Grid Based Boltzmann Solver Research Articles

Practical experience with initial quality assurance of a high dose rate brachytherapy grid-based Boltzmann solver algorithm.

The purpose of this study was to validate the use of a model-based dose calculation algorithm (MBDCA), Acuros BV, for high dose rate brachytherapy treatment planning for a community-based hospital with a Bravos afterloader. Based on published AAPM recommendations, this work details a practical approach for community-based clinics to complete initial validation of Acuros BV, in order to add a MBDCA to a TG-43 based brachytherapy treatment planning program. Source dimensions and materials used in Acuros BV and TG-43 source models were compared to the physical source. TG-186 testing was completed with standardized test cases externally calculated with Monte Carlo compared to locally calculated with Acuros BV. Point doses calculated using TG-43 were compared to those calculated with Acuros BV in water at various dose grid settings. Secondary dose check software was used to evaluate dose distributions resembling clinical patient plans, both in water and on CT datasets representative of patient anatomy. The major source of discrepancy of source models was the length of modeled steel cable. TG-186 testing showed that the largest differences between Monte Carlo and Acuros BV dose distributions were located along the source axis for cases calculated in water, as well as located in regions of high dose gradients and within the applicator for the case calculated with a generic shielded applicator. An audit of point doses calculated with both TG-43 and Acuros BV in water found that dose grid settings significantly affected agreement. Secondary dose check software indicated that Acuros BV functioned satisfactorily, and a 5% threshold was adopted for secondary dose checks on gynecologic plans. This validation process indicated that Acuros BV met expected standards and affirmed its suitability for integration into this clinical practice's brachytherapy treatment planning.

Feasibility of energy adaptive angular meshing for perpendicular and parallel magnetic fields in a grid based Boltzmann solver

Purpose: To develop the enabling algorithmic techniques which allow forward-peaked adaptive angular meshing to be compatible with angular advection of magnetic fields within a deterministic Grid Based Boltzmann Solver (GBBS) for MRI-guided radiotherapy, and establish appropriate energy adaptive meshing schemes which minimize total numerical degrees of freedom while preserving high dosimetric accuracy for parallel and perpendicular magnetic fields. Methods: A framework to independently adapt angular mesh resolution and basis function refinement of forward and backscattering hemispheres is developed, uniquely accommodating angular advection introduced by magnetic fields. Upwind stabilization techniques to accurately transfer fluence between hemispheres having different discretization are established. To facilitate oblique beam and magnetic field orientations, cardinal forward-peaked mesh orientations were devised to balance requirements for acyclic space-angle sweep ordering, while ensuring the beam predominantly overlaps the forward hemisphere. Energy-dependent fluence anisotropy is investigated, leading to adaptive angular meshing schemes for parallel and perpendicular magnetic fields. Calculated dose distributions were validated against GEANT4 Monte Carlo calculations on slab geometry and anthropomorphic phantoms. Results: Forward-peaked and isotropic energy adaptive angular meshing schemes were developed for parallel and perpendicular magnetic fields respectively, which reduce the number of elements solved by 52.8% and 47.7% respectively compared to static discretization using 32 quadratic elements while retaining over 97% of points passing the gamma 1%/1 mm criterion against Monte Carlo. Conclusions: Techniques to preserve angular upwind-stabilization between hemispheres of a forward-peaked mesh and establish an acyclic directed space-angle sweep graph enabled energy-adaptive meshing schemes to be developed while accurately solving for magnetic fields. This substantially reduced the numerical degrees of freedom while retaining excellent dosimetric agreement with Monte Carlo. These algorithmic underpinnings contribute towards a fast deterministic GBBS for MRI-guided radiotherapy.

A novel transport sweep architecture for efficient deterministic patient dose calculations in MRI-guided radiotherapy

Accurate and efficient patient dose calculations are essential for treatment planning in magnetic resonance imaging guided radiotherapy (MRIgRT). Achieving reasonable performance for a space-angle discontinuous finite element method (DFEM) grid based Boltzmann solver (GBBS) with magnetic fields for clinical MRIgRT applications largely depends on how the transport sweep is orchestrated. Compared to classical Discrete Ordinates, DFEM in angle introduces increased angular degrees of freedom and eliminates ray-effect artifacts. However, the inclusion of magnetic fields introduces additional serial dependencies such that parallelization of the space-angle transport sweeps becomes more challenging.Novel techniques for the transport sweep and right-hand source assembly are developed, predicated on limiting the number of bulk material densities modeled in the transport sweep scatter calculations. Specifically, k-means clustering is used to assign sub-intervals of mass-density for each spatial element to execute the scatter-dose calculations using batched multiplication by pre-inverted transport sweep matrices. This is shown to be two orders of magnitude more efficient than solving each elemental system individually at runtime.Even with discrete material densities used in the transport sweep scatter calculations, accuracy is maintained by optimizing the material density assignments using k-means clustering, and by performing the primary photon fluence calculations (ray-tracing) using the underlying continuous density of the computed tomography (CT) image. In the presence of 0.5 T parallel and 1.5 T perpendicular magnetic fields, this approach demonstrates high levels of accuracy with gamma 1%/1 mm passing rates exceeding 94% across a range of anatomical sites compared to GEANT4 Monte Carlo dose calculations which used continuous densities.This deterministic GBBS approach maintains unconditional stability, produces no ray-effect artifacts, and has the benefit of no statistical uncertainty. Runtime on a non-parallelized Matlab implementation averaged 10 min per beam averaging 80 000 spatial elements, paving way for future development based on this algorithmically efficient paradigm.

