Model-based Dose Calculation Algorithms 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.

The effect of vaginal cylinder inhomogeneity on the HDR brachytherapy dose calculations using Monte Carlo simulations.

To analytically assess the heterogeneity effect of vaginal cylinders (VC) made of high-density plastics on dose calculations, considering the prescription point (surface or 5mm beyond the surface), and benchmark the accuracy of a commercial model-based dose calculation (MBDC) algorithm using Monte Carlo (MC) simulations. The GEANT4 MC code was used to simulate a commercial 192 Ir HDR source and VC, with diameters ranging from 20 to 35mm, inside a virtual water phantom. Standard plans were generated from a commercial treatment planning system [TPS-BrachyVision ACUROS (BV)] optimized for a treatment length of 5cm through two dose calculation approaches: (1) assuming all the environment as water (i.e., Dw,w-MC & Dw,w-TG43 ) and (2) accounting for the heterogeneity of VC applicators (i.e., Dw,w-App-MC & Dw,w-App-MBDC ). The compared isodose lines, and dose & energy difference maps were extracted for analysis. In addition, the dose difference on the peripheral surface, along the applicator and at middle of treatment length, as well as apical tip was evaluated. The Dw,w-App-MC results indicated that the VC heterogeneity can cause a dose reduction of (up to) % 6.8 on average (for all sizes) on the peripheral surface, translating to 1mm shrinkage of the isodose lines compared to Dw,w-MC . In addition, the results denoted that BV overestimates the dose on the peripheral surface and apical tip of about 3.7% and 17.9%, respectively, (i.e., Dw,w-App-MBDC vs Dw,w-App-MC ) when prescribing to the surface. However, the difference between the two were negligible at the prescription point when prescribing to 5mm beyond the surface. The VCs' heterogeneity could cause dose reduction when prescribing dose to the surface of the applicator, and hence increases the level of uncertainty. Thus, reviewing the TG43 results, in addition to ACUROS, becomes prudent, when evaluating the surface coverage at the apex.

Direction Modulated Brachytherapy Tandem Model Applicators for Treatment Planning of Multi-Institutional Cervical Cancer Cases: Removing Needles in Intracavitary-Interstitial Techniques

To evaluate the potential of nine direction modulated brachytherapy (DMBT) tandem applicator models of various designs to obviate the need for needles during intracavitary-interstitial (IC-IS) cervical cancer brachytherapy. A cohort of 33 retrospective clinical high dose-rate (HDR) brachytherapy plans, from three institutions, were re-planned with Varian's BrachyVision® (v16.1) treatment planning system (BV-TPS), using the latest VEGO® inverse optimization algorithm, with dose heterogeneity accounted for through the AcurosBV® model-based dose calculation algorithm. All plans consisted of IC-IS cases, with a range of 2-4 freehand-loaded needles, with an average prescription dose of 706±54 cGy and average high-risk clinical target volume (HRCTV) of 36.0±17.4 [range 9.8-69.6] cm3. Nine novel DMBT tandem models of varying physical dimensions were integrated for the first time into the BV-TPS, with thicknesses (4-8 mm). During re-planning, the conventional tandems and all of the needles were replaced by one of the nine DMBT tandem models while leaving the ovoids/rings in place. An optimization process was performed such that the lowest possible organs at risk (OAR) D2cc doses could be achieved while keeping equivalent target coverage (ΔHRCTV-D90 to within ±0.5%) and maintaining a pear-shape dose distribution. The process was repeated for each of the nine DMBT tandem models resulting in (33 × 9 =) 297 plans. Average ΔHRCTV-D90 was +0.35±0.39% (+2.8±3.1 cGy). OAR D2cc reductions were achieved by all models for all plans. The performance of the thickest DMBT model (8 mm) was the best in terms of achieving the lowest D2cc for all OARs, with 31 out of 33 plans (94%) achieving lower D2cc doses for all three OARs. The two cases in which the D2cc doses could not be reduced had HRCTV volumes ranging between 50 cm3 and 60 cm3. Additionally, D2cc reductions in terms of EQD2 [Gy] were calculated assuming each re-plan was delivered throughout the entire course of a patient's treatment, which included the external beam radiotherapy dose of 45 Gy, showed significant reductions of -2.64±2.67 Gy, -1.65±1.97 Gy, and -2.80±2.20 Gy for bladder, rectum, and sigmoid, respectively. According to the results, it is clinically feasible to replace the conventional IC-IS cases, with 2-4 freehand-loaded needles, with the DMBT tandem technology, effectively avoiding the need for needle involvement.

