Conventional Dose Calculation Research Articles

Assessment of dose to vaginal mucosa for gynecologic template interstitial high-dose-rate brachytherapy using Monte Carlo simulation.

This study investigated reliable vaginal mucosa dose-volume histogram (DVH) metrics in gynecologic template interstitial high-dose-rate brachytherapy (HDR-BT) for the purpose of standardized dose reporting. Gynecologic template (Syed/Neblett) interstitial HDR-BT patients treated from September 2016 to November 2022 at the study institute were included in the cohort. Each patient implant included a vaginal mucosa contour defined by a 5 mm expansion from vaginal cylinder, then another volume with clinical target volume subtracted. DVH metrics were investigated between D0.1cc to D4cc. Clinical plans were re-calculated using Monte Carlo (MC) simulations both in heterogeneous material and in water. The patient cohort included 61 patients with clinical plans using conventional homogeneous dose calculation (TG43). Heterogeneous vs. water MC dose differences were between -1.1% and -1.4% for all metrics investigated. DVH metrics D1cc and smaller resulted in > 5% discrepancies between TG43 and MC dose (to water) calculation due to the proximity of source positions in/nearby the vaginal mucosa. Reliability improved when DVH metric volume was larger (D2cc and D4cc). Both D2cc and D4cc presented very high linear correlation between TG43 and MC reported doses for the vagina, and average ± standard deviation dose difference was 4.6 ±2.9% and -3.0 ±1.9%, respectively. Dose differences decreased when the clinical target volume was removed: -1.5 ±3.5% and -0.8 ±2.1% for D2cc and D4cc, respectively. For perineal template gynecologic HDR-BT procedures, the 2 cc volume is the smallest representative volume that reliably reports vaginal dose and at minimum should be reported to establish dose and outcome evaluation.

Development of a simple calculation tool of dose distributions in a phantom for boron neutron capture therapy

A simple dose calculation tool, SiDE, was developed for dose evaluation in a water phantom for boron neutron capture therapy, which makes the calculation time much shorter compared with the conventional particle transportation Monte Carlo codes and is applicable to any type of incident neutron spectra to the phantom. As the SiDE can not only calculate quantitatively the dose distribution in the phantom but also output dose indexes such as advantage depth and peak tumor dose, a comparison between different boron neutron capture therapy neutron sources can be easily performed. Consistency with a Monte Carlo transportation code was verified through comparison with the conventional dose calculation with the Particle and Heavy Ion Transport Code System, and the calculation time was nearly 1/90 in the SiDE. The dose distributions for a reactor and accelerator-based neutron sources were compared, and the differences were found to be small although large differences existed between the incident spectra.

Technical Note: The development of a multi-physics simulation tool to estimate the background dose by systemic targeted alpha therapy.

PurposeTo predict biological effects of targeted alpha therapy (TAT) in preclinical studies, dosimetry calculations based on the micro‐level distributions of emitters are essential. Due to the saturation of the tumor antigenic sites and bonding breaks by decay, some of Alpha‐immuno‐conjugate and decay daughters may inevitably be transported by convection and diffusion along with blood or lymphatic circulation. This results in highly nonuniform and unsteady distributions of irradiation sources. Since the micro‐level distribution of emitters cannot be measured and obtained in patients with current technology, a modeling toolset to give more insight of the internal dose could be an alternative.MethodsA multi‐physics model based on a Monte Carlo microdosimetry technique and computational fluid dynamics (CFD) modeling was developed and applied to multiple internal irradiation sources. The CFD model tracks the path of the radionuclides and the dose model is capable of evaluating the time‐dependent absorbed dose to the target.ResultsThe conceptual model is capable of handling complex nonuniform irradiation sources in vasculature. The results from the simulations indicate that the assumption of homogeneous and motionless distribution of the administered activity used in the conventional dose calculation tends to significantly underestimate or overestimate the absorbed dose to the vascular system in various scenarios.ConclusionModeling the in vivo transport of radionuclides has the potential to improve the accuracy of TAT dose estimates. It could be the first step to develop a simulation tool set for assessing absorbed dose to tumor or normal tissues and predict the corresponding biological responses in the future.

