2 mm distance to agreement and 5% dose difference criteria. As shown in this work, these results mark a significant improvement relative to dosimetry algorithms that disregard the presence of the shielded applicator since the use of the latter leads to dosimetry errors on the order of 20%-30% at the edge of the "unshielded" segment of the geometry and even 2%-6% at points corresponding to the potential location of the target volume in clinical applications using the applicator (points in the unshielded segment at short distances from the applicator). Results of this work attest the capability of the TPS to accurately account for the scatter conditions and the increased attenuation involved in HDR brachytherapy applications employing multiple source dwell positions and partially shielded applicators.

Commissioning of Brachytherapy TPS Using a 2D-Array of Ion Chambers

Purpose/Objective(s): Unlike external beam treatment planning system (TPS) commissioning, a brachytherapy TPS requires little input, no modeling with most brachytherapy TPS calculations are based on the AAPM TG-43 formalism. The dose distribution using the AAPM TG-43 dose formalism is usually compared with the calculations using the Sievert summation, Monte Carlo simulation or dose distributions measured by with GAFCHROMIC film. These methods have shown an agreement of within 5% compared to the AAPM TG-43 dose formalism. The purpose of this study is to report our experience of using a 2D-array of ion chambers (MatriXX Evolution, IBA Dosimetry) for dosimetric verification of conformal CT-based high dose rate (HDR) brachytherapy.Material/Methods: After benchmarking the new TPS against the old TPS used in the clinical for several years and comparing with MATLAB calculated dose distribution, the dose calculation accuracy of TPS systems was investigated by measuring dose distributions experimentally using MatriXX Evolution. The phantom used for this technique consists of multiple catheters, the IBA MatriXX detector and a slab of RW3 to provide full scattering conditions. The TPS dose distribution was calculated on the CT scan of this phantom. The measured and TPS calculated distributions were compared in IBA Dosimetry OmniPro-I'mRT software. Results: The average absolute dose difference over the ROI was 1.67% and the gamma agreement index computed for a distance to agreement of 3 mm and a dose difference of 3% showed agreement for 98.7% of all pixels, with gamma ≤ 1. Conclusion: We have found that MatriXX 2D dosimetric technique provides a fast and accurate way to validate a brachytherapy TPS for both commissioning and quality assurance.

RapidArc treatment verification in 3D using polymer gel dosimetry and Monte Carlo simulation

The aim of this study was to verify the advanced inhomogeneous dose distribution produced by a volumetric arc therapy technique (RapidArc™) using 3D gel measurements and Monte Carlo (MC) simulations. The TPS (treatment planning system)-calculated dose distribution was compared with gel measurements and MC simulations, thus investigating any discrepancy between the planned dose delivery and the actual delivery. Additionally, the reproducibility of the delivery was investigated using repeated gel measurements. A prostate treatment plan was delivered to a 1.3 liter nPAG gel phantom using one single arc rotation and a target dose of 3.3 Gy. Magnetic resonance imaging of the gel was carried out using a 1.5 T scanner. The MC dose distributions were calculated using the VIMC-Arc code. The relative absorbed dose differences were calculated voxel-by-voxel, within the volume enclosed by the 90% isodose surface (VOI90), for the TPS versus gel and TPS versus MC. The differences between the verification methods, MC versus gel, and between two repeated gel measurements were investigated in the same way. For all volume comparisons, the mean value was within 1% and the standard deviation of the differences was within 2.5% (1SD). A 3D gamma analysis between the dose matrices were carried out using gamma criteria 3%/3 mm and 5%/5 mm (% dose difference and mm distance to agreement) within the volume enclosed by the 50% isodose surface (VOI50) and the 90% isodose surface (VOI90), respectively. All comparisons resulted in very high pass rates. More than 95% of the TPS points were within 3%/3 mm of both the gel measurement and MC simulation, both inside VOI50 and VOI90. Additionally, the repeated gel measurements showed excellent consistency, indicating reproducible delivery. Using MC simulations and gel measurements, this verification study successfully demonstrated that the RapidArc™ plan was both accurately calculated and delivered as planned.