Generation and comparison of 3D dosimetric reference datasets for COMS eye plaque brachytherapy using model-based dose calculations.

A joint Working Group of the American Association of Physicists in Medicine (AAPM), the European Society for Radiotherapy and Oncology (ESTRO), and the Australasian Brachytherapy Group (ABG) was created to aid in the transition from the AAPM TG-43 dose calculation formalism, the current standard, to model-based dose calculations. This work establishes the first test cases for low-energy photon-emitting brachytherapy using model-based dose calculation algorithms (MBDCAs). Five test cases are developed: (1) a single model 6711 125 I brachytherapy seed in water, 13 seeds (2) individually and (3) in combination in water, (4) the full Collaborative Ocular Melanoma Study (COMS) 16 mm eye plaque in water, and (5) the full plaque in a realistic eye phantom. Calculations are done with four Monte Carlo (MC) codes and a research version of a commercial treatment planning system (TPS). For all test cases, local agreement of MC codes was within ∼2.5% and global agreement was ∼2% (4% for test case 5). MC agreement was within expected uncertainties. Local agreement of TPS with MC was within 5% for test case 1 and ∼20% for test cases 4 and 5, and global agreement was within 0.4% for test case 1 and 10% for test cases 4 and 5. Dose distributions for each set of MC and TPS calculations are available online (https://doi.org/10.52519/00005) along with input files and all other information necessary to repeat the calculations. These data can be used to support commissioning of MBDCAs for low-energy brachytherapy as recommended by TGs 186 and 221 and AAPM Report 372. This work additionally lays out a sample framework for the development of test cases that can be extended to other applications beyond eye plaque brachytherapy.

AAPMWGDCAB Report 372:A joint AAPM, ESTRO, ABG, and ABS report oncommissioning of model-based dose calculation algorithms in brachytherapy.

The introduction of model-based dose calculation algorithms (MBDCAs) in brachytherapy provides an opportunity for a more accurate dose calculation and opens the possibility for novel, innovative treatment modalities. The joint AAPM, ESTRO, and ABG Task Group 186 (TG-186) report provided guidance to early adopters. However, the commissioning aspect of these algorithms was described only in general terms with no quantitative goals. This report, from the Working Group on Model-Based Dose Calculation Algorithms in Brachytherapy, introduced a field-tested approach to MBDCA commissioning. It is based on a set of well-characterized test cases for which reference Monte Carlo (MC) and vendor-specific MBDCA dose distributions are available in a Digital Imaging and Communications in Medicine-Radiotherapy (DICOM-RT) format to the clinical users. The key elements of the TG-186 commissioning workflow are now described in detail, and quantitative goals are provided. This approach leverages the well-known Brachytherapy Source Registry jointly managed by the AAPM and the Imaging and Radiation Oncology Core (IROC) Houston Quality Assurance Center (with associated links at ESTRO) to provide open access to test cases as well as step-by-step user guides. While the current report is limited to the two most widely commercially available MBDCAs and only for 192 Ir-based afterloading brachytherapy at this time, this report establishes a general framework that can easily be extended to other brachytherapy MBDCAs and brachytherapy sources. The AAPM, ESTRO, ABG, and ABS recommend that clinical medical physicists implement the workflow presented in this report to validate both the basic and the advanced dose calculation features of their commercial MBDCAs. Recommendations are also given to vendors to integrate advanced analysis tools into their brachytherapy treatment planning system to facilitate extensive dose comparisons. The use of the test cases for research and educational purposes is further encouraged.

