SU‐E‐T‐509: DICOM Test Case Plans for Model‐Based Dose Calculations Methods in Brachytherapy

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

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.

Supplementary Materials referenced in the text containing dosimetric comparisons for all test cases were not previously published alongside the paper and are attached here. Single seed in water phantom 13 seeds in water phantom, no plaque, no interseed attenuation (TG-43) 13 seeds in water phantom, no plaque, with interseed attenuation Full plaque in water phantom Full plaque in realistic eye phantom. Calculations for each test case were performed using three Monte Carlo (MC) codes: egs_brachy (based on EGSnrc), MCNP, and TOPAS (based on Geant4). Calculations for test cases 1, 4, and 5 are also performed using a research version of the OncentraBrachy ACE treatment planning system. The figures presented here are summarized in Table VII of the original paper. They demonstrate that overall local agreement ( % Δ D l o c a l $\% \Delta {D}_{local}$ ) between MC codes is approximately 2.5% and local agreement ( % Δ D l o c a l $\% \Delta {D}_{local}$ ) between ACE and MC is approximately 5% (test case 1) to 20% (test cases 4 and 5); global agreement ( % Δ D g l o b a l $\% \Delta {D}_{global}$ ) between MC codes is approximately 2% (4% for test case 5) and global agreement between ACE and MC is approximately 0.4% (test case 1) to 10% (test cases 4 and 5). Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Relationships between macroscopic (bulk tissue) and microscopic (cellular) dose descriptors are investigated using cavity theory and Monte Carlo (MC) simulations. Small, large, and multiple intermediate cavity theory (SCT, LCT, and ICT, respectively) approaches are considered for 20 to 370 keV incident photons; ICT is a sum of SCT and LCT contributions weighted by parameter d. Considering μm-sized cavities of water in bulk tissue phantoms, different cavity theory approaches are evaluated via comparison of (where Dw,m is dose-to-water-in-medium and Dm,m is dose-to-medium-in-medium) with MC results. The best overall agreement is achieved with an ICT approach in which , where L is the mean chord length of the cavity and β is given by (RCSDA is the continuous slowing down approximation range of an electron of energy equal to that of incident photons). Cell nucleus doses, Dnuc, computed with this ICT approach are compared with those from MC simulations involving multicellular soft tissue models considering a representative range of cell/nucleus sizes and elemental compositions. In of cases, ICT and MC predictions agree within ; disagreement is at most 8.8%. These results suggest that cavity theory may be useful for linking doses from model-based dose calculation algorithms (MBDCAs) with energy deposition in cellular targets. Finally, based on the suggestion that clusters of water molecules associated with DNA are important radiobiological targets, two approaches for estimating dose-to-water by application of SCT to MC results for Dm,m or Dnuc are compared. Results for these two estimates differ by up to , demonstrating the sensitivity of energy deposition within a small volume of water in nucleus to the geometry and composition of its surroundings. In terms of the debate over the dose specification medium for MBDCAs, these results do not support conversion of Dm,m to Dw,m using SCT.

There is an acknowledged need for the design and implementation of physical phantoms appropriate for the experimental validation of model-based dose calculation algorithms (MBDCA) introduced recently in 192Ir brachytherapy treatment planning systems (TPS), and this work investigates whether it can be met. A PMMA phantom was prepared to accommodate material inhomogeneities (air and Teflon), four plastic brachytherapy catheters, as well as 84 LiF TLD dosimeters (MTS-100M 1 × 1 × 1 mm3 microcubes), two radiochromic films (Gafchromic EBT3) and a plastic 3D dosimeter (PRESAGE). An irradiation plan consisting of 53 source dwell positions was prepared on phantom CT images using a commercially available TPS and taking into account the calibration dose range of each detector. Irradiation was performed using an 192Ir high dose rate (HDR) source. Dose to medium in medium, , was calculated using the MBDCA option of the same TPS as well as Monte Carlo (MC) simulation with the MCNP code and a benchmarked methodology. Measured and calculated dose distributions were spatially registered and compared. The total standard (k = 1) spatial uncertainties for TLD, film and PRESAGE were: 0.71, 1.58 and 2.55 mm. Corresponding percentage total dosimetric uncertainties were: 5.4–6.4, 2.5–6.4 and 4.85, owing mainly to the absorbed dose sensitivity correction and the relative energy dependence correction (position dependent) for TLD, the film sensitivity calibration (dose dependent) and the dependencies of PRESAGE sensitivity. Results imply a LiF over-response due to a relative intrinsic energy dependence between 192Ir and megavoltage calibration energies, and a dose rate dependence of PRESAGE sensitivity at low dose rates (<1 Gy min−1). Calculations were experimentally validated within uncertainties except for MBDCA results for points in the phantom periphery and dose levels <20%. Experimental MBDCA validation is laborious, yet feasible. Further work is required for the full characterization of dosimeter response for 192Ir and the reduction of experimental uncertainties.

Objective To investigate the impact of metal plate on radiation dose distribution by Monte Carlo (MC) code. Methods The metal plates with 0. 4 thick were placed in water at 5 cm, all the plate irradiated with 6 MV X-ray were simulated by MC code, SSD =100 cm. The percentage depth dose with or without metal implants were compared. Results The surface absorbed doses on incident plane of stainless steel plate and titanium plate were increased by 19. 6％ and 15.7％ respectively as compared water,the dose influence was less than 1.5％ more than 0. 3 cm outside the incidence plane. The doses on the exit surface of stainless steel plate and titanium plate were lowered by 8. 6％ and 8. 2％ when compared with water, the dose impacts of this places where were from the exit surface of stainless steel plate 、titanium plate more than 1.2 cm,0. 9 cm were less than 1.5％. The surface absorbed doses on incident plane of stainless steel plate were increased by 3.9％ respectively as titanium plate, the dose of the exit surface of stainless steel plate and titanium plate was similar. Conclusions MC method is a fast and accurate calculation method. The influence of metal plate on the radiotherapy dose distribution is significant. Under the such condition, the impact of stainless steel plate is much more than that of titanium alloy plate. Key words: Monte Carlo; Radiation dose; Metal implants

TPSDose43: A new TG-43 based dose calculation code for brachytherapy

Clinical transition to model-based dose calculation algorithm: A retrospective analysis of high-dose-rate tandem and ring brachytherapy of the cervix

Traditional engineering-based core analyses generally rely on a two-stage approach, involving a transport code and a diffusion (e.g. nodal) solver. The accuracy and fidelity of the core solution strongly depend on the generation of few-group constants, which is a topic that has been extensively studied. However, the practice of calculating group constants with Monte Carlo (MC) codes