Monte Carlo Dose Distributions Research Articles

A critical evaluation of secondary cancer risk models applied to Monte Carlo dose distributions of 2-dimensional, 3-dimensional conformal and hybrid intensity-modulated radiation therapy for breast cancer

The comparison of radiotherapy techniques regarding secondary cancer risk has yielded contradictory results possibly stemming from the many different approaches used to estimate risk. The purpose of this study was to make a comprehensive evaluation of different available risk models applied to detailed whole-body dose distributions computed by Monte Carlo for various breast radiotherapy techniques including conventional open tangents, 3D conformal wedged tangents and hybrid intensity modulated radiation therapy (IMRT). First, organ-specific linear risk models developed by the International Commission on Radiological Protection (ICRP) and the Biological Effects of Ionizing Radiation (BEIR) VII committee were applied to mean doses for remote organs only and all solid organs. Then, different general non-linear risk models were applied to the whole body dose distribution. Finally, organ-specific non-linear risk models for the lung and breast were used to assess the secondary cancer risk for these two specific organs. A total of 32 different calculated absolute risks resulted in a broad range of values (between 0.1% and 48.5%) underlying the large uncertainties in absolute risk calculation. The ratio of risk between two techniques has often been proposed as a more robust assessment of risk than the absolute risk. We found that the ratio of risk between two techniques could also vary substantially considering the different approaches to risk estimation. Sometimes the ratio of risk between two techniques would range between values smaller and larger than one, which then translates into inconsistent results on the potential higher risk of one technique compared to another. We found however that the hybrid IMRT technique resulted in a systematic reduction of risk compared to the other techniques investigated even though the magnitude of this reduction varied substantially with the different approaches investigated. Based on the epidemiological data available, a reasonable approach to risk estimation would be to use organ-specific non-linear risk models applied to the dose distributions of organs within or near the treatment fields (lungs and contralateral breast in the case of breast radiotherapy) as the majority of radiation-induced secondary cancers are found in the beam-bordering regions.

A simplified methodology to produce Monte Carlo dose distributions in proton therapy.

The purpose of this study was to develop a simplified methodology that will produce Monte Carlo (MC) dose distribution for proton therapy which can be used as a clinical aid in determining the adequacy of proton plans produced from the treatment planning system (TPS). The Geant4 Monte Carlo toolkit was used for all simulations. The geometry of the double scatter nozzle in the simulation was a simplification of the treatment nozzle. The proton source was modeled as discrete energy layers, each with a unique energy distribution and weighting factor. The simplified MC system was designed to give the same dose distribution as the measured data used to commission the TPS. After the simplified MC system was finalized, a series of verification comparisons were made between it, measurements, and the clinically used TPS. Comparisons included the lateral profile of a stair‐shaped compensator that simulated a sharp lateral heterogeneity and depth‐dose measurements through heterogeneous materials. The simplified MC system matched measurements to within 2% or 2 mm for all commissioning data under investigation; moreover, the distal edge and lateral penumbra was within 1 mm of the measurements. The simplified MC system was able to better reproduce the measured profiles for a stair‐shaped compensator than the TPS. Both MC and TPS matched the measured depth dose through heterogeneous materials to within 2% or 2 mm. The simplified MC system was straightforward to implement, and produced accurate results when compared to measurements. Therefore, it holds promise as a clinically useful methodology to validate the relative dose distribution of patient treatment plans produced by the treatment planning systems.PACS number: 87.55.K‐, 87.55.ne

Development and reproducibility evaluation of a Monte Carlo-based standard LINAC model for quality assurance of multi-institutional clinical trials.