MO-FG-CAMPUS-JeP2-03: Clinical Commissioning of MR-Only Prostate Treatment Planning Workflow

Purpose: To quantify the effect of MR-only treatment planning on dose distributions for prostate cancer for the clinical validation of MR-only workflow. Methods: For pre-clinical validation of the MR workflow where five-value stratified synthetic CT (Magnetic Resonance for Calculating Attenuation (MRCAT) algorithm Philips Ingenia 3.0T) are used for dose calculation instead of CT we (1) converted conventional 12-bit CT scans to five-value stratified CT and (2) applied the measured geometric distortions (deformation vector field measured with a dedicated phantom) from the MR scanner to the CT scans of 10 patients to quantify the effect on dose to target and organs-at-risk (OAR) for IMAT prostate plans. For the clinical validation we calculated the gamma index of the 3D dose distribution for MR-only and conventional CT dose calculations for the same patient. Results: Five-value perturbed CT and conventional CT dose distributions were equivalent. No difference was observed for DVH-parameters for standard CT compared to five-value CT with MR distortion vector field applied. DVH-parameters were generally higher for five-density CT/CT dose calculations; PTV mean dose was 100.5% compared to 100% for CT-based plans. For rectum the effect of CT HU histogram reduction on the mean dose and D2cc was not significant (rectum Dmean_CT=31.2 Gy, rectum Dmean_5valCT=31.6 Gy). MR-only compared to CT 3D dose distribution 3D gamma analysis (1mm/2%) pass rates were >95% except for one patient (92%) in the 50% and 90% isodose volumes. Conclusion: Synthetic CT dose distributions are equivalent to CT for OARs and slightly overestimate the dose to the target. The differences due to the simplified CT composition and deformation are small. 3D gamma analysis (1mm/2%) pass rates for MR-only dose distributions compared to CT are >95% for comparable organ filling at both scans.

Mean Organ Doses Resulting From Non-Human Primate Whole Thorax Lung Irradiation Prescribed to Mid-Line Tissue.

Multi-organ dose evaluations and the effects of heterogeneous tissue dose calculations have been retrospectively evaluated following irradiation to the whole thorax and lung in non-human primates (NHP). A clinical-based approach was established to evaluate actual doses received in the heart and lungs during whole thorax lung irradiation. Anatomical structure and organ densities have been introduced in the calculations to show the effects of dose distribution through heterogeneous tissue. Mean organ doses received by non-human primates undergoing whole thorax lung irradiations were calculated using a treatment planning system that is routinely used in clinical radiation oncology. The doses received by non-human primates irradiated following conventional dose calculations have been retrospectively reconstructed using computerized tomography-based, heterogeneity-corrected dose calculations. The use of dose volume descriptors for irradiation to organs at risk and tissue exposed to radiation is introduced. Mean and partial-volume doses to lung and heart are presented and contrasted. The importance of exact dose definitions is highlighted, and the relevance of precise dosimetry to establish organ-specific dose response relationships in NHP models of acute and delayed effects of acute radiation exposure is emphasized.

The Importance of Quasi-4D Path-Integrated Dose Accumulation for More Accurate Risk Estimation in Stereotactic Liver Radiotherapy.