A MC-based anthropomorphic test case for commissioning model-based dose calculation in interstitial breast 192-Ir HDR brachytherapy.

To provide the first clinical test case for commissioning of 192 Ir brachytherapy model-based dose calculation algorithms (MBDCAs) according to the AAPM TG-186 report workflow. A computational patient phantom model was generated from a clinical multi-catheter 192 Ir HDR breast brachytherapy case. Regions of interest (ROIs) were contoured and digitized on the patient CT images and the model was written to a series of DICOM CT images using MATLAB. The model was imported into two commercial treatment planning systems (TPSs) currently incorporating an MBDCA. Identical treatment plans were prepared using a generic 192 Ir HDR source and the TG-43-based algorithm of each TPS. This was followed by dose to medium in medium calculations using the MBDCA option of each TPS. Monte Carlo (MC) simulation was performed in the model using three different codes and information parsed from the treatment plan exported in DICOM radiation therapy (RT) format. Results were found to agree within statistical uncertainty and the dataset with the lowest uncertainty was assigned as the reference MC dose distribution. The dataset is available online at http://irochouston.mdanderson.org/rpc/BrachySeeds/BrachySeeds/index.html,https://doi.org/10.52519/00005. Files include the treatment plan for each TPS in DICOM RT format, reference MC dose data in RT Dose format, as well as a guide for database users and all files necessary to repeat the MC simulations. The dataset facilitates the commissioning of brachytherapy MBDCAs using TPS embedded tools and establishes a methodology for the development of future clinical test cases. It is also useful to non-MBDCA adopters for intercomparing MBDCAs and exploring their benefits and limitations, as well as to brachytherapy researchers in need of a dosimetric and/or a DICOM RT information parsing benchmark. Limitations include specificity in terms of radionuclide, source model, clinical scenario, and MBDCA version used for its preparation.

DICOM CT-based Monte Carlo dose calculations for prostate permanent implant using 125I seed sources

Dose distributions are calculated for a clinical prostate implant consisting of multiple 125I sources using Monte Carlo-based egs_brachy code. A voxelised heterogeneous CT phantom, generated from DICOM CT images is used. Dose distributions vary significantly with scoring and transport media. About 3% interseed attenuation and 12% dose differences between prostate tissue and water are observed at D90. Monte Carlo-calculated dose values are smaller as compared to the TG-43 formalism-based TPS-generated dose values at a given location. Hence, patient-specific model-based dose calculation algorithm is preferred over TG-43 formalism-based TPS.

Modern Tools for Modern Brachytherapy

This review aims to showcase the brachytherapy tools and technologies that have emerged during the last 10 years. Soft-tissue contrast using magnetic resonance and ultrasound imaging has seen enormous growth in use to plan all forms of brachytherapy. The era of image-guided brachytherapy has encouraged the development of advanced applicators and given rise to the growth of individualised 3D printing to achieve reproducible and predictable implants. These advances increase the quality of implants to better direct radiation to target volumes while sparing normal tissue. Applicator reconstruction has moved beyond manual digitising, to drag and drop of three-dimensional applicator models with embedded pre-defined source pathways, ready for auto-recognition and automation. The simplified TG-43 dose calculation formalism directly linked to reference air kerma rate of high-energy sources in the medium water remains clinically robust. Model-based dose calculation algorithms accounting for tissue heterogeneity and applicator material will advance the field of brachytherapy dosimetry to become more clinically accurate. Improved dose-optimising toolkits contribute to the real-time and adaptive planning portfolio that harmonises and expedites the entire image-guided brachytherapy process. Traditional planning strategies remain relevant to validate emerging technologies and should continue to be incorporated in practice, particularly for cervical cancer. Overall, technological developments need commissioning and validation to make the best use of the advanced features by understanding their strengths and limitations. Brachytherapy has become high-tech and modern by respecting tradition and remaining accessible to all.