Technical developments in radiotherapy (RT) have created a need for systematic quality assurance (QA) to ensure that clinical institutions deliver prescribed radiation doses consistent with the requirements of clinical protocols. For QA, an ideal dose verification system should be independent of the treatment-planning system (TPS). This paper describes the development and reproducibility evaluation of a Monte Carlo (MC)-based standard LINAC model as a preliminary requirement for independent verification of dose distributions. The BEAMnrc MC code is used for characterization of the 6-, 10- and 15-MV photon beams for a wide range of field sizes. The modeling of the LINAC head components is based on the specifications provided by the manufacturer. MC dose distributions are tuned to match Varian Golden Beam Data (GBD). For reproducibility evaluation, calculated beam data is compared with beam data measured at individual institutions. For all energies and field sizes, the MC and GBD agreed to within 1.0% for percentage depth doses (PDDs), 1.5% for beam profiles and 1.2% for total scatter factors (Scps.). Reproducibility evaluation showed that the maximum average local differences were 1.3% and 2.5% for PDDs and beam profiles, respectively. MC and institutions' mean Scps agreed to within 2.0%. An MC-based standard LINAC model developed to independently verify dose distributions for QA of multi-institutional clinical trials and routine clinical practice has proven to be highly accurate and reproducible and can thus help ensure that prescribed doses delivered are consistent with the requirements of clinical protocols.

SU-E-T-616: Comparison of Plan Dose Accuracy for Anterior Vs. Lateral Fields in Proton Therapy of Prostate Cancer

Purpose: With the anticipated introduction of in vivo range verification methods, the use of anterior fields for proton therapy of prostate cancer may become an attractive treatment option, and improve upon the dose distributions achievable with conventional lateral-opposed fields. This study aimed to evaluate and compare the planned dose accuracy for lateral versus anterior oblique field arrangements. Methods: Four patients with low/intermediate risk prostate cancer, participating in a clinical trial at our institution, were selected for this study. All patients were treated using lateral-opposed fields (LAT). The clinical target volume (CTV) received a total dose of 79.2 Gy in 44 fractions. Anterior oblique research plans (ANT) were created using the clinical planning system, and featured beams with ±35-degree gantry angle, 1.2 cm aperture margins, 3-mm range compensator smearing and no range uncertainty margins. Monte Carlo (MC) simulations were performed for both beam arrangements using TOPAS. Dose volume histograms were analyzed and compared for planned and MC dose distributions. Differences between MC and planned DVH parameters were computed as a percentage of the total prescribed dose. Results: For all patients, CTV dose was systematically lower (∼2–2.5%) for MC than the plan. This discrepancy was slightly larger (∼0.5%) for LAT compared to ANT plans for all cases. Although the dose differences for bladder and anterior rectal wall remained within 0.7% for all LAT cases, they were slightly larger for ANT plans, especially for case 3 due to larger patient size and MC-plan range difference. The EUD difference for femoral heads was within 0.6% for both LAT and ANT cases. Conclusion: The dose calculated by the treatment planning system using pencil beam algorithm agrees with MC to within 2.5% and is comparable for lateral and anterior scenarios. The dose agreement in the anterior rectal wall is range- and hence, patient-dependent for ANT treatments.

TH‐E‐BRE‐07: Development of Dose Calculation Error Predictors for a Widely Implemented Clinical Algorithm

Purpose:Several shortcomings of the current implementation of the analytic anisotropic algorithm (AAA) may lead to dose calculation errors in highly modulated treatments delivered to highly heterogeneous geometries. Here we introduce a set of dosimetric error predictors that can be applied to a clinical treatment plan and patient geometry in order to identify high risk plans. Once a problematic plan is identified, the treatment can be recalculated with more accurate algorithm in order to better assess its viability.Methods:Here we focus on three distinct sources dosimetric error in the AAA algorithm. First, due to a combination of discrepancies in smallfield beam modeling as well as volume averaging effects, dose calculated through small MLC apertures can be underestimated, while that behind small MLC blocks can overestimated. Second, due the rectilinear scaling of the Monte Carlo generated pencil beam kernel, energy is not properly transported through heterogeneities near, but not impeding, the central axis of the beamlet. And third, AAA overestimates dose in regions very low density (< 0.2 g/cm3). We have developed an algorithm to detect the location and magnitude of each scenario within the patient geometry, namely the field‐size index (FSI), the heterogeneous scatter index (HSI), and the lowdensity index (LDI) respectively.Results:Error indices successfully identify deviations between AAA and Monte Carlo dose distributions in simple phantom geometries. Algorithms are currently implemented in the MATLAB computing environment and are able to run on a typical RapidArc head & neck geometry in less than an hour.Conclusion:Because these error indices successfully identify each type of error in contrived cases, with sufficient benchmarking, this method can be developed into a clinical tool that may be able to help estimate AAA dose calculation errors and when it might be advisable to use Monte Carlo calculations.