Intrafraction organ deformation may be accounted for by inclusion of temporal information in dose calculation models. In this article, we demonstrate a quasi-4-dimensional method for improved risk estimation. Conventional 3-dimensional and quasi-4-dimensional calculations employing dose warping for dose accumulation were undertaken for patients with liver metastases planned for 42 Gy in 6 fractions of stereotactic body radiotherapy. Normal tissue complication probabilities and stochastic risks for radiation-induced carcinogenesis and cardiac complications were evaluated for healthy peripheral structures. Hypothetical assessments of other commonly employed dose/fractionation schedules on normal tissue complication probability estimates were explored. Conventional 3-dimensional dose computation may result in significant under- or overestimation of doses to organ at risk. For instance, doses differ (on average) by 17% (σ = 14%) for the left kidney, by 14% (σ = 7%) for the right kidney, by 7% (σ = 9%) for the large bowel, and by 10% (σ = 14%) for the duodenum. Discrepancies in the excess relative risk range up to about 30%. The 3-dimensional approach was shown to result in cardiac complication risks underestimated by >20%. For liver stereotactic body radiotherapy, we have shown that conventional 3-dimensional dose calculation may significantly over-/underestimate dose to organ at risk (90%-120% of the 4-dimensional estimate for the mean dose and 20%-150% for D2%). Providing dose estimates that most closely represent the actual dose delivered will provide valuable information to improve our understanding of the dose response for partial volume irradiation using hypofractionated schedules. Excess relative risks of radiocarcinogenesis were shown to range up to approximately excess relative risk = 4 and the prediction thereof depends greatly on the use of either 3-dimensional or 4-dimensional methods (with corresponding results differing by tens of percent).

SU‐E‐J‐200: A Dosimetric Analysis of 3D Versus 4D Image‐Based Dose Calculation for Stereotactic Body Radiation Therapy in Lung Tumors

Purpose:To evaluate the dosimetric difference between 3D and 4Dweighted dose calculation using patient specific respiratory trace and deformable image registration for stereotactic body radiation therapy in lung tumors.Methods:Two dose calculation techniques, 3D and 4D‐weighed dose calculation, were used for dosimetric comparison for 9 lung cancer patients. The magnitude of the tumor motion varied from 3 mm to 23 mm. Breath‐hold exhale CT was used for 3D dose calculation with ITV generated from the motion observed from 4D‐CT. For 4D‐weighted calculation, dose of each binned CT image from the ten breathing amplitudes was first recomputed using the same planning parameters as those used in the 3D calculation. The dose distribution of each binned CT was mapped to the breath‐hold CT using deformable image registration. The 4D‐weighted dose was computed by summing the deformed doses with the temporal probabilities calculated from their corresponding respiratory traces. Dosimetric evaluation criteria includes lung V20, mean lung dose, and mean tumor dose.Results:Comparing with 3D calculation, lung V20, mean lung dose, and mean tumor dose using 4D‐weighted dose calculation were changed by −0.67% ± 2.13%, −4.11% ± 6.94% (−0.36 Gy ± 0.87 Gy), −1.16% ± 1.36%(−0.73 Gy ± 0.85 Gy) accordingly.Conclusion:This work demonstrates that conventional 3D dose calculation method may overestimate the lung V20, MLD, and MTD. The absolute difference between 3D and 4D‐weighted dose calculation in lung tumor may not be clinically significant.This research is supported by Siemens Medical Solutions USA, Inc and Iowa Center for Research By Undergraduates

SU‐E‐T‐345: Validation of a Patient‐Specific Monte Carlo Targeted Radionuclide Therapy Dosimetry Platform

Purpose:There is a compelling need for personalized dosimetry in targeted radionuclide therapy given that conventional dose calculation methods fail to accurately predict dose response relationships. To address this need, we have developed a Geant4‐based Monte Carlo patient‐specific 3D dosimetry platform for TRT. This platform calculates patient‐specific dose distributions based on serial CT/PET or CT/SPECT images acquired after injection of the TRT agent. In this work, S‐values and specific absorbed fractions (SAFs) were calculated using this platform and benchmarked against reference values.Methods:S‐values for 1, 10, 100, and 1000g spherical tumors with uniform activity distributions of I‐124, I‐125, I‐131, F‐18, and Ra‐223 were calculated and compared to OLINDA/EXM reference values. SAFs for monoenergetic photons of 0.01, 0.1, and 1 MeV and S factors for monoenergetic electrons of 0.935 MeV were calculated for the liver, kidneys, lungs, pancreas, spleen, and adrenals in the Zubal Phantom and compared with previously published values. Sufficient particles were simulated to keep the voxel statistical uncertainty below 5%.Results:The calculated spherical S‐values agreed within a few percent of reference data from OLINDA/EXM for each radionuclide and sphere size. The comparison of photon SAFs and electron S‐values with previously published values showed good agreement with the previously published values. The S‐values and SAFs of the source organs agreed within 1%.Conclusion:Our platform has been benchmarked against reference values for a variety of radionuclides and over a wide range of energies and tumor sizes. Therefore, this platform could be used to provide accurate patientspecific dosimetry for use in radiopharmaceutical clinical trials.