SU‐GG‐T‐390: Dose to Proximal Bronchial Tree in Lung SBRT Treatments: Comparison of Pencil Beam and Monte Carlo Dose Distributions

Purpose: Increased dose to proximal bronchial tree (PBT) has been associated with higher incidence of toxicities in prior lung SBRT trials. In this study, the effect of pencil beam (PB) dose calculations on the PBT dose for targets within the PBT zone was evaluated using Monte Carlo (MC). Methods: Lung SBRT plans with PTVs within the PBT zone were generated for 10 patients using a commercial treatment planning software equipped with PB and MC algorithms. PBT within 2 cm of the PTV region was contoured on CT datasets for all patients. Treatment plans were generated using two different dose calculation methods: i) Pencil beam algorithm without inhomogeneity correction (PB‐IHC), and ii) PB with inhomogeneity correction (PB+IHC). Both plans for each patient had identical beam geometry, but different MLC shapes were used to achieve similar conformai dose coverage of PTV. All plans were normalized to have 98% of PTV receive 60Gy in 3 fx. Dose distribution for each plan was recalculated with MC by keeping the monitor units the same in respective plans. Dosimetric comparisons were performed between PB‐IHC, PB+IHC, and MC based dose distributions, where MC dose distributions were assumed to be the gold standard. Results: The MC mean values for PTVD95, PTVDmax, PBTDmean, and PBTDmax outside the PTV region were 67.7Gy, 83.1Gy, 38.5Gy, and 64.5Gy respectively. These values were higher than PB‐IHC by 9%±4.5%, 17%±9%, 19%±6%, and 16%±5.8%, and were lower than PB+IHC by 13%±4.7%, 9%±1.5%, 5%±1.8%, and 8%±2.4%, respectively. Conclusion: Compared to MC, PB+IHC overestimates the dose to both PTV and PBT, and PB‐IHC underestimates the dose to both PTV and PBT. Unrecognized dose hotspots in this region might influence the risk of toxicity to proximal airways. This issue should be considered when analyzing clinical toxicity data calculated using PB‐based algorithms without heterogeneity corrections.

MO‐E‐BRA‐02: Computer Aided Design and Monte Carlo Validation of a Patient‐Specific Co‐60 TBI Treatment Unit

Purpose: To modify and characterize a teletherapy 60Co unit for total body irradiation (TBI) treatments at extended SSD using experiments and a Monte Carlo (MC) model and to propose the design of custom compensators based on MC dose in the patient. Methods and Materials: An existing Eldorado T78 60Co teletherapy unit was stripped from its original collimator and equipped with beam‐defining cerrobend blocks for extended SSD TBI treatments. An acrylic flattening filter was numerically designed based on detailed mapping of the dose distribution of the large open field at 10 cm depth in water and using a primary radiation attenuation calculation. An EGSnrc MC model of the resulting unit was developed and validated. Dose distributions measured using ionization chambers were compared to MC dose distribution in air and phantom. The validated model was used to calculate dose in a whole‐body patient CT image. A compensator, designed to make the patient mid‐plane dose uniform was proposed. Results: The designed filter flattens the beam to within ±2% over an area of 200 × 70 cm2 at patient mid‐plane. The agreement between measured and calculated dose profiles in the open and the filtered beams is at the sub 2% and sub 1% level, respectively. Surface dose in the filtered beam is 78.5% and mean photon energy of the primary fluence is 0.94 MeV independent of position in the field. Patient‐specific calculations show excess dose in lung and the area of neck and extremities relative to prescription dose, by up to 20% and 30%, respectively. These excess doses can be reduced by introducing compensators, designed from the mid‐plane dose distribution, following similar techniques as for the design of the filter. Conclusions: This work shows that extended SSD 60Co irradiation equipment and patient‐specific compensators designed based on realistic dose distributions, can improve TBI delivery.

An integrated 6 MV linear accelerator model from electron gun to dose in a water tank