SU‐E‐CAMPUS‐T‐02: Exploring Radiation Acoustics CT Dosimeter Design Aspects for Proton Therapy

Purpose:Investigate the design aspects and imaging dose capabilities of the Radiation Acoustics Computed Tomography (RA CT) dosimeter for Proton induced acoustics, with the objective to characterize a pulsed pencil proton beam. The focus includes scanner geometry, transducer array, and transducer bandwidth on image quality.Methods:The geometry of the dosimeter is a cylindrical water phantom (length 40cm, radius 15cm) with 71 ultrasound transducers placed along the length and end of the cylinder to achieve a weighted set of projections with spherical sampling. A 3D filtered backprojection algorithm was used to reconstruct the dosimetric images and compared to MC dose distribution. First, 3D Monte Carlo (MC) Dose distributions for proton beam energies (range of 12cm, 16cm, 20cm, and 27cm) were used to simulate the acoustic pressure signal within this scanner for a pulsed proton beam of 1.8×107 protons, with a pulse width of 1 microsecond and a rise time of 0.1 microseconds. Dose comparison within the Bragg peak and distal edge were compared to MC analysis, where the integrated Gaussian was used to locate the 50% dose of the distal edge. To evaluate spatial fidelity, a set of point sources within the scanner field of view (15×15×15cm3) were simulated implementing a low‐pass bandwidth response function (0 to 1MHz) equivalent to a multiple frequency transducer array, and the FWHM of the point‐spread‐function determined.Results:From the reconstructed images, RACT and MC range values are within 0.5mm, and the average variation of the dose within the Bragg peak are within 2%. The spatial resolution tracked with transducer bandwidth and projection angle sampling, and can be kept at 1.5mm.Conclusion:This design is ready for fabrication to start acquiring measurements. The 15 cm FOV is an optimum size for imaging dosimetry. Currently, simulations comparing transducer sensitivity, bandwidth, and proton beam parameters are being evaluated to assess signal‐to‐noise.

SU‐E‐I‐28: Evaluating the Organ Dose From Computed Tomography Using Monte Carlo Calculations

Purpose:To evaluate organ doses from computed tomography (CT) using Monte Carlo (MC) calculations.Methods:A Philips Brilliance CT scanner (64 slice) was simulated using the GMctdospp (IMPS, Germany) based on the EGSnrc user code. The X‐ray spectra and a bowtie filter for MC simulations were determined to coincide with measurements of half‐value layer (HVL) and off‐center ratio (OCR) profile in air. The MC dose was calibrated from absorbed dose measurements using a Farmer chamber and a cylindrical water phantom. The dose distribution from CT was calculated using patient CT images and organ doses were evaluated from dose volume histograms.Results:The HVLs of Al at 80, 100, and 120 kV were 6.3, 7.7, and 8.7 mm, respectively. The calculated HVLs agreed with measurements within 0.3%. The calculated and measured OCR profiles agreed within 3%. For adult head scans (CTDIvol) =51.4 mGy), mean doses for brain stem, eye, and eye lens were 23.2, 34.2, and 37.6 mGy, respectively. For pediatric head scans (CTDIvol =35.6 mGy), mean doses for brain stem, eye, and eye lens were 19.3, 24.5, and 26.8 mGy, respectively. For adult chest scans (CTDIvol=19.0 mGy), mean doses for lung, heart, and spinal cord were 21.1, 22.0, and 15.5 mGy, respectively. For adult abdominal scans (CTDIvol=14.4 mGy), the mean doses for kidney, liver, pancreas, spleen, and spinal cord were 17.4, 16.5, 16.8, 16.8, and 13.1 mGy, respectively. For pediatric abdominal scans (CTDIvol=6.76 mGy), mean doses for kidney, liver, pancreas, spleen, and spinal cord were 8.24, 8.90, 8.17, 8.31, and 6.73 mGy, respectively. In head scan, organ doses were considerably different from CTDIvol values.Conclusion:MC dose distributions calculated by using patient CT images are useful to evaluate organ doses absorbed to individual patients.