Comparison of Absorbed Dose to Medium and Absorbed Dose to Water for Spine IMRT Plans Using a Commercial Monte Carlo Treatment Planning System

Dose in radiation therapy has been reported as the water-equivalent dose using conventional dose calculation algorithms. The Monte Carlo (MC) algorithm employs characterization of human tissues by elemental composition and mass density. It enables more accurate dose calculation for radiation therapy treatment planning and typically reports absorbed dose to medium. Whether one should use dose to medium or tissue (Dm) in place of dose to water (Dw) for MC treatment planning remains the subject of debate. The aim of the current study is to evaluate the differences between dose-volume indices for Dm and Dw MC-calculated IMRT plans. Thirty-seven spine patients were selected for this study. The IMRT optimization and MC calculations were performed using the iPlan RT DoseTM ver 4.1.2 (Brainlab, Munich, Germany) treatment planning system (TPS) with an X-ray Voxel Monte Carlo (XVMC) dose calculation engine. Dw and Dm results for target and critical structures were evaluated using the dose-volume-based indices. Systematic differences between dose-volume indices computed with Dw and Dm were up to 5.2%, 4.2%, and 4.5% for D2, D50 and D98 indices of the clinical target volume (CTV), respectively and up to 1% for the critical structure dose indices. Our study demonstrates that employing Dm in place of Dw in MC-calculated IMRT treatment plans introduces a significant systematic difference in target DVHs. We recommend that for diffused target structures (such as spine tumors), dose to water is a better quantity for dose prescription in photon beam treatment planning using existing MC TPS. While for critical structures, it would be reasonable to report Dm always. However in future with the availability of finer spatial resolution, Dm will be the most suitable variable for both target and critical structures’ dose prescription and reporting in MC treatment planning.

SU-F-500-01: Towards Personalized Dosimetry Using Diapeutic Radiopharmaceuticals

Purpose: he efficacy of targeted radionuclide therapy (TRT) depends on the ability to accurately characterize the absorbed dose distribution within tumors as well as other potential dose limiting organs. Clinical data suggests a compelling need for personalized dosimetry given that conventional dose calculation methods fail to accurately predict dose response relationships. This work describes the implementation of a Monte Carlo patient‐specific 3D dosimetry platform for TRT. Methods: CLR1404, a small‐molecule, phospholipid ether analog can serve as a ‘diapeutic’ (diagnostic and therapeutic) agent for the detection and treatment of multiple solid tumors. Because both the I–124 labeled imaging and I–131 labeled therapy agent are administered in similar mass doses, they are expected to display equivalent in vivo pharmacokinetic profiles. Therefore, the 124I‐CLR1404 distribution can be converted to the therapeutic 131I‐CLR1404 distribution by simply accounting for the difference in decay rates. In this study, high grade Glioma patients were scanned at 4.5, 24, and 48 hours post injection of 124I‐CLR1404. PET, CT and MRI images were co‐registered, resampled to the CT resolution and contoured. The 131I‐CLR1404 dose deposition throughout the CT volume was simulated using the Monte Carlo code Geant4.9.4 assuming a source strength in each voxel was proportional to the PET activity. The total cumulated absorbed dose was calculated by numerically integrating the activity over all time on a voxel‐by‐voxel basis. Results: Spatial distributions of the absorbed dose rates, DVHs, and mean, minimum, maximum, and total dose rate values for each contoured region were generated for each time point, clearly illustrating CLR1404's pharmacokinetic properties. The mean cumulated absorbed dose within tumor was 52.3 mGy per injected activity of 131I‐CLR1404. Conclusion: This work demonstrates that a Geant4 based Monte Carlo radiopharmaceutical dose calculation platform is not only feasible, but can serve as invaluable tool for personalized dosimetry in TRT. JJG and JPW are affiliated with Novelos Therapeutics which owns the licensing rights to CLR1404 and related compounds.