PO-2006 Evaluation of a Model Based Dose Calculation Algorithm for skin interventional radiation therapy

PO-2006 Evaluation of a Model Based Dose Calculation Algorithm for skin interventional radiation therapy

Dosimetric impact of applying a model-based dose calculation algorithm for skin cancer brachytherapy (interventional radiotherapy).

Brachytherapy (BT, interventional radiotherapy) is a well-established radiotherapy technique capable of delivering high doses to tumors while sparing organs at risk (OARs). Currently, the clinically accepted dose calculation algorithm used is TG-43. In the TG-186 report, new model-based dose calculation algorithms (MBDCA), such as Elekta's advanced collapsed cone engine (ACE), have been introduced, although their clinical application is yet to be fully realized. This study aimed to investigate two aspects of TG-186: firstly, a comparison of dose distributions calculated with TG-43 and TG-186 for skin tumors; and secondly, an exploration of the impact of using a water bolus on the coverage of clinical target volume (CTV) and OARs. Ten treatment plans for high-dose-rate IRT were developed. All plans were initially calculated using the TG-43 algorithm, and were subsequently re-calculated with TG-186. In addition, one of the treatment plans was assessed with both TG-43 and TG-186, using 10 different water bolus thicknesses ranging from 0 to 5 cm. To assess dose variations, the following dose-volume histogram (DVH) parameters were compared: D2cc and D0.01cc for OARs, and V150, V100, V95 and V90 for CTV coverage. The average dosimetric results for CTV and OARs, as calculated by both algorithms, revealed statistically significant lower values for TG-186 when compared with TG-43. The presence of a bolus was observed to enhance CTV coverage for the TG-186 algorithm, with a bolus thickness of 2 cm being the point at which ACE calculations matched those of TG-43. This study identified significant differences in dosimetric parameters for skin tumors when comparing the TG-43 and TG-186 algorithms. Moreover, it was demonstrated that the inclusion of a water bolus increased CTV coverage in TG-186 calculations.

A versatile physical phantom design and construction for I-125 dose measurements and dose-to-medium determination

In this paper we present a phantom designed to provide conditions to generate set of "true" independent reference data as requested by TG-186, and mitigating the scarcity of experimental studies on brachytherapy validation. It was used to perform accurate experimental measurements of dose of 125I brachytherapy seeds using LiF dosimeters, with the objective of experimentally validating Monte Carlo (MC) calculations with model-based dose calculation algorithm (MBDCA). In addition, this work intends to evaluate a methodology to convert the experimental values from LiF into dose in the medium. The proposed PMMA physical phantom features cavities to insert a LiF dosimeter and a 125I seed, adjusted in different configurations with variable thickness. Monte Carlo calculations performed with MCNP6.2 code were used to score the absorbed dose in the LiF and the dose conversion parameters. A sensitivity analysis was done to verify the source of possible uncertainties and quantify their impact on the results. The proposed phantom and experimental procedure developed in this work provided precise dose data within 5.68% uncertainty (k = 1). The achieved precision made it possible to convert the LiF responses into absorbed dose to medium and to validate the dose conversion factor methodology. The proposed phantom is simple both in design and as in its composition, thus achieving the demanded precision in dose evaluations due to its easy reproducibility of experimental setup. The results derived from the phantom measurements support the dose conversion methodology. The phantom and the experimental procedure developed here can be applied for other materials and radiation sources.