SU‐E‐J‐194: Dynamic Tumor Tracking End‐To‐End Testing Using a 4D Thorax Phantom and EBT3 Films

Purpose:To quantify the Vero linac dosimetric accuracy of the tumor dynamic tracking treatment using EBT3 film embedded in a 4D thorax phantom.Methods:A dynamic thorax phantom with tissue equivalent materials and a film insert were used in this study. The thorax phantom was scanned in 4DCT mode with a viscoil embedded in its film insert composed of lung equivalent material. Dynamic tracking planning was performed using the 50% phase CT set with 5 conformal beams at gantry angles of 330, 15, 60, 105 and 150 degrees. Each field is a 3cm by 3cm square centered at viscoil since there was no solid mass target. Total 3 different 1–2cos4 motion profiles were used with varied motion magnitude and cycle frequency. Before treatment plan irradiation, a 4D motion model of the target was established using a series of acquired fluoroscopic images and infrared markers motion positions. During irradiation, fluoroscopic image monitoring viscoil motion was performed to verify model validity. The irradiated films were scanned and the dose maps were compared to the planned Monte Carlo dose distributions. Gamma analyses using 3%–3mm, 2%–3mm, 3%–2mm, 2%–2mm criteria were performed and presented.Results:For each motion pattern, a 4D motion model was built successfully and the target tracking performance was verified with fluoroscopic monitoring of the viscoil motion and its model predicted locations. The film gamma analysis showed the average pass rates among the 3 motion profiles are 98.14%, 96.2%, 91.3% and 85.61% for 3%–3mm, 2%–3mm, 3%–2mm, 2%–2mm criteria.Conclusion:Target dynamic tracking was performed using patient‐like breathing patterns in a 4D thorax phantom with EBT3 film insert and a viscoil. There was excellent agreement between acquired and planned dose distributions for all three target motion patterns. This study performed end‐to‐end testing and verified the treatment accuracy of tumor dynamic tracking.

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

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

128: 4D dose calculations and 4D PET image reconstruction using deformable tetrahedral models of moving organs

Purpose: Respiratory-induced organ motion is a technical challenge to nuclear imaging and to particle therapy dose calculations for lung cancer treatment in particular. Internal organ tissue displacements and deformations induced by breathing need to be taken into account when calculating Monte Carlo dose distributions or when performing tomographic reconstructions for PET imaging. Current techniques based on Deformable Image Registration (DIR) cannot fully take into account complex internal deformations of the lungs nor the fact that respiratory motion is not reproducible. This study proposes a method to calculate dose distribution and to reconstruct PET activities over adaptive tetrahedral meshes which deform with time. As an illustration, biomechanical modeling based on Finite Element Analysis (FEA) was used for motion field computation.