33 Entropic model for real-time dose calculation

Introduction This work proposes a completely new Grid-Based Boltzmann Solver (GBBS) developed for the transport and energy deposition by energetic particles. Its entropic closure and mathematical formulation allow our entropic model to calculate the delivered dose with an accuracy comparable to Monte Carlo (MC) codes with a computational time that is reduced to the order of few seconds without any special processing power requirement. Methods In contrast to discrete ordinates angular discretization methods, such as Acuros®, our method is based on a reduced number of moment equations closed with Boltzmann’s H-theorem. Keeping a good accuracy of calculations, the algorithm can simulate different treatment techniques such as the external radiotherapy even in presence of magnetic field [1] , [2] (e.g., MRI-guided radiotherapy), brachytherapy or intra-operative radiation therapy. The first validation step consists in simulating dose distributions in complex numerical phantoms including a large number of heterogeneity shapes such as bone, lung and air. For both brachytherapy and external beam radiotherapy, simulations based on CT scan, using the real phase-space of the source, have been performed. The entropic model is validated by a direct comparison with the reference MC code PENELOPE. Results The code is capable of calculating 3D dose distributions with 1 mm3voxels without statistical uncertainties in few seconds instead of several minutes like PENELOPE. In brachytherapy applications the dose distributions significantly differ from those calculated with the TG-43 approximations, thanks to its capability to account for inhomogeneities and strong density gradients. Moreover, for both applications the code shows an excellent agreement with PENELOPE within the 1%/1 mm gamma-index criterion. Conclusions In the comparison with the MC results the excellent accuracy of the model is demonstrated. Thanks to its reduced computational time and its accuracy, this model is a promising candidate to become a real-time dose calculation algorithm.

High performance modelling of the transport of energetic particles for photon radiotherapy

This work consists of the validation of a new Grid Based Boltzmann Solver (GBBS) conceived for the description of the transport and energy deposition by energetic particles for radiotherapy purposes. The entropic closure and a compact mathematical formulation allow our code (M1) to calculate the delivered dose with an accuracy comparable to the Monte-Carlo (MC) codes with a computational time that is reduced to the order of few minutes without any special processing power requirement. A validation protocol with heterogeneity inserts has been defined for different photon sources. The comparison with the MC calculated depth-dose curves and transverse profiles of the beam at different depths shows an excellent accuracy of the M1 model.

Evaluation of a deterministic grid-based Boltzmann solver (GBBS) for voxel-level absorbed dose calculations in nuclear medicine

To evaluate the 3D Grid-based Boltzmann Solver (GBBS) code ATTILA® for coupled electron and photon transport in the nuclear medicine energy regime for electron (beta, Auger and internal conversion electrons) and photon (gamma, x-ray) sources. Codes rewritten based on ATTILA are used clinically for both high-energy photon teletherapy and 192Ir sealed source brachytherapy; little information exists for using the GBBS to calculate voxel-level absorbed doses in nuclear medicine.We compared DOSXYZnrc Monte Carlo (MC) with published voxel-S-values to establish MC as truth. GBBS was investigated for mono-energetic 1.0, 0.1, and 0.01 MeV electron and photon sources as well as 131I and 90Y radionuclides. We investigated convergence of GBBS by analyzing different meshes (), energy group structures () for each radionuclide component, angular quadrature orders (, and scattering order expansions (–); higher indices imply finer discretization. We compared GBBS to MC in (1) voxel-S-value geometry for soft tissue, lung, and bone, and (2) a source at the interface between combinations of lung, soft tissue, and bone.Excluding Auger and conversion electrons, MC agreed within ≈5% of published source voxel absorbed doses. For the finest discretization, most GBBS absorbed doses in the source voxel changed by less than 1% compared to the next finest discretization along each phase space variable indicating sufficient convergence. For the finest discretization, agreement with MC in the source voxel ranged from −3% to −20% with larger differences at lower energies (−3% for 1 MeV electron in lung to −20% for 0.01 MeV photon in bone); similar agreement was found for the interface geometries. Differences between GBBS and MC in the source voxel for 90Y and 131I were −6%.The GBBS ATTILA was benchmarked against MC in the nuclear medicine regime. GBBS can be a viable alternative to MC for voxel-level absorbed doses in nuclear medicine. However, reconciliation of the differences between GBBS and MC at lower energies requires further investigation of energy deposition cross-sections.