The details of a full simulation of an inline side-coupled 6 MV linear accelerator (linac) from the electron gun to the target are presented. Commissioning of the above simulation was performed by using the derived electron phase space at the target as an input into Monte Carlo studies of dose distributions within a water tank and matching the simulation results to measurement data. This work is motivated by linac-MR studies, where a validated full linac simulation is first required in order to perform future studies on linac performance in the presence of an external magnetic field. An electron gun was initially designed and optimized with a 2D finite difference program using Child's law. The electron gun simulation served as an input to a 6 MV linac waveguide simulation, which consisted of a 3D finite element radio-frequency field solution within the waveguide and electron trajectories determined from particle dynamics modeling. The electron gun design was constrained to match the cathode potential and electron gun current of a Varian 600C, while the linac waveguide was optimized to match the measured target current. Commissioning of the full simulation was performed by matching the simulated Monte Carlo dose distributions in a water tank to measured distributions. The full linac simulation matched all the electrical measurements taken from a Varian 600C and the commissioning process lead to excellent agreements in the dose profile measurements. Greater than 99% of all points met a 1%/1mm acceptance criterion for all field sizes analyzed, with the exception of the largest 40 x 40 cm2 field for which 98% of all points met the 1%/1mm acceptance criterion and the depth dose curves matched measurement to within 1% deeper than 1.5 cm depth. The optimized energy and spatial intensity distributions, as given by the commissioning process, were determined to be non-Gaussian in form for the inline side-coupled 6 MV linac simulated. An integrated simulation of an inline side-coupled 6 MV linac has been completed and benchmarked matching all electrical and dosimetric measurements to high accuracy. The results showed non-Gaussian spatial intensity and energy distributions for the linac modeled.

Planning tools for modulated electron radiotherapy

To develop tools to plan modulated electron radiotherapy (MERT) and to compare the MERT plans to conventional or intensity modulated radiotherapy (IMRT) treatment plans. Monte Carlo dose calculations of electron fields shaped with the inherent photon multileaf collimators (MLCs) were investigated in this study. Treatment plans for four postmastectomy breast cancer patients were generated using MERT. The distances from the patient skin surfaces to the distal planning target volume surfaces were computed along the beam axis direction to determine the physical depth. Electron beam energies were selected to provide target coverage at these depths and energy bins were generated. A custom built MERT treatment planning graphical user interface (MERTgui) was used to shape the electron bins into deliverable electron segments. Monte Carlo dose distribution simulations were performed using the MLC-defined segments generated from the MERTgui. A custom built superposition gui was used to combine doses for each segment using relative weights and final MERT treatment plans were compared to the conventional or IMRT treatment plans. In addition, a demonstration of combined MERT and IMRT treatment plans was performed. The MERT treatment plans provided acceptable target organ coverage in all cases. Relative to 3D conventional or IMRT treatment plans, the MERT plans predicted lower heart doses in all cases; average of the heart D20 of all plans was reduced from 14.1 to 3.3 Gy. The contralateral breast and contralateral lung doses decreased substantially with MERT planning compared to IMRT (on average, contralateral breast heart D20 was reduced from 8.7 to 0.7 Gy and contralateral lung D20 was reduced from 8.4 to 1.2 Gy with MERT). Ipsilateral lung D20 was lower with MERT than with the conventional plans (44.6 vs 29.2 Gy with MERT), but greater when compared against IMRT treatment plans (25.4 vs 28.9 Gy with MERT). A MERT and IMRT combination plan was generated to benefit from the complementary advantages of MERT and IMRT, resulting in satisfactory target coverage and reduced organ at risk doses. MERT tools can facilitate treatment planning and provide plans for treatment of shallow targets such as the postmastectomy chest wall.

Research on the Digital Mixed Denoising Techniques for Accelerating Monte Carlo Simulations

The Monte Carlo method is a stochastic statistic algorithm. It is one of the most accurate dose calculation methods, but its application in clinic is limited because of the long computation time. Generally, to accelerate Monte Carlo simulation and reduce stochastic noise, a digital filtering technique is used to smooth a rough dose distribution (includes evident noise) to a satisfied one. Different types of filters have been applied, such as Gaussian filters, Savitzky-Golay filters, etc., but the ability of a single filtering filter is limited. Therefore, a hybrid filter combining those filtering techniques was used. Two types of mixture methods—parallel and cascade—with three-dimensional Gaussian and Savitzky-Golay filters were researched. In addition, a method that simplifies the mixture filter structure using an equivalent convolution kernel based on convolution theory was introduced. With simulation data from a standard phantom, the rough dose distributions and the dose distribution smoothed by the two types of mixture filters were compared with that of the “benchmark” one. Test results showed that the two types of mixture filters can suppress much of the noise added in Monte Carlo dose distributions and enhance its visualization. As for the research’s test cases, the filtering effect of the cascade mixture filter was slightly better than that of the parallel mixture filter. Filter combinations can provide favorable filtering effects. The filtering effects of different mixture methods are not uniform. The cascade mixture filter has a better filtering effect than the parallel mixture filter.