Dosimetry of 125I and 103Pd COMS eye plaques for intraocular tumors: Report of Task Group 129 by the AAPM and ABS

Dosimetry of eye plaques for ocular tumors presents unique challenges in brachytherapy. The challenges in accurate dosimetry are in part related to the steep dose gradient in the tumor and critical structures that are within millimeters of radioactive sources. In most clinical applications, calculations of dose distributions around eye plaques assume a homogenous water medium and full scatter conditions. Recent Monte Carlo (MC)-based eye-plaque dosimetry simulations have demonstrated that the perturbation effects of heterogeneous materials in eye plaques, including the gold-alloy backing and Silastic insert, can be calculated with reasonable accuracy. Even additional levels of complexity introduced through the use of gold foil "seed-guides" and custom-designed plaques can be calculated accurately using modern MC techniques. Simulations accounting for the aforementioned complexities indicate dose discrepancies exceeding a factor of ten to selected critical structures compared to conventional dose calculations. Task Group 129 was formed to review the literature; re-examine the current dosimetry calculation formalism; and make recommendations for eye-plaque dosimetry, including evaluation of brachytherapy source dosimetry parameters and heterogeneity correction factors. A literature review identified modern assessments of dose calculations for Collaborative Ocular Melanoma Study (COMS) design plaques, including MC analyses and an intercomparison of treatment planning systems (TPS) detailing differences between homogeneous and heterogeneous plaque calculations using the American Association of Physicists in Medicine (AAPM) TG-43U1 brachytherapy dosimetry formalism and MC techniques. This review identified that a commonly used prescription dose of 85 Gy at 5 mm depth in homogeneous medium delivers about 75 Gy and 69 Gy at the same 5 mm depth for specific (125)I and (103)Pd sources, respectively, when accounting for COMS plaque heterogeneities. Thus, the adoption of heterogeneous dose calculation methods in clinical practice would result in dose differences >10% and warrant a careful evaluation of the corresponding changes in prescription doses. Doses to normal ocular structures vary with choice of radionuclide, plaque location, and prescription depth, such that further dosimetric evaluations of the adoption of MC-based dosimetry methods are needed. The AAPM and American Brachytherapy Society (ABS) recommend that clinical medical physicists should make concurrent estimates of heterogeneity-corrected delivered dose using the information in this report's tables to prepare for brachytherapy TPS that can account for material heterogeneities and for a transition to heterogeneity-corrected prescriptive goals. It is recommended that brachytherapy TPS vendors include material heterogeneity corrections in their systems and take steps to integrate planned plaque localization and image guidance. In the interim, before the availability of commercial MC-based brachytherapy TPS, it is recommended that clinical medical physicists use the line-source approximation in homogeneous water medium and the 2D AAPM TG-43U1 dosimetry formalism and brachytherapy source dosimetry parameter datasets for treatment planning calculations. Furthermore, this report includes quality management program recommendations for eye-plaque brachytherapy.