Impact of heterogeneity-corrected dose calculation using a grid-based Boltzmann solver on breast and cervix cancer brachytherapy.

PurposeTo analyze the impact of heterogeneity-corrected dose calculation on dosimetric quality parameters in gynecological and breast brachytherapy using Acuros, a grid-based Boltzmann equation solver (GBBS), and to evaluate the shielding effects of different cervix brachytherapy applicators.Material and methodsCalculations with TG-43 and Acuros were based on computed tomography (CT) retrospectively, for 10 cases of accelerated partial breast irradiation and 9 cervix cancer cases treated with tandem-ring applicators. Phantom CT-scans of different applicators (plastic and titanium) were acquired. For breast cases the V20Gyαβ3 to lung, the D0.1cm3, D1cm3, D2cm3 to rib, the D0.1cm3, D1cm3, D10cm3 to skin, and Dmax for all structures were reported. For cervix cases, the D0.1cm3, D2cm3 to bladder, rectum and sigmoid, and the D50, D90, D98, V100 for the CTVHR were reported. For the phantom study, surrogates for target and organ at risk were created for a similar dose volume histogram (DVH) analysis. Absorbed dose and equivalent dose to 2 Gy fractionation (EQD2) were used for comparison.ResultsCalculations with TG-43 overestimated the dose for all dosimetric indices investigated. For breast, a decrease of ~8% was found for D10cm3 to the skin and 5% for D2cm3 to rib, resulting in a difference ~ –1.5 Gy EQD2 for overall treatment. Smaller effects were found for cervix cases with the plastic applicator, with up to –2% (–0.2 Gy EQD2) per fraction for organs at risk and –0.5% (–0.3 Gy EQD2) per fraction for CTVHR. The shielding effect of the titanium applicator resulted in a decrease of 2% for D2cm3 to the organ at risk versus 0.7% for plastic.ConclusionsLower doses were reported when calculating with Acuros compared to TG-43. Differences in dose parameters were larger in breast cases. A lower impact on clinical dose parameters was found for the cervix cases. Applicator material causes systematic shielding effects that can be taken into account.

TH-AB-BRA-10: Clinical Implementation of a Grid-Based Boltzmann Solver with Adaptive Meshing for Nuclear Medicine Dosimetry

Purpose: We previously compared the Grid-Based Boltzmann Solver (GBBS) with DOSXYZnrc Monte Carlo (MC) for beta and gamma sources in uniform materials and along material interfaces. The current standard in nuclear medicine is MIRD, which only calculates mean organ doses assuming uniform uptake in anthropomorphic phantoms. This work extends GBBS to clinical nuclear medicine for fast and accurate patient-specific voxel-level dosimetry. Methods: We compared the GBBS with MC using a patient’s post-therapy quantitative 90Y microsphere bremsstrahlung SPECT/CT.The GBBS (Attila™ v8.0.0) requires a tetrahedral mesh as input instead of voxels. Adaptive tetrahedral meshes were generated with TetGen v1.5.0 by folding CT and SPECT images into a mesh sizing function; the function was defined at each node on a background mesh with nodes defined at each SPECT/CT voxel. Target tetrahedral edge lengths varied from 0.5 to 8 cm and were adapted on SPECT activity level, CT material, and material gradient.GBBS used 30 electron energy groups, 32 angles, and 67,000 tetrahedrons. GBBS reported point data aligned with the center of SPECT/CT voxels which match MC. We report: 1) run time of the GBBS, 2) average difference in non-zero absorbed dose voxels, and 3) percent difference in mean absorbed dose to tumor and normal liver. DVHs and isodose curves were visually compared, and the tetrahedral mesh was compared to SPECT/CT. Results: GBBS calculation required 2 minutes. The average difference over all non-zero absorbed dose voxels (N=318,000) was −0.2 Gy. GBBS mean doses agreed within 1% of MC for both normal liver (39.1 Gy vs 39.3 Gy) and tumor (317 Gy vs 320 Gy). Adaptive meshing allowed sufficient material assignment and yielded smaller tetrahedrons near high activity regions and bone. DVHs had excellent visual agreement. Conclusions: GBBS with adaptive meshing is practical for fast and accurate clinical nuclear medicine dosimetry. Research reported in this publication was supported by the National Cancer Institute of the National Institutes of Health under Award Number R01CA138986. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.