PO26 Presentation Time: 7:40 AM: Dosimetry Accuracy of a Model-Based Dose Calculation Algorithm in Modeling a Novel Direction Modulated Brachytherapy Tandem Applicator for the Cervical Cancer

<h3>Purpose</h3> A novel shielded tandem applicator for the direction modulated brachytherapy (DMBT) was previously introduced, which showed capability to improve the coverage of nonuniform targets and organs at risk in cervical cancer. This applicator has been recently modeled in a commercial treatment planning system (TPS - BrachyVision ACUROS^TM (BVA)), a Model based Dose Calculation algorithm (MBDCA), for the clinical research. In this study, we aim to benchmark the accuracy of the TPS in modeling the DMBT tandem using Monte Carlo (MC) simulations. <h3>Materials and Methods</h3> The DMBT tandem applicator, made of an MR compatible tungsten alloy with six symmetrical grooves covered by a sheath of PEEK plastic, was simulated in GEANT4 MC code. Subsequently, two scenarios were created using the BVA TPS and reproduced by GEANT4 MC simulations, including ‘source at the center of the water phantom' and ‘source at the middle of the applicator' for the cubical phantoms with dimensions of (20 cm)³, (30 cm)³, and (40 cm)³. A track length estimator (TLE) was then utilized to estimate the dose deposited inside the water phantoms and 2D/3D scoring were performed. Volumetric dose comparisons of the TPS against MC results were further implemented. <h3>Results</h3> There were different level of agreements between the TPS and MC results for the different phantom sizes. The best agreement, however, was achieved for the phantom (40 cm)³, such that the dose difference fell in the intervals [0% 8.5%] and [-6.5% 6.5%] in 95% of the voxels, for the ‘source at the center of the water phantom' and ‘source at the middle of the applicator', respectively. In addition, the dose difference was within for all phantoms, within 5 cm from the center of the HDR sources. <h3>Conclusions</h3> The results of this study indicated that the accuracy of the TPS in dose calculation depends on the phantom size. Nevertheless, it seems the TPS could still provide satisfactory accuracy inside the area of clinical importance.

PHSOR08 Presentation Time: 1:05 PM: Retrospective Comparison of TG43 vs. AcurosBV Model-Based Dose Calculation Algorithm (MBDCA) in Cervical Cancer Patients Treated with HDR Brachytherapy Boost