Dosimetric effects of a high-density spinal implant

In this study, a treatment plan for a spinal lesion, with all beams transmitted though a titanium vertebral reconstruction implant, was used to investigate the potential effect of a high-density implant on a three-dimensional dose distribution for a radiotherapy treatment. The BEAMnrc/DOSXYZnrc and MCDTK Monte Carlo codes were used to simulate the treatment using both a simplified, recltilinear model and a detailed model incorporating the full complexity of the patient anatomy and treatment plan. The resulting Monte Carlo dose distributions showed that the commercial treatment planning system failed to accurately predict both the depletion of dose downstream of the implant and the increase in scattered dose adjacent to the implant. Overall, the dosimetric effect of the implant was underestimated by the commercial treatment planning system and overestimated by the simplified Monte Carlo model. The value of performing detailed Monte Carlo calculations, using the full patient and treatment geometry, was demonstrated.

SU‐C‐108‐06: Monte Carlo Simulations for Trajectory‐Based Beam Delivery in Varian TrueBeam Developer Mode

Purpose: Varian experimental trajectory based beam delivery mode on the TrueBeam linac comes well in advance of treatment planning software capable of managing the range of trajectories the linac is able to deliver. The goal of this study is to present a Monte Carlo based dose calculation system for trajectory based treatment delivered using Varian TrueBeam Developer Mode. Methods: The Varian TrueBeam linear accelerator advanced control features are designed to allow complex three dimensional trajectory beam delivery. In Developer Mode, the machine is capable of dynamically moving all its components simultaneously while the beam is on. Dose delivery is executed via a set of instructions written in XML which use the “Trajectory Model”, a central concept of the TrueBeam control system. In this model all beams are characterized by the relationship between monitor units (MU) and position of the moving components of the linear accelerator. A trajectory based beam was created which includes simultaneous collimator rotation, couch translation and rotation. An in‐house C++ code converts the XML file into DOSXYZnrc input file for source 20. This source supports all of the degrees of freedom available in Developer Mode. Therefore, it is possible to simulate any complex trajectory‐based beam delivery continuously, in a single run. The trajectory beam was delivered on a Varian TrueBeam linac in developer mode using a 6XFFF photon beam with a 1.5cm2 field size defined by the MLC. Monte Carlo dose distribution was compared to Gafchromic film measurement using FilmQA Pro. Results: Good agreement is observed between Monte Carlo and film measurement with over 90% of the points passing the 3%/3mm Gamma test. Conclusion: This work created a tool that can be employed by any user of the TrueBeam Developer Mode to calculate the dose for complex trajectory‐based beam delivery with the accuracy of Monte Carlo simulations. This work is supported by the Varian Master Research Agreement

Comparison of methods for the measurement of radiation dose distributions in high dose rate (HDR) brachytherapy: Ge‐doped optical fiber, EBT3 Gafchromic film, and PRESAGE® radiochromic plastic