TU-AB-201-06: Evaluation of Electromagnetically Guided High- Dose Rate Brachytherapy for Ablative Treatment of Lung Metastases

Purpose: The advent of electromagnetic navigation bronchoscopy has enabled minimally invasive access to peripheral lung tumors previously inaccessible by optical bronchoscopes. As an adjunct to Stereotactic Ablative Radiosurgery (SABR), implantation of HDR catheters can provide focal treatments for multiple metastases and sites of retreatments. The authors evaluate a procedure to deliver ablative doses via Electromagnetically-Guided HDR (EMG-HDR) to lung metastases, quantify the resulting dosimetry, and assess its role in the comprehensive treatment of lung cancer. Methods: A retrospective study was conducted on ten patients, who, from 2009 to 2011, received a hypo-fractionated SABR regimen with 6MV VMAT to lesions in various lobes ranging from 1.5 to 20 cc in volume. A CT visible pathway was delineated for EM guided placement of an HDR applicator (catheter) and dwell times were optimized to ensure at least 98% prescription dose coverage of the GTV. Normal tissue doses were calculated using inhomogeneity corrections via a grid-based Boltzmann solver (Acuros_BV_1.5.0). Results: With EMG-HDR, an average of 83% (+/−9% standard deviation) of each patient's GTV received over 200% of the prescription dose, as compared to SABR where the patients received an average maximum dose of 125% (+/−5%). EMG-HDR enabled a 59% (+/−12%) decrease in the aorta maximum dose, a 63% (+/−26%) decrease in the spinal cord max dose, and 57% (+/−23%) and 70% (+/−17%) decreases in the volume of the body receiving over 50% and 25% of the prescription dose, respectively. Conclusion: EMG-HDR enables delivery of higher ablative doses to the GTV, while concurrently reducing surrounding normal tissue doses. The single catheter approach shown here is limited to targets smaller than 20 cc. As such, the technique enables ablation of small lesions and a potentially safe and effective retreatment option in situations where external beam utility is limited by normal tissue constraints.

Dosimetric comparison of Acuros™ BV with AAPM TG43 dose calculation formalism in breast interstitial high-dose-rate brachytherapy with the use of metal catheters.

PurposeRadiotherapy for breast cancer includes different techniques and methods. The purpose of this study is to compare dosimetric calculations using TG-43 dose formalism and Varian Acuros™ BV (GBBS) dose calculation algorithm for interstitial implant of breast using metal catheters in high-dose-rate (HDR) brachytherapy, using 192Ir.Material and methodsTwenty patients who were considered for breast conservative surgery (BCS), underwent lumpectomy and axillary dissection. These patients received perioperative interstitial HDR brachytherapy as upfront boost using rigid metal implants. Whole breast irradiation was delivered TG-43 after a gap of two weeks. Standard brachytherapy dose calculation was done by dosimetry. This does not take into account tissue heterogeneity, attenuation and scatter in the metal applicator, and effects of patient boundary. Acuros™ BV is a Grid Based Boltzmann Solver code (GBBS), which takes into consideration all the above, was used to compute dosimetry and the two systems were compared.ResultsComparison of GBBS and TG-43 formalism on interstitial metal catheters shows difference in dose prescribed to CTV and other OARs. While the estimated dose to CTV was only marginally different with the two systems, there is a significant difference in estimated doses of starting from 4 to 53% in the mean value of all parameters analyzed.ConclusionsTG-43 algorithm seems to significantly overestimate the dose to various volumes of interest; GBBS based dose calculation algorithm has impact on CTV, heart, ipsilateral lung, heart, contralateral breast, skin, and ribs of the ipsilateral breast side; the prescription changes occurred due to effect of metal catheters, inhomogeneities, and scatter conditions.

Commissioning of a grid-based Boltzmann solver for cervical cancer brachytherapy treatment planning with shielded colpostats

We sought to commission a gynecologic shielded colpostat analytic model provided from a treatment planning system (TPS) library. We have reported retrospectively the dosimetric impact of this applicator model in a cohort of patients. A commercial TPS with a grid-based Boltzmann solver (GBBS) was commissioned for (192)Ir high-dose-rate (HDR) brachytherapy for cervical cancer with stainless steel-shielded colpostats. Verification of the colpostat analytic model was verified using a radiograph and vendor schematics. MCNPX v2.6 Monte Carlo simulations were performed to compare dose distributions around the applicator in water with the TPS GBBS dose predictions. Retrospectively, the dosimetric impact was assessed over 24 cervical cancer patients' HDR plans. Applicator (TPS ID #AL13122005) shield dimensions were within 0.4mm of the independent shield dimensions verification. GBBS profiles in planes bisecting the cap around the applicator agreed with Monte Carlo simulations within 2% at most locations; differing screw representations resulted in differences of up to 9%. For the retrospective study, the GBBS doses differed from TG-43 as follows (meanvalue ±standard deviation [min, max]): International Commission on Radiation units [ICRU]rectum (-8.4±2.5% [-14.1, -4.1%]), ICRUbladder (-7.2±3.6% [-15.7, -2.1%]), D2cc-rectum (-6.2±2.6% [-11.9, -0.8%]), D2cc-sigmoid (-5.6±2.6% [-9.3, -2.0%]), and D2cc-bladder (-3.4±1.9% [-7.2, -1.1%]). As brachytherapy TPSs implement advanced model-based dose calculations, the analytic applicator models stored in TPSs should be independently validated before clinical use. For this cohort, clinically meaningful differences (>5%) from TG-43 were observed. Accurate dosimetric modeling of shielded applicators may help to refine organ toxicity studies.