<h3>Purpose</h3> To evaluate dosimetric impact of heterogeneity from applicators and patient tissue between plans treated with tandem and ovoid applicators with interstitial needles calculated with the TG43 formalism and Acuros®BV MBDCA (Varian Medical Systems (VMS), Inc). <h3>Materials Methods</h3> For this retrospective analysis, HDR plans prescribing 5.5 Gy to high-risk clinical target volume (CTV) D_90% for locally advanced cervical cancer patients were selected. The implants consisted of tandem and ovoid (T&O) with or without interstitial needles, treated using a GammaMedPlus™ iX afterloader with a ¹⁹²Ir source. Implants were stabilized by Alatus vaginal balloons (Angiodynamics) filled with a 5% IsoVue to 95% saline ratio. Imaging for applicator digitization and planning was done using either Acuity CBCT (VMS, Inc), or AIRO CT (BrainLab AG). For standard TG43 clinical dose calculation, all applicators were manually digitized on the CT or CBCT images. Solid renderings of Fletcher-Suit Delclos applicators included in BrachyVision (VMS, Inc) were matched to those used clinically. The composition of the applicators was assigned from the applicator library. Titanium needles were left unchanged, as there is no solid applicator model available. The dose was recalculated using Varian's Acuros®BV 13.5 and dose-to-medium-in-medium (D_m,m) was reported. Equivalent dose in 2 Gy per fraction (EQD2) values for targets and organs at risk (OARs) were compared between TG43 formalism and the Acuros®BV plans. D_90% and D_98% values were reported for the high and intermediate-risk CTVs, and D_2cm3 values were reported for OARs including bladder, rectum, sigmoid, bowel, and vagina. In a separate analysis, we compared dosimetric endpoint differences between TG43 and Acuros®BV plans imaged with CT versus CBCT, and also between plans with T&O applicators with and without needles. Linear regression and Bland Altman Analysis (BAA) were performed between the two plans to show the correlation and dosimetric differences between both methods. <h3>Results</h3> Thirty-two patients with a total of 156 fractions were included in this study, 70 planned on CBCT, and 86 planned on AIRO CT. Consistently, Acuros®BV plans showed lower dosimetric values for all plans compared to TG43. The average ± standard deviation of dosimetric reduction in D_90% was 4.33 ± 0.09% (0.41 ± 0.13 Gy) for CTV_HR and 4.12 ± 0.09% (0.21 ± 0.07Gy) for CTV_IR . Fig.1 shows an example BAA plot for differences between TG43 and Acuros®BV D_90% for the targets. The reduction to OARs D_2cm3 was: 4.99 ± 0.15% (0.34 ± 0.17 Gy) for bladder, 7.87 ± 0.16% (0.20 ± 0.09 Gy) for rectum, 5.79 ± 0.17% (0.21 ± 0.09 Gy) for sigmoid, 6.91 ± 0.14% (0.25 ± 0.13 Gy) for bowel, and 4.55 ± 0.14% (0.46 ± 0.27 Gy) for vagina. There was no statistically significant difference between TG43 and Acuros®BV plans calculated on CBCT versus CT images (p value = 0.24). A set of 5 patients with a total of 24 fractions planed with T&O applicators without needles was used for comparison with plans with interstitial needles, and no statistically significant difference between TG43 and Acuros®BV plans was observed (p value = 0.35). <h3>Conclusion</h3> This large patient cohort study confirms that plans calculated using Acuros®BV MBDCA show a reduction in dose to all structures when compared to the standard TG43 formalism. This is expected mainly due to more photon attenuation by high-density material of the applicators, as well as tissue heterogeneities. Clinical implementation of Acuros®BV can be challenging due to increased calculation time, alteration of the clinical flow, and increased planning complexity. These factors must be evaluated, and further investigation should be conducted in order to understand the effects of its utilization as a primary dose calculation method.

PO28 Presentation Time: 7:40 AM: The Effect of Vaginal Cylinder Inhomogeneity on the HDR Brachytherapy Dose Calculations Using Monte Carlo Simulations and a Commercial Model-Based Dose Calculation Algorithm

<h3>Purpose</h3> Some of the commercial vaginal cylinder (VC) applicators are made of high-density plastic materials. When an HDR source is placed inside the applicator's lumen(s), these applicators create a heterogeneous environment around the sources that could affect the dose distribution, contrary to the TG43 formalism. In addition, the prescription/normalization point, popularly placed either on the surface or 5 mm beyond the applicator's surface, can intensify this effect. This study utilizes Monte Carlo (MC) simulations to assess this heterogeneity effect and benchmarks the accuracy of a commercial model-based dose calculation algorithm (MBDCA) against the MC simulation results. <h3>Materials and Methods</h3> The GEANT4 MC code was used to simulate a commercial ¹⁹²Ir HDR source and a commercial VC with diameters ranging from 20-35 mm inside a virtual water phantom. Standard plans were generated from a commercial treatment planning system (TPS - BrachyVision ACUROS^TM (BVA)) optimized through two dose calculation approaches: (1) TG43 protocol assuming all environment as water and (2) MC & BVA approaches accounting for the heterogeneity of VC applicators. The dose and energy deposited profiles were extracted for analysis. <h3>Results</h3> The MC simulation results showed that the VC inhomogeneity can cause shrinkage of the isodose lines by up to 1 mm at the lateral surfaces compared to TG43, leading to the reduction of the dose when prescribing to the surface. The BVA was not sensitive to show the effect, though. In addition, the BVA overestimated the dose on the surface of about 5.0% and 8.5% at the periphery and the apex, respectively, when prescribing to the surface. However, the difference between the BVA and MC simulations were negligible at the prescription point when prescribing to 5 mm beyond the surface. <h3>Conclusions</h3> The uncertainty due to the VC applicators on dose calculations depends on the VC material/design, and the prescription point location. The incident of the airgaps around the applicators can add to the complexity, which needs further study.