Dose distribution measurement in clinical high dose rate (HDR) brachytherapy is challenging, because of the high dose gradients, large dose variations, and small scale, but it is essential to verify accurate treatment planning and treatment equipment performance. The authors compare and evaluate three dosimetry systems for potential use in brachytherapy dose distribution measurement: Ge-doped optical fibers, EBT3 Gafchromic film with multichannel analysis, and the radiochromic material PRESAGE(®) with optical-CT readout. Ge-doped SiO2 fibers with 6 μm active core and 5.0 mm length were sensitivity-batched and their thermoluminescent properties used via conventional heating and annealing cycles. EBT3 Gafchromic film of 30 μm active thickness was calibrated in three color channels using a nominal 6 MV linear accelerator. A 48-bit transmission scanner and advanced multichannel analysis method were utilized to derive dose measurements. Samples of the solid radiochromic polymer PRESAGE(®), 60 mm diameter and 100 mm height, were analyzed with a parallel beam optical CT scanner. Each dosimetry system was used to measure the dose as a function of radial distance from a Co-60 HDR source, with results compared to Monte Carlo TG-43 model data. Each system was then used to measure the dose distribution along one or more lines through typical clinical dose distributions for cervix brachytherapy, with results compared to treatment planning system (TPS) calculations. Purpose-designed test objects constructed of Solid Water and held within a full-scatter water tank were utilized. All three dosimetry systems reproduced the general shape of the isolated source radial dose function and the TPS dose distribution. However, the dynamic range of EBT3 exceeded those of doped optical fibers and PRESAGE(®), and the latter two suffered from unacceptable noise and artifact. For the experimental conditions used in this study, the useful range from an isolated HDR source was 5-40 mm for fibers, 3-50 mm for EBT3, and 4-21 mm for PRESAGE(®). Fibers demonstrated some over-response at very low dose levels, suffered from volume averaging effects in the dose distribution measurement, and exhibited up to 9% repeatability variation over three repeated measurements. EBT3 demonstrated excellent agreement with Monte Carlo and TPS dose distributions, with up to 3% repeatability over three measurements. PRESAGE(®) gave promising results, being the only true 3D dosimeter, but artifacts and noise were apparent. The comparative response of three emerging dosimetry systems for clinical brachytherapy dose distribution measurement has been investigated. Ge-doped optical fibers have excellent spatial resolution for single-direction measurement but are currently too large for complex dose distribution assessment. The use of PRESAGE(®) with optical-CT readout gave promising results in the measurement of true 3D dose distributions but further development work is required to reduce noise and improve dynamic range for brachytherapy dose distribution measurements. EBT3 Gafchromic film with multichannel analysis demonstrated accurate and reproducible measurement of dose distributions in HDR brachytherapy. Calibrated dose measurements were possible with agreement within 1.5% of TPS dose calculations. The suitability of EBT3 as a dosimeter for 2D quality control or commissioning work has been demonstrated.

Monte Carlo optimization of total body irradiation in a phantom and patient geometry

The objective of this work is to apply a Monte Carlo (MC) accelerator model, validated by experimental data at isocentre distances, to a large-field total body irradiation (TBI) technique and to develop a strategy for individual patient treatment on the basis of MC dose distributions. Calculations are carried out using BEAMnrc/DOSXYZnrc code packages for a 15 MV Varian accelerator. Acceptable agreement is obtained between MC data and measurements in a large water phantom behind a spoiler at source–skin distances (SSD) = 460 cm as well as in a CIRS® thorax phantom. Dose distributions in patients are studied when simulating bilateral beam delivery at a distance of 480 cm to the patient central sagittal plane. A procedure for individual improvement of the dose uniformity is suggested including the design of compensators in a conventional treatment planning system (TPS) and a subsequent update of the dose distribution. It is demonstrated that the dose uniformity for the simple TBI technique can be considerably improved. The optimization strategy developed is straightforward and suitable for clinics where the TPS available is deficient to calculate 3D dose distributions at extended SSD.

OC-0065: Small field dosimetry in flattening filter free (FFF) beams: comparison of diode, film and scintillation dosimeters

Fig 1 shows a screenshot of Smart-Plan of the module where CT images of animal specimens are inspected for their electron density, and where a range of tissues is assigned to the very small voxels. The bottom panel shows a Monte Carlo dose distribution in a mouse thorax from 3 stationary photon beams in the range of 1-5 mm field size. The results are presented as dose-to-medium, meaning the photon transport was done in the proper media and doses are expressed in the media as well (an alternative is to express doses in water). The calculation times for a typical treatment plan are about 5 minutes. The dose verification in the heterogeneous phantom revealed good agreement between measured and calculated dose distributions. Conclusions: A novel radiotherapy treatment planning system for small animals was developed and validated. The system may play an important role in pre-clinical translational studies.