SU‐E‐T‐345: Dose Comparison of TG‐43 and Acuros Algorithm to Account Lateral Scatter in Dose Distribution

Purpose: To study Effect of lateral scatter in dose distribution in Ir192 HDRBTMethods: Brachytherapy phantom was fabricated using tissue‐equivalent material E.D.1.045gm/cm3 Phantom has provision to place plastic catheters at every 5mm and TLD and Gafchromic film at every 10 mm distance along vertical direction.Phantom CT scan was performed, images were transferred to Brachyvision TPS, which has option of analytical dose calculation algorithm Acuros.Acuros is Grid Based Boltzmann Solver code which solves Linear Boltzmann equation. We reconstructed single catheter at 5 mm from edge of phantom with three source dwell positions, 10mm apart. Dose was prescribed to single point at 10 mm from central source along transverse axis.Dose calculation was done using TG43 and Acuros algorithm. Dose was measured at 10, 20and 30 mm depth using TLDs and GafchromicEBT3 films. Same procedure was repeated by placing catheter at 10mm, 15mm, 20mm and 30mm from the edge of phantom. Results: For prescription point 5mm away from edge of phantom, where scatter contribution to total dose is minimum, we found TG43 overestimates dose by 4.5% compared to Acuros. As we move away from edge of phantom lateral scatter contribution increases, dose calculation variation between two algorithms decreases and is within 1% at 20mm from edge of phantom.At greater depths, catheter close to phantom edge we observed TG43 overestimates dose by 7% at 20mm and 10 % at 30mm depth. For catheter at 20mm from phantom edge, TG43 overestimates dose by 3% at 20mm and 4 % at 30mm depth. These results were confirmed by measurements of TLD and Gafchromic filmsConclusion: Non consideration of lateral scatter contribution by TG43 can leads to dose variation up to 5% to target which is nearby skin. Overestimation of dose must be considered while deciding dose to critical structures which are at greater depths.

Head and neck 192 Ir HDR-brachytherapy dosimetry using a grid-based Boltzmann solver

PurposeTo compare dosimetry for head and neck cancer patients, calculated with TG-43 formalism and a commercially available grid-based Boltzmann solver.Material and methodsThis study included 3D-dosimetry of 49 consecutive brachytherapy head and neck cancer patients, computed by a grid-based Boltzmann solver that takes into account tissue inhomogeneities as well as TG-43 formalism. 3D-treatment planning was carried out by using computed tomography.ResultsDosimetric indices D90 and V100 for target volume were about 3% lower (median value) for the grid-based Boltzmann solver relative to TG-43-based computation (p < 0.01). The V150 dose parameter showed 1.6% increase from grid-based Boltzmann solver to TG-43 (p < 0.01).ConclusionsDose differences between results of a grid-based Boltzmann solver and TG-43 formalism for high-dose-rate head and neck brachytherapy patients to the target volume were found. Distinctions in D90 of CTV were low (2.63 Gy for grid-based Boltzmann solver vs. 2.71 Gy TG-43 in mean). In our clinical practice, prescription doses remain unchanged for high-dose-rate head and neck brachytherapy for the time being.

SU-E-T-425: Impact of Model Based Dose Calculation Algorithm for Ir-192 Intracavitary Brachytherapy with Shielded Applicator.