Monte Carlo vs. Pencil Beam based optimization of stereotactic lung IMRT

BackgroundThe purpose of the present study is to compare finite size pencil beam (fsPB) and Monte Carlo (MC) based optimization of lung intensity-modulated stereotactic radiotherapy (lung IMSRT).Materials and methodsA fsPB and a MC algorithm as implemented in a biological IMRT planning system were validated by film measurements in a static lung phantom. Then, they were applied for static lung IMSRT planning based on three different geometrical patient models (one phase static CT, density overwrite one phase static CT, average CT) of the same patient. Both 6 and 15 MV beam energies were used. The resulting treatment plans were compared by how well they fulfilled the prescribed optimization constraints both for the dose distributions calculated on the static patient models and for the accumulated dose, recalculated with MC on each of 8 CTs of a 4DCT set.ResultsIn the phantom measurements, the MC dose engine showed discrepancies < 2%, while the fsPB dose engine showed discrepancies of up to 8% in the presence of lateral electron disequilibrium in the target. In the patient plan optimization, this translates into violations of organ at risk constraints and unpredictable target doses for the fsPB optimized plans. For the 4D MC recalculated dose distribution, MC optimized plans always underestimate the target doses, but the organ at risk doses were comparable. The results depend on the static patient model, and the smallest discrepancy was found for the MC optimized plan on the density overwrite one phase static CT model.ConclusionsIt is feasible to employ the MC dose engine for optimization of lung IMSRT and the plans are superior to fsPB. Use of static patient models introduces a bias in the MC dose distribution compared to the 4D MC recalculated dose, but this bias is predictable and therefore MC based optimization on static patient models is considered safe.

Modulated electron radiotherapy treatment planning using a photon multileaf collimator for post-mastectomized chest walls

Background and purpose To evaluate the feasibility of using a photon MLC (xMLC) for modulated electron radiotherapy treatment (MERT) as an alternative to conventional post-mastectomy chest wall (CW) irradiation. A Monte Carlo (MC) based planning system was developed to overcome the inaccuracy of the ‘pencil beam’ algorithm. MC techniques are known to accurately calculate the dose distributions of electron beams, allowing the explicit simulation of electron interactions within the MLC. Materials and methods Four real clinical CW cases were planned using MERT which were compared with the conventional electron treatments based on blocks and by a straightforward approach using the MLC, and not the blocks (as an intermediate step to MERT) to shape the same segments with SSD between 60 and 70 cm depending on PTV size. MC calculations were verified with an array of ionization chambers and radiochromic films in a solid water phantom. Results Tests based on gamma analysis between MC dose distributions and radiochromic film measurements showed an excellent agreement. Differences in the absolute dose measured with a plane-parallel chamber at a reference point were below 3% for all cases. MERT solution showed a better PTV coverage and a significant reduction of the doses to the organs at risk (OARs). Conclusion MERT can effectively improve the current electron treatments by obtaining a better PTV coverage and sparing healthy tissues. More directly, block-shaped treatments could be replaced by MLC-shaped non-modulated segments providing similar results.

A CT‐based analytical dose calculation method for HDR brachytherapy

This article presents an analytical dose calculation method for high-dose-rate 192Ir brachytherapy, taking into account the effects of inhomogeneities and reduced photon backscatter near the skin. The adequacy of the Task Group 43 (TG-43) two-dimensional formalism for treatment planning is also assessed. The proposed method uses material composition and density data derived from computed tomography images. The primary and scatter dose distributions for each dwell position are calculated first as if the patient is an infinite water phantom. This is done using either TG-43 or a database of Monte Carlo (MC) dose distributions. The latter can be used to account for the effects of shielding in water. Subsequently, corrections for photon attenuation, scatter, and spectral variations along medium- or low-Z inhomogeneities are made according to the radiological paths determined by ray tracing. The scatter dose is then scaled by a correction factor that depends on the distances between the point of interest, the body contour, and the source position. Dose calculations are done for phantoms with tissue and lead inserts, as well as patient plans for head-and-neck, esophagus, and MammoSite balloon breast brachytherapy treatments. Gamma indices are evaluated using a dose-difference criterion of 3% and a distance-to-agreement criterion of 2 mm. PTRAN_CT MC calculations are used as the reference dose distributions. For the phantom with tissue and lead inserts, the percentages of the voxels of interest passing the gamma criteria (Pgamma > or = 1) are 100% for the analytical calculation and 91% for TG-43. For the breast patient plan, TG-43 overestimates the target volume receiving the prescribed dose by 4% and the dose to the hottest 0.1 cm3 of the skin by 9%, whereas the analytical and MC results agree within 0.4%. Pgamma > or = 1 are 100% and 48% for the analytical and TG-43 calculations, respectively. For the head-and-neck and esophagus patient plans, Pgamma > or = 1 are > or = 99% for both calculation methods. A correction-based dose calculation method has been validated for HDR 192Ir brachytherapy. Its high calculation efficiency makes it feasible for use in treatment planning. Because tissue inhomogeneity effects are small and primary dose predominates in the near-source region, TG-43 is adequate for target dose estimation provided shielding and contrast solution are not used.