Accuracy of Acuros^{text {TM}} BV as determined from GATE monte-carlo simulation

The American Association of Physicists in Medicine’s Task Group No.43 has provided a standardised dose calculation methodology that is now the international benchmark for all brachytherapy dosimetry publications and treatment planning systems. However, limitations in this methodology has seen the development of Model-Based Dose Calculation Algorithms (MBDCA). In 2009, Varian (Varian Medical Systems, Palo Alto, CA, USA) released Acuros^{text {TM}} BrachyVision (ABV) which calculates dose by explicitly solving the Linear Boltzmann Transport Equation. In this study we have assessed the accuracy of ABV dose calculations within a range of materials relevant to high dose rate brachytherapy with an iridium-192 (^{text {192}}Ir) source. Accuracy assessment has been achieved by implementing a modelled GamaMed Plus ^{text {192}}Ir source within a series of phantoms using the GEANT4 Application for Emission Tomography (GATE) to calculate dose for comparison with dose as determined by ABV. Comparisons between GATE and ABV were made using point-to-point profile comparisons and 1D gamma analysis. Source validation results yielded good agreement with published data. Spectrum and TG43U1 comparisons showed no major differences, with TG43U1 comparisons agreeing within ± 1%. Point-to-point comparisons showed large differences between GATE and ABV near the source and in low density materials. 1D gamma analysis pass criteria of 2%/1 mm and 2%/2 mm yielded passing rates ranging between 51.72–100% and 62.07–100% respectively. A critical analysis of this study’s results suggest that ABV is unable to accurately calculate doses in low density materials. Furthermore, spatial accuracy of dose near the source is within 2 mm.

Dosimetric study of Varian Universal Multi-Channel Cylinder System for High-Dose-Rate 192Ir Brachytherapy

Recently, the Varian multichannel vaginal cylinder (MCVC) set for high-dose-rate 192Ir brachytherapy was commercially released. This MCVC was distinct from our existing MCVC in its peripheral channel layout and tip design. This investigation sought to assess the dosimetric impact of these changes. The dimensions of the virtual model for each applicator were compared against both physical and radiographic measurements. Volumetric dose distributions were generated in silico using a model-based dose calculation algorithm (MBDCA). To characterize the effects of the new peripheral channel layout on dose to adjacent areas ("dose-spill"), point doses were compared using two sets of applicator-based reference points: at surface or 5 mm radially from surface. To evaluate the dose-shaping capabilities, a dose distribution was generated for the new applicator and assessed against a representative dose distribution for a patient previously treated with existing equipment. Based on both physical and radiographic measurements, virtual models were representative of each applicator within ±1 mm. Commissioning of the MBDCA was benchmarked based on AAPM Working Group on Dose Calculation Algorithms in Brachytherapy. The layout of the new applicator reduced dose-spill to other reference points significantly, as much as a factor of 16.3, compared with the existing equipment. The rounded tip shape and curve of the peripheral channels in the new applicator produced more conformity to its HR-CTV than existing equipment. Compared with our existing equipment, the design changes in the new Varian MCVC set offered improved control of dose spill and better conformality to HR-CTV.

Effect of a lead block on alveolar bone protection in image-guided high-dose-rate interstitial brachytherapy for tongue cancer: using model-based dose calculation algorithms to correct for inhomogeneity.