To determine the impact of a grid based Boltzmann solver (GBBS) on a cohort of cervical cancer patients treated with Ir-192 intracavitary brachytherapy with shielded applicators. Retrospective plans were generated using BrachyVision v8.8 (TPS) with GBBS Acuros v1.3.1. The study includes 24 patients that had CT planning images acquired with CT/MR compatible applicators. Using the TPS applicator library, shielded colpostats and tandem (#AL13122005) were virtually positioned to replace the applicators seen on CT. Dwell weights were based on TG43 delivering 6 Gy to point A. Four GBBS calculations were performed to assess differences from the standard of practice TG43. The different GBBS calculations were: 1) no applicator modeled, body= 1 g/cc muscle, 2) applicator modeled, body=1g/cc muscle, 3) applicator modeled, CT-to-material mapping with contrast (vaginal packing, rectal, Foley balloon) = 1g/cc muscle, and 4) applicator model, CT-to-material mapping without material overrides. The multiple GBBS calculations allow differences from TG43 to be attributed to factors representing the modeling of source and patient boundaries (scatter conditions), tissue heterogeneities, and applicators. Differences between GBBS4 and TG43 at clinical dosimetric points were as follows: [mean ± standard (min, max)], Point A: - 2.5% ± 0.5% (-3.8%, -1.2%), Point B: -1.5% ± 1.0% (-3.2%, 1.1%), ICRU rectum: -8.4% ± 2.5% (-14.0%, -4.1%), D2cc rectum: -6.2% ± 2.6% (- 11.9%, -0.8%), ICRU bladder: -7.2% ± 3.6% (-15.7%, -2.1%); D2cc bladder: -3.4% ± 1.8% (-7.2%, -1.1%). Bar plots comparing the modeling factors previously listed show that applicator modeling is the largest contributor to differences from TG43. Clinically significant dose differences (>5%) relative to TG43 exist when using a model-based dose calculation algorithm such as the GBBS with shielded applicators. Differences were largely due to applicator modeling, not tissue heterogeneities, source modeling, or patient boundary modeling.

Experimental validation of deterministic Acuros XB algorithm for IMRT and VMAT dose calculations with the Radiological Physics Center's head and neck phantom.

The purpose of this study was to verify the dosimetric performance of Acuros XB (AXB), a grid-based Boltzmann solver, in intensity-modulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT). The Radiological Physics Center (RPC) head and neck (H&N) phantom was used for all calculations and measurements in this study. Clinically equivalent IMRT and VMAT plans were created on the RPC H&N phantom in the Eclipse treatment planning system (version 10.0) by using RPC dose prescription specifications. The dose distributions were calculated with two different algorithms, AXB 11.0.03 and anisotropic analytical algorithm (AAA) 10.0.24. Two dose report modes of AXB were recorded: dose-to-medium in medium (D(m,m)) and dose-to-water in medium (D(w,m)). Each treatment plan was delivered to the RPC phantom three times for reproducibility by using a Varian Clinac iX linear accelerator. Absolute point dose and planar dose were measured with thermoluminescent dosimeters (TLDs) and GafChromic® EBT2 film, respectively. Profile comparison and 2D gamma analysis were used to quantify the agreement between the film measurements and the calculated dose distributions from both AXB and AAA. The computation times for AAA and AXB were also evaluated. Good agreement was observed between measured doses and those calculated with AAA or AXB. Both AAA and AXB calculated doses within 5% of TLD measurements in both the IMRT and VMAT plans. Results of AXB_D(m,m) (0.1% to 3.6%) were slightly better than AAA (0.2% to 4.6%) or AXB_D(w,m) (0.3% to 5.1%). The gamma analysis for both AAA and AXB met the RPC 7%/4 mm criteria (over 90% passed), whereas AXB_D(m,m) met 5%/3 mm criteria in most cases. AAA was 2 to 3 times faster than AXB for IMRT, whereas AXB was 4-6 times faster than AAA for VMAT. AXB was found to be satisfactorily accurate when compared to measurements in the RPC H&N phantom. Compared with AAA, AXB results were equal to or better than those obtained with film measurements for IMRT and VMAT plans. The AXB_D(m,m) reporting mode was found to be closer to TLD and film measurements than was the AXB_D(w,m) mode. AXB calculation time was found to be significantly shorter (× 4) than AAA for VMAT.

SU‐E‐T‐377: Evaluation of Calculated and Measured HDR Brachytherapy Doses in the Presence of Heterogeneities

Purpose: With the recent introduction of the Grid‐Based Boltzmann Solver (GBBS) in commercial HDR brachytherapy treatment planning (Acuros™, Varian), there is now a rapid and accurate alternative to Monte Carlo calculations to attain improved dosimetric accuracy. The validation of Acuros™ has been performed by comparison to Monte Carlo calculations in water phantom. In this investigation, we compare calculated and measured doses in the presence of both high and low density heterogeneities. Methods: Phantoms were created incorporating both low and high density heterogeneities including representative densities of bone and air cavities. For the comparisons HDR plans were developed to deliver 100 cGy with TG‐43 formalism to optically stimulated luminescence (OSL) detectors placed in or near the heterogeneities. The detectors were calibrated by exposure to a known dose. The plans were then recalculated using the Acuros™ algorithm, and the dose to the detectors was determined both by calculation and measurement. Results: The OSL detectors measured an average dose of 105 cGy when placed after a low density insert in the phantom, as compared to a calculated average dose of 107 cGy. When the OSL detectors were placed after three different higher density inserts ranging from 1.3 to 1.9 g/cm3, the measured doses were 78, 77, and 75 cGy, compared to the calculated doses of 84, 82, and 75 cGy. Conclusions: The Acuros™ algorithm has been demonstrated to perform rapid and accurate dosimetry in homogenous water phantom. In this study we demonstrate that it performs accurately in the vicinity of heterogeneities expected to be encountered in the clinic. This work shows good qualitative agreement between measured and calculated high dose rate brachytherapy doses in the presence of both low and high density heterogeneities, and shows that the confidence‐building step of comparing calculated to measured dose can be readily made with simple phantom geometries.