SU‐FF‐T‐408: Tissue Inhomogeneities in Monte Carlo Treatment Planning for Proton Therapy

Purpose: To investigate the effect of tissue segmentation and metal streaking artifacts for Monte Carlo (MC) dose calculations in proton therapy. Method and Materials: CT images of a phantom with 9 tissue equivalent inserts were segmented into material and mass density maps using the conventional single‐energy CT and a more accurate dual‐energy (DECT) material extraction. MC dose calculations for a broad 200 MeV proton beam were performed in the exact geometry and in the single‐energy and dual‐energy CT geometries in the MCNPX code. The dose calculation errors for the two tissue segmentation approaches were quantified. MC dose calculations were performed for a 147 MeV proton beam treatment plan of a patient with metal bilateral hip prostheses based on water‐only geometry, on original CT images with severe streaking artifacts and based on artifact corrected images. The effect of the artifacts and their correction on MC dose distribution was evaluated. Results: The materials of three inserts were incorrectly assigned using the conventional approach. The conventional tissue segmentation yielded dose calculation errors below 2%. In both the single‐energy CT and DECT geometry, there was a 0.7 cm shift in the position of the Bragg peak suggesting that density assignment is more important than correct tissue segmentation in proton beam MC dose calculations. The patient dose calculations using CT images with streaking artifacts showed large statistical errors in the artifact corrupted voxels and differences up to 1.5 cm in the 20% and 30% isodose lines due to the artifacts. Conclusions: The shift in the Bragg peak demonstrates the need for careful mass density assignment in proton beam MC dose calculations. The use of DECT tissue segmentation might therefore have only a small added benefit. The patient study shows that a metal artifact correction is necessary for patients with bilateral hip prostheses.

SU‐FF‐T‐324: Comparison of RTPS and Monte Carlo Dose Distributions in Heterogeneous Phantoms for Clinical Photon Beams

Purpose: To compare dose distributions from three different radiotherapy treatment planning systems (RTPS) with those from Monte Carlo (MC) calculations and measurements, in heterogeneous phantoms for photon beams. Methods and Materials: This study used four algorithms for RTPS: AAA (analytical anisotropic algorithm) implemented in the Eclipse (Varian Medical Systems) treatment planning system, CC (collapsed cone) superposition from the Pinnacle (Philips), and MGS (multigrid superposition) and FFT (fast Fourier transform) convolution from XiO (CMS). The dose distributions from these algorithms were compared with those from MC and measurements in a set of heterogeneous phantoms. The phantom inhomogeneities mimic relevant clinical treatment sites, which include lung slab, lung‐bone slab, bone‐lung slab, mediastinum, and tumor geometries. The benchmark comparisons were performed in lung densities of 0.30 g/cm3 and a bone density 1.819 g/cm3 for 3×3, 5×5, and 10×10 cm2 field sizes, 4, 6, 10, and 15 MV photon beams. Results: MC dose distributions agreed much better with measured data than the superposition algorithm in a set of heterogeneous phantoms. The difference in dose distributions between Eclipse‐AAA and MC were up to 15%. Eclipse‐AAA underestimated the dose inside the lung region for low energies of 4 and 6 MV, and in constant, overestimated in the tissue region. The dose distributions from Pinnacle‐CC and XiO‐MGS almost agree with those of MC and measurements but increase errors at15 MV, especially for a small field of 3×3 cm2. The FFT convolution extremely overestimated the dose inside the lung slab compared to MC. Conclusions: The dose distributions from the superposition algorithms almost agree with those from MC as well as measured values at 4 and 6 MV. The dose errors for Eclipse‐AAA are lager in lung model phantoms for 4 and 6 MV. The FFT convolution should not use for dose calculations in heterogeneous regions.