PurposeThe purpose of this study was to evaluate the effect of a lead block for alveolar bone protection in image-guided high-dose-rate interstitial brachytherapy for tongue cancer.Material and methodsWe treated 6 patients and delivered 5,400 cGy in 9 fractions using a lead block. Effects of lead block (median thickness, 4 mm) on dose attenuation by distance were visually examined using TG-43 formalism-based dose distribution curves to determine whether or not the area with the highest dose is located in the alveolar bone, where there is a high-risk of infection. Dose re-calculations were performed using TG-186 formalism with advanced collapsed cone engine (ACE) for inhomogeneity correction set to cortical bone density for the whole mandible and alveolar bone, water density for clinical target volume (CTV), air density for outside body and lead density, and silastic density for lead block and its’ silicon replica, respectively.ResultsThe highest dose was detected outside the alveolar bone in five of the six cases. For dose-volume histogram analysis, median minimum doses delivered per fraction to the 0.1 cm3 of alveolar bone (D0.1cm3TG-43, ACE-silicon, and ACE-lead) were 344.3 (range, 262.9-427.4) cGy, 336.6 (253.3-425.0) cGy, and 169.7 (114.9-233.3) cGy, respectively. D0.1cm3ACE-lead was significantly lower than other parameters. No significant difference was observed between CTV-related parameters.ConclusionsThe results suggested that using a lead block for alveolar bone protection with a thickness of about 4 mm, can shift the highest dose area to non-alveolar regions. In addition, it reduced D0.1cm3 of alveolar bone to about half, without affecting tumor dose.

P76 - The dosimetric impact of applying a model-based dose calculation algorithm for non-melanoma skin cancer interventional radiotherapy

P76 - The dosimetric impact of applying a model-based dose calculation algorithm for non-melanoma skin cancer interventional radiotherapy

Technical note: Extraction of proton pencil beam energy spectrum from measured integral depth dose in a cyclotron proton beam system.

Proton pencil beam energy spectrum is an essential parameter for calculations of dose and linear energy transfer. We extract the energy spectrum from measured integral depth dose (IDD). A measured IDD (measIDD) in water is decomposed into many IDDs of mono-energetic pencil beams (monoIDDs) in water. A simultaneous iterative technique is used to do the decomposition that extracts the energy spectrum of protons from the measIDD. The monoIDDs are generated by our analytic random walk model-based proton dose calculation algorithm. The linear independence of the monoIDDs is considered and high spatial resolution monoIDDs are used to improve their linear independence. To validate the extraction, first we use synthesized IDDs (synIDD) with Gaussian energy spectrum and compare the extracted energy spectrum with the Gaussian; second, for the energy spectrum extracted from measIDDs, the accuracy of the extraction is validated by comparing the calculated IDD from the energy spectrum with the measIDD. The measIDDs are derived from commissioning of a cyclotron proton pencil beam system with a Bragg peak ionization chamber. The nominal energy of the pencil beams is from 70 to 245MeV. The monoIDDs are generated for energies from 0.05 to 275MeV in steps of 0.05MeV with a spatial resolution of 1mm. The difference of the extracted and original Gaussian energy spectrum peaked at 75 and 80MeV was <1%. As the energy decreased, the difference increased but was reduced by using 0.1-mm monoIDDs. The difference was not sensitive to the energy interval of monoIDDs when the interval increased from 0.05 to 1MeV. For the energy spectrum extraction from measIDDs, there was a main peak near the nominal energy but the spectrum was not in Gaussian distribution. In three example cases (70, 160, and 245MeV), the relative differences of the measIDDs and calculated IDDs were within 3.4%, 2.9%, and 4.7% of the Bragg peak value, respectively. Fitting the spectrum by Gaussian distribution, we had σ=0.87, 1.51, and 0.86MeV, respectively, for these three examples, and the relative differences of the resultant calculated IDDs from the measIDDs were within 4.7%, 7.4%, and 4.5%, respectively. Our algorithm accurately extracted the energy spectrum from the synIDDs and measIDDs. High-resolution monoIDDs are necessary to extract low-energy spectrum. Energy spectrum extraction from measIDD reveals important information for beam modeling.

SP-0372 Advancement of model-based dose calculation algorithms in brachytherapy and their clinical impact

SP-0372 Advancement of model-based dose calculation algorithms in brachytherapy and their clinical impact