Monte Carlo Verification of a CT/MR Ovoid Model Used by a Grid-Based Boltzmann Solver in a Commercial Treatment Planning System

Purpose: A commercial TPS is available that accounts for patient boundaries, applicator materials, and tissue heterogeneities. A library of HDR brachytherapy applicators is included with the TPS. The purpose of this work is to verify the geometric and dosimetric properties for a CT/ MR compatible ovoid. Materials and Methods: A model of the ovoid was constructed using the MCNPX visual editor. The Monte Carlo (MC) model was based on CAD drawings (part #:AL07334001 v1.0) obtained from the vendor and independent measurement of the ovoid structure from CT images. The ovoid model was then placed in a 30 cm diameter sphere of water. Materials in the ovoid include Titanium tubing, an Acetal cap, and dry air. The commercial TPS BrachyVision v8.8 with grid-based Boltzmann solver (GBBS) Acuros v1.3.1 was used in this work. MC simulations for a single source centered in the ovoid were performed using MCNPX v2.6. A MC model of the VS2000 Ir HDR source was taken from our previously benchmarked MC model of this source. A source cable length of 1 mm was chosen to match the cable length modeled in the GBBS. The absorbed dose was approximated by collisional kerma. Cross sections were based on MCPLIB04. A rectangular mesh tally 150 1 with 2 mm isotropic voxels was setup for both the xz and xy plane. 6.5 10 histories were simulated (1s!0.5%). A virtual sphere of water was created in IDL and imported as a DICOM CT into the TPS. The right-side (diameter52.5 cm) FSD CT/MR compatible ovoid was placed at the center of the sphere. The dose grid was set to 2 mm isotropic for 150 voxels. The GBBS calculation was done using the CT option. Both MC and GBBS output were converted to Gy/U to allow comparisons. Geometric and dosimetric differences between the two models were reported. Results: The MC ovoid model included the Titanium set screw and an air pocket on the back of the Acetal cap where the Titanium ovoid tubing enters, while the library applicator model used for GBBS does not account for these structures. The hole for the screw and air pocket are visible in Fig 1a. Other dimensions of the ovoid were in good agreement. The ratio of GBBS/MC dose predictions are shown in Fig 1. The white dashed line (r53.25 cm) shows the physical location of the profile shown in Fig 1b. At q5180 the profile is 2 cm from the ovoid surface where disease would usually be present. Points 1 & 8 in Fig 1b: GBBS models Acetal whereas MC includes a Ti screw. Point 2 in Fig 1b: These are artificial differences below the display range due to the Ti ovoid arm. The MC conversion to Gy/U assumed all materials were water. Point 3 in Fig 1b: GBBS is less than MC due to Acetal in place of air. Point 4 in Fig 1b: Good agreement is found here because differences would be attributed to Ti tubing, not air. Point 5 in Fig 1b: The difference is due to the Acetal (GBBS) and air (MC). Agreement between GBBS and MC is within 2% for the clinical disease region from point 6 to 7 in Fig 1b.

SU-GG-T-39: Grid-Based Boltzmann Solver (GBBS) vs TG-43 for Ir-192 HDR Intracavitary Brachytherapy: A Retrospective Dosimetric Study