SU‐FF‐T‐405: Dose to Water Versus Dose to Medium in Proton Beam Therapy

Purpose: Dose in radiation therapy is traditionally reported as dose‐to‐water, Dw. Monte Carlo (MC) dose calculations report dose‐to‐medium, Dm. A methodology is needed to convert Dm into Dw for comparing MC and planning system results. This work addressed the following questions for proton therapy: What formalism allows the conversion from Dm to Dw?1. Can we convert MC calculated Dm into Dw retroactively or do we have to convert during particle tracking?2. What is the difference between Dm and Dw when analyzing patient data in terms of dose distributions, dose volume histograms and absolute doses?3. Is there a difference in the predicted beam range between Dm and Dw?Method: We did develop a formalism to convert Dm into Dw for proton beam dose calculations. Three different methods are introduced, an approximate method based on relative stopping power which allows retroactive conversion, a method considering energy dependent relative stopping powers, and a method incorporating nuclear interaction events as well. A total of 33 patient fields were analyzed. MC calculated dose distributions, dose volume histograms and absolute doses to assess the clinical significance of differences between Dm and Dw were compared. Results: We found that the difference between the three conversion methods was within 1% in most cases when analyzing mean doses to contoured structures. Further, we found that the difference between Dm and Dw can be up to 10% for high CT numbers. The difference is clinically insignificant for soft tissues. For proton beams stopping in bony anatomy, the predicted beam range can differ by 2–3 mm when comparing Dm and Dw. Conclusion: For comparison between MC and analytical dose distributions in proton therapy, a dose conversion is required. However, retroactive conversion seems to be sufficiently accurate in most cases, except for the end of range.

Patient-Specific Monte Carlo Dose Calculations for High-Dose-Rate Endorectal Brachytherapy With Shielded Intracavitary Applicator

An integrated software platform was developed to perform a patient-specific dosimetric study on high-dose-rate (192)Ir endorectal brachytherapy. Monte Carlo techniques were used to examine the perturbation effects of an eight-channel intracavitary applicator with shielding and a liquid-inflatable balloon. Such effects are ignored in conventional treatment planning systems that assume water-equivalent geometries. A total of 40 Task Group 43-based rectal patient plans were calculated using the PTRAN_CT Monte Carlo photon transport code. The silicone applicator, tungsten or lead shielding, contrast solution-filled balloon, and patient anatomy were included in the simulations. The dose to water and dose to medium were scored separately. The effects of heterogeneities and uncertainties in source positioning were examined. A superposition calculation method using pregenerated Monte Carlo dose distributions about the shielded applicator in water was developed and validated for efficient treatment planning purposes. On average, metal shielding decreases the mean dose to the contralateral normal tissues by 24% and reduces the target volume covered by the prescribed dose from 97% to 94%. Tissue heterogeneities contribute to dose differences of <1% relative to the prescribed dose. The differences in the dose volume indices between dose to water and dose to medium-based calculations were <1% for soft tissues, <2% for bone marrow, and >20% for cortical bone. A longitudinal shift of +/-2.5 mm and a rotational shift of +/-15 degrees in applicator insertion reduced the target volume receiving the prescribed dose by </=4%. The shielded applicator improved dose conformity and normal tissue sparing; however, Task Group 43-based treatment planning might compromise target coverage by not accounting for shielding.

A Monte Carlo Model for Independent Dose Verification in Serial Tomotherapy

Due to the very high complexity of IMRT treatment plans, it is imperative to perform dose verification, preferably before patient delivery. The aim of this project is to develop a Monte-Carlo-based model to verify the final dose distributions of plans developed using the Peacock system (CORVUS Treatment Planning System and MIMiC collimator). The system delivers radiation through arc therapy and uses sinogram files to determine the state of each of the multileaf collimator leaves. In-house software was developed using Matlab to decode the sinograms and create blocklets that are used as input in an MCSIM model of the MIMiC collimator attached to a Varian Clinac 600C. After validating the model, a prostate and head and neck case were simulated. The CORVUS-predicted dose distributions were compared with the Monte Carlo dose distributions. As expected, the results agreed very closely for the homogeneous case of the prostate but there were large discrepancies observed for the more heterogeneous head and neck case.

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.