Lack of backscatter factor measurements in HDR applications with MOSkins

Measurements of backscatter correction factors for intra operative (IOBT) HDR brachytherapy applicators were made using Centre for Medical Radiation Physics (CMRP), MOSFET devices. In clinical use there is an absence of backscatter material above the IOBT applicator, leading to a lower dose than predicted by conventional TG-43 dose calculations. To estimate the uncertainty in the MOSFET measurements, the dosimetric characteristics, including reproducibility, stability, linearity, and angular and energy response were measured using a HDR Ir-192 source, kilovoltage treatment unit and a high energy linac. Measurements were compared with previously published Monte Carlo data. Variability of the response of the MOSFETs due to angular variation contributed the largest uncertainty in dose measurements. Using the IOBT applicator without adequate scatter material resulted in a reduction of delivered dose of on average 10%, but was dependent on the location on the applicator and the treatment field size. Theoretical calculations based on previously published study indicated an expected reduced dose of on average 4%. MOSFET devices provide an ideal measurement tool in the presence of high dose gradients, however, the dosimetric characteristics of the detector must be accounted for when estimating the uncertainty.

Dose specification for radiation therapy: dose to water or dose to medium?

The Monte Carlo method enables accurate dose calculation for radiation therapy treatment planning and has been implemented in some commercial treatment planning systems. Unlike conventional dose calculation algorithms that provide patient dose information in terms of dose to water with variable electron density, the Monte Carlo method calculates the energy deposition in different media and expresses dose to a medium. This paper discusses the differences in dose calculated using water with different electron densities and that calculated for different biological media and the clinical issues on dose specification including dose prescription and plan evaluation using dose to water and dose to medium. We will demonstrate that conventional photon dose calculation algorithms compute doses similar to those simulated by Monte Carlo using water with different electron densities, which are close (<4% differences) to doses to media but significantly different (up to 11%) from doses to water converted from doses to media following American Association of Physicists in Medicine (AAPM) Task Group 105 recommendations. Our results suggest that for consistency with previous radiation therapy experience Monte Carlo photon algorithms report dose to medium for radiotherapy dose prescription, treatment plan evaluation and treatment outcome analysis.

SU-GG-T-221: Clinical Implementation of Monte Carlo Simulation for RapidArc Dosimetry Verification

Purpose: The RapidArc technique, which increases the treatment efficiency with reduced monitor units (MU), has been increasingly applied clinically. However, the small isolated fields and the highly irregular field shapes are challenging for conventional dose calculation algorithms employed in commercial treatment planning systems (TPS). Owing to the ability to handle heterogeneity and small fields, Monte Carlo simulation has been recognized as the benchmark method for radiotherapydose calculation. This study aims to demonstrate the performance of TPS calculation on head and neck, abdomen, and pelvis RapidArc treatments using Monte Carlo simulations.Method and Materials: To simplify the simulation process, Monte Carlodose computation was based on the reconstructed fluence distributions between control points. The dosimetric features of the MLC including rounded leaf ends, intra‐leaf transmission and the tongue and groove effect were included in our simulation. Ten RapidArc patients with different treatment sites were recruited for this retrospective study. The dose distributions and the dose volume histograms (DVH) calculated by the Monte Carlo simulations were compared with TPS calculations. Results: The isodose comparisons between TPS and Monte Carlo simulation showed good agreement except for the highly heterogeneous region and extremely complex treatment geometry. For the DVH comparison, overall, the mean dose of the PTV calculated by the Monte Carlo agreed with the TPS calculation to within 2%. The dose to the critical organs calculated by the TPS was higher than that by Monte Carlo simulations.Conclusion: This study evaluated ten patients treated by the RapidArc technique to demonstrate the performance of the TPS dose calculation for different treatment sites. It is suggested that for the treatment sites with severe heterogeneity, Monte Carlo simulations are necessary.