The use of non-standard CT conversion ramps for Monte Carlo verification of 6 MV prostate IMRT plans

Monte Carlo (MC) dose calculation algorithms have been widely used to verify the accuracy of intensity-modulated radiotherapy (IMRT) dose distributions computed by conventional algorithms due to the ability to precisely account for the effects of tissue inhomogeneities and multileaf collimator characteristics. Both algorithms present, however, a particular difference in terms of dose calculation and report. Whereas dose from conventional methods is traditionally computed and reported as the water-equivalent dose (Dw), MC dose algorithms calculate and report dose to medium (Dm). In order to compare consistently both methods, the conversion of MC Dm into Dw is therefore necessary.This study aims to assess the effect of applying the conversion of MC-based Dm distributions to Dw for prostate IMRT plans generated for 6 MV photon beams. MC phantoms were created from the patient CT images using three different ramps to convert CT numbers into material and mass density: a conventional four material ramp (CTCREATE) and two simplified CT conversion ramps: (1) air and water with variable densities and (2) air and water with unit density. MC simulations were performed using the BEAMnrc code for the treatment head simulation and the DOSXYZnrc code for the patient dose calculation. The conversion of Dm to Dw by scaling with the stopping power ratios of water to medium was also performed in a post-MC calculation process.The comparison of MC dose distributions calculated in conventional and simplified (water with variable densities) phantoms showed that the effect of material composition on dose-volume histograms (DVH) was less than 1% for soft tissue and about 2.5% near and inside bone structures. The effect of material density on DVH was less than 1% for all tissues through the comparison of MC distributions performed in the two simplified phantoms considering water. Additionally, MC dose distributions were compared with the predictions from an Eclipse treatment planning system (TPS), which employed a pencil beam convolution (PBC) algorithm with Modified Batho Power Law heterogeneity correction. Eclipse PBC and MC calculations (conventional and simplified phantoms) agreed well (<1%) for soft tissues. For femoral heads, differences up to 3% were observed between the DVH for Eclipse PBC and MC calculated in conventional phantoms. The use of the CT conversion ramp of water with variable densities for MC simulations showed no dose discrepancies (0.5%) with the PBC algorithm. Moreover, converting Dm to Dw using mass stopping power ratios resulted in a significant shift (up to 6%) in the DVH for the femoral heads compared to the Eclipse PBC one.Our results show that, for prostate IMRT plans delivered with 6 MV photon beams, no conversion of MC dose from medium to water using stopping power ratio is needed. In contrast, MC dose calculations using water with variable density may be a simple way to solve the problem found using the dose conversion method based on the stopping power ratio.

TH‐C‐BRA‐06: Measurement Based Source Model for On‐Board Imager Monte Carlo Dosimetry Simulations

Purpose: The purpose of this study is to investigate and develop a systematic approach to quantify the dose impact of the radiographic and cone‐beam computed tomography (CBCT) modes of operation of the onboard imager (OBI) on radiotherapy treatment machines, independent of manufacturer's data specifications. Methods: In order to facilitate dose calculations using Monte Carlo (MC) methods, we have developed an equivalent source model for OBI systems consisting of an equivalent spectrum module incorporative of intrinsic filtration and an equivalent filter module characterizing the added bowtie filter. The equivalent spectrum was generated by iteratively updating the source spectrum estimate until the calculated beam behavior matched the measured half‐value layer and quarter‐value layer values. The equivalent filter was established by obtaining a series of exposure readings along the bowtie profile and calculating the equivalent path lengths of a particular filter material. The equivalent spectrum module was validated by assessing the consistency of the estimation results obtained from half‐ and full‐ bowtie measurements. The end‐to‐end system was validated by comparing dose distributions from MC simulations based on an equivalent spectrum and an equivalent filter with dose distributions from film measurements for both the full‐ and half‐bowtie scenario. Results: Results were generated using a 95 kVp beam. Agreement was observed between the equivalent source spectrums derived from measurements using either full‐ or half‐ bowtie filters. This consistency indirectly validates the consistency and accuracy of the equivalent source approach, as the experimental data are mutually independent, with source and internal filtration being the intrinsic common factor. A root‐mean‐square error of approximately 3% was observed between the MC and film measurement dose distributions for both bowtie filters. Conclusions: We have developed a source model that generates valid input for OBI MC dosimetry simulations. The method uses relatively simple measurement data, instead of manufacturer's specifications.