Purpose: To determine dosimetric differences between a grid‐based Boltzmann solver (GBBS) and TG‐43 for a cohort of HDR cervical cancer cases treated using the CT/MR Fletcher‐Suit‐Delclos (FSD) type applicator. Method and Materials:Dose distributions from a cohort (n=10) of cervical cancer patients treated with the VS2000 192Ir HDR source were analyzed retrospectively using the BrachyVision‐Acuros v8.8 TPS. A single physician contoured the rectum and bladder. 6 Gy was prescribed to point A. Doses were recorded for ICRU rectal and bladder points as well as vaginal surfaces. Rectum, bladder, and entire dose grid DVHs were visually assessed and compared at relevant points. Dose grids were exported to IDL to calculate mean voxel percent differences. Average, minimum, and maximum percent dose differences are reported for 1) D2cc and D90 for rectum and bladder and for 2) dose grid mean voxel percent difference. Results: Visual inspection of DVH showed lateral shifting of GBBS curves to lower doses. The shifting was noticeable for dose levels within 4 Gy. Percent difference results from the DVH output: D2cc‐Rectum: −2.1%(mean) [−3.0%(min), −0.5%(max)]; D90‐Rectum: −2.3% [−7.5%, 4.6%]; D2cc‐Bladder: −0.7% [−1.8%, 0.7%]; D90‐Bladder: −4.4% [−7.0%, −2.2%]. All ICRU points studied had dose differences within −3.3% relative to TG‐43. The average of the mean voxel percent difference for the entire dose grid was −1.3% and ranged from −3.7% to 1.2%. Visualization of individual percent difference maps overlaid on CT revealed lower GBBS doses behind contrast filled balloon in the bladder. Conclusions: For this cohort of patients that underwent intracavitary HDR brachytherapy with the CT/MR FSD applicator, the GBBS calculated dose to clinical reference points and hot spots (D2cc) was within 3.3% of TG‐43. Over the entire dose grid, however, lateral shifting of DVHs to lower doses was observed. This may be partially due to contrast filled balloon in bladder.

TU-D-BRB-08: Comparison of TLD Measured Dose, TG-43 Dose, and Grid-Based Boltzmann Solver Dose from Intracavitary APBI for Multiple Reference Locations in a Phantom

Purpose: To quantify the difference between inhomogeneity‐corrected brachytherapytreatment planningdose calculations and regular homogeneous (TG‐43) dose calculations and to compare the results from both calculation methods with TLD measurements for well‐defined phantom geometries. Methods and Materials: Well‐characterized TLD‐100 chips were used to measure the dose at multiple reference locations on a torso phantom with gel breasts attached simulating a HDR 192Ir intracavitary APBI treatment. A CT scan of the phantom was obtained and the dose to each reference location was calculated in the treatment planning software, both with the standard TG‐43 dosimetry formalism and with a grid‐based Boltzmann solver (GBBS) that calculates the dose distribution for the actual patient dimensions and tissue compositions. Results: The TLD measured doses ranged from approximately 40 cGy to 300 cGy and were 5% lower, on average, than the GBBS calculated doses and 17% lower, on average, than the TG‐43 calculated doses. The GBBS doses were always lower than the TG‐43 doses, with the discrepancy becoming more pronounced at lower doses (further from the source) and at the surface of the phantom. The discrepancy between the TLD measured doses and the GBBS doses did not appear to trend with dose.Conclusion: The application of inhomogeneity corrections reduces the discrepancy between the TPS calculated and TLD measured doses. When inhomogeneity corrections were not applied, the TPS overestimated the dose at each reference location.

WE-D-BRB-02: Comparing a Grid-Based Boltzmann Solver with Monte Carlo Simulation for Voxel-Based Therapeutic Radionuclide Dose Calculations

Purpose: To compare the accuracy and speed of a deterministic grid‐based Boltzmann solver (GBBS) with Monte Carlo(MC) simulations for calculating voxel‐based absorbed dose rates from SPECT/CT imaging.Methods: A SPECT/CT image of a breast cancer patient with metastatic osteosarcoma was obtained using a tracer administration of 153 Sm EDTMP. The therapeutic activity administered was determined from MIRD estimates. DOSXYZnrc and the GBBS Acuros were used to calculate dose rate maps from the activity distribution over the full CT grid. The GBBS photondose rate was calculated using the collisional KERMA approximation rather than explicitly transporting the generated electrons; an energy cutoff was used to neglect spatial transport of electrons below a threshold of 200 keV for Acuros and 189 keV for DOSXYZnrc. The photon, beta‐particle, and total absorbed dose rates calculated using GBBS were compared with the gold standard MC simulations using the gamma index. Gamma index parameters evaluated were 3%/3mm and 5%/5mm; both used a step size of 0.5mm with a 10 mm search distance. A patient mask was created from the CT to report pixels within/near the patient. The gamma failure points (γ>1.0) within the patient mask were viewed overlaid on the activity map and on the CT for the calculated beta, photon, and total dose rates. Results: For total dose rate, 90.1% and 99.6% of pixels within the patient mask had γ⩽1.0 for 3%/3mm and 5%/5mm respectively. γ failures were noticed at the edges of the activity distribution for beta‐particles, and at various interfaces for the photons. For Acuros, the beta‐particle and photon transport required about 10 minutes each. DOSXYZnrc took approximately 15,000 times longer. Conclusions: GBBS has the potential to provide accuracy similar to MC in a much shorter time; this could be useful for voxel‐based radionuclide absorbed dose estimates in a clinical setting.