Comparing the accuracy of four‐dimensional photon dose calculations with three‐dimensional calculations using moving and deforming phantoms

Four-dimensional (4D) dose calculation algorithms, which explicitly incorporate respiratory motion in the calculation of doses, have the potential to improve the accuracy of dose calculations in thoracic treatment planning; however, they generally require greater computing power and resources than currently used for three-dimensional (3D) dose calculations. The purpose of this work was to quantify the increase in accuracy of 4D dose calculations versus 3D dose calculations. The accuracy of each dose calculation algorithm was assessed using measurements made with two phantoms. Specifically, the authors used a rigid moving anthropomorphic thoracic phantom and an anthropomorphic thoracic phantom with a deformable lung insert. To incorporate a clinically relevant range of scenarios, they programed the phantoms to move and deform with two motion patterns: A sinusoidal motion pattern and an irregular motion pattern that was extracted from an actual patient's breathing profile. For each combination of phantom and motion pattern, three plans were created: A single-beam plan, a multiple-beam plan, and an intensity-modulated radiation therapy plan. Doses were calculated using 4D dose calculation methods as well as conventional 3D dose calculation methods. The rigid moving and deforming phantoms were irradiated according to the three treatment plans and doses were measured using thermoluminescent dosimeters (TLDs) and radiochromic film. The accuracy of each dose calculation algorithm was assessed using measured-to-calculated TLD doses and a gamma analysis. No significant differences were observed between the measured-to-calculated TLD ratios among 4D and 3D dose calculations. The gamma results revealed that 4D dose calculations had significantly greater percentage of pixels passing the 5%/3 mm criteria than 3D dose calculations. These results indicate no significant differences in the accuracy between the 4D and the 3D dose calculation methods inside the gross tumor volume. On the other hand, the film results demonstrated that the 4D dose calculations provided greater accuracy than 3D dose calculations in heterogeneous dose regions. The increase in accuracy of the 4D dose calculations was evident throughout the planning target volume.

A Preliminary Study of In-House Monte Carlo Simulations: An Integrated Monte Carlo Verification System

To develop an infrastructure for the integrated Monte Carlo verification system (MCVS) to verify the accuracy of conventional dose calculations, which often fail to accurately predict dose distributions, mainly due to inhomogeneities in the patient's anatomy, for example, in lung and bone. The MCVS consists of the graphical user interface (GUI) based on a computational environment for radiotherapy research (CERR) with MATLAB language. The MCVS GUI acts as an interface between the MCVS and a commercial treatment planning system to import the treatment plan, create MC input files, and analyze MC output dose files. The MCVS consists of the EGSnrc MC codes, which include EGSnrc/BEAMnrc to simulate the treatment head and EGSnrc/DOSXYZnrc to calculate the dose distributions in the patient/phantom. In order to improve computation time without approximations, an in-house cluster system was constructed. The phase-space data of a 6-MV photon beam from a Varian Clinac unit was developed and used to establish several benchmarks under homogeneous conditions. The MC results agreed with the ionization chamber measurements to within 1%. The MCVS GUI could import and display the radiotherapy treatment plan created by the MC method and various treatment planning systems, such as RTOG and DICOM-RT formats. Dose distributions could be analyzed by using dose profiles and dose volume histograms and compared on the same platform. With the cluster system, calculation time was improved in line with the increase in the number of central processing units (CPUs) at a computation efficiency of more than 98%. Development of the MCVS was successful for performing MC simulations and analyzing dose distributions.

SU‐FF‐T‐35: A Monte Carlo Study for Evaluation of Tissue Heterogeneity Effect of Inversely Optimized Intracavitary High Dose Rate Brachytherapy Plan

Purpose: To investigate tissue heterogeneity effect of an intracavitary high dose rate brachytherapy plan which is optimized by the inverse planning option. Method and Materials: A CT and VOI based brachytherapy plan for an intra‐uterine and cervix cancer treatment was made using inverse planning simulated‐annealing (IPSA) module provided by Plato TPS. The microSelectron‐HDR Ir‐192 afterloader and 3 catheters (2 ovoids and 1 tandem) were used for the treatment. For a verification of the plan result, EGSnrc based dosxyznrc Monte Carlo code was employed. Patient CT data was converted to egsphant data by the ctcreate code and each source position and weight from the plan data was used for a Monte Carlo simulation as well. Ir‐192 source was approximately modeled by 3.6 mm length and 0.6 mm diameter and it was assumed to be located at the center of the catheters, while parallel to the z‐axis at each position because of restricted freedom of source modeling. The microSelectron‐HDR Ir‐192 spectrum data could be obtained from the EGSnrc code, however, number of history was determined empirically based on known air‐kerma and dose rate value. Conventional point A and B doses and DVHs of PTV, bladder and rectum were evaluated for a quantitative comparison. Results: Significant decrease of dose at point A and B more than 10% was observed in the Monte Carlo simulation. PTV coverage by 100% isodose surface was degraded while rectum and bladder mean dose was reduced up to 15%. Resultant dose rate decrease was also observed. Conclusion: Conventional brachytherapy dose calculation method could not appropriately consider tissue heterogeneity effect and inverse planning optimization could invoke serious erroneous result. Therefore inverse planning optimization should be carefully implemented for a HDR brachytherapy planning.

TU‐A‐350‐01: Electron Monte Carlo: Clinical Impact

This presentation will facilitate participants with the clinical application of Monte Carlo dose calculation algorithms in radiotherapy treatment planning and dosimetry verification. Following a brief introduction to the clinical implementation and commissioning of the Monte Carlo dose calculation software detailed discussions will be given on the actual and potential impact of Monte Carlo dose calculation algorithms on conventional electron beam therapy and advanced mixed beam treatments.Comparisons of patient treatment plans generated using conventional dose calculation algorithms and Monte Carlo methods will be made with an emphasis on the causes of the dose discrepancies. Further discussions will be conducted on the use of Monte Carlo dose calculation as a radiotherapy treatment QA tool to validate individual patient plans and as an investigation tool to improve target dose conformity and normal tissue sparing using novel treatment techniques.Educational Objectives:1. To describe clinical implement and commissioning of Monte Carlo dose calculation algorithms for radiotherapy treatment planning.2. To discuss the impact of Monte Carlo dose calculation in electron therapy and advanced mixed beam therapy treatment planning.3. To describe the applications of Monte Carlo dose calculation in treatment planning and beam delivery QA for advanced radiotherapy treatments.

Underestimation of Low-Dose Radiation in Treatment Planning of Intensity-Modulated Radiotherapy

To investigate potential dose calculation errors in the low-dose regions and identify causes of such errors for intensity-modulated radiotherapy (IMRT). The IMRT treatment plans of 23 patients with lung cancer and mesothelioma were reviewed. Of these patients, 15 had severe pulmonary complications after radiotherapy. Two commercial treatment-planning systems (TPSs) and a Monte Carlo system were used to calculate and compare dose distributions and dose-volume parameters of the target volumes and critical structures. The effect of tissue heterogeneity, multileaf collimator (MLC) modeling, beam modeling, and other factors that could contribute to the differences in IMRT dose calculations were analyzed. In the commercial TPS-generated IMRT plans, dose calculation errors primarily occurred in the low-dose regions of IMRT plans (<50% of the radiation dose prescribed for the tumor). Although errors in the dose-volume histograms of the normal lung were small (<5%) above 10 Gy, underestimation of dose <10 Gy was found to be up to 25% in patients with mesothelioma or large target volumes. These errors were found to be caused by inadequate modeling of MLC transmission and leaf scatter in commercial TPSs. The degree of low-dose errors depends on the target volumes and the degree of intensity modulation. Secondary radiation from MLCs contributes a significant portion of low dose in IMRT plans. Dose underestimation could occur in conventional IMRT dose calculations if such low-dose radiation is not properly accounted for.

Comparison of TG43 and Monte Carlo dosimetry for seed-in-mesh brachytherapy for thoracic malignancies

Purpose: It is well known that the dose distributions of commonly used permanent LDR brachytherapy sources may be significantly affected by material following seed-in-mesh permanent implants for resected thoracic malignancies because of the photoelectric effect. The conventional dose calculation using TG-43 and most commercially available brachytherapy dosimetry systems do not account for density and material correction. This study was designed to test the feasibility and significance of patient-specific Monte Carlo dose calculations for permanent thoracic brachytherapy implants.