OC-0378 HELICAL TOMOTHERAPY DYNAMICS ARE NOT AN ISSUE TO TREAT LUNG TUMORS FOR PATIENTS COACHED TO ENSURE REGULAR BREATHING

Purpose/objective: To evaluate helical tomotherapy (HT) treatment delivery and breathing motion interplay effect on dose distributions using a comprehensive 4D Monte Carlo (MC) dose engine including non-rigid deformation of dose maps computed on a 4D CT scan. Material/methods: Treatment planning is usually performed on a 3DCT image, even though lung tumors may move during delivery. To mitigate this issue, 4DCT-based delineation and appropriate margin definition are used to ensure good tumor coverage. However, treatment beam and breathing-induced motions interplay may lead to clinically unacceptable delivered doses, irrespective of the margin definition technique used. The particular delivery scheme of HT, with synchronous motion of both the gantry and the treatment couch, raise concerns specific to this modality for treating lung tumors. To evaluate the effect of motion interplay on dose distributions, a 4D MC dose computation engine was devised: 1) the MC core was TomoPen, a previously validated MC model of tomotherapy; 2) dose maps were computed on a 4D CT scan (10 phases); 3) resulting dose maps were accumulated through non-rigid deformation. Regular breathing was ensured with patient audio coaching. The treatment sinogram was correlated to the measured breathing period. 4D dose distributions (“interplay simulated”) were computed for 7 patients with a large range of motion amplitudes (up to 11.4 mm in the superior-inferior direction). Those were compared to MC dose distributions calculated on the 3DCT (“planned” dose distributions) and also on the 4DCT with the entire sinogram computed on every single phase, thus accounting only for breathing motion assuming an infinitely slow treatment (“no interplay”). Generalized equivalent uniform doses (gEUDs) and typical DVHs metrics (D95, D2, V95, Dmean, …) were computed for the tumor volumes (CTV and GTV) and compared for all simulation modalities. Results: For all modalities and all 7 patients, dose distributions complied with local clinical requirements (reported as in ICRU 83, RTOG 0618 and 0236 depending on the cases studied). Between “interplay simulated” and “no interplay”, D95 and D2 for tumor volumes were within 2.2% whereas Dmean and gEUDs were within 1% . The maximum difference between planned dose and “interplay simulated” was a 3% increase in Dmean and gEUD (see figure (b)). Since the “no interplay” effect was in agreement within 0.1 % with “interplay simulated”, the difference was attributed to the different quality of images of the 3D and 4D CT data sets. Conclusions: For the patients included in this study, coached to ensure regular breathing, HT delivery and breathing motion interplay was found clinically negligible, confirming in practical clinical routine previous theoretical analysis. Thus, as with 3D conformal static delivery, the problem of covering moving lung tumors may be restricted to the sole definition of margins if gating or tracking technologies are not available.

Mathematical solutions of the TG-43 geometry function for curved line, ring, disk, sphere, dome and annulus sources, and applications for quality assurance

Analytic solutions for the TG-43 geometry function for curved line, ring, disk, sphere, dome and annulus shapes containing uniform distributions of air-kerma are derived. These geometry functions describe how dose distributions vary strictly due to source geometry and not including attenuation or scatter effects. This work extends the use of geometry functions for individual sources to applicators containing multiple sources. Such geometry functions may be used to verify dose distributions computed using advanced techniques, including QA of model-based dose calculation algorithms. The impact of source curvature on linear and planar implants is considered along with the specific clinical case of brachytherapy eye plaques. For eye plaques, the geometry function for a domed distribution is used with published Monte Carlo dose distributions to determine a radial dose function and anisotropy function which includes all the scatter and attenuation effects due to the phantom, eye plaque and sources. This TG-43 model of brachytherapy eye plaques exactly reproduces azimuthally averaged Monte Carlo calculations, both inside and outside the eye.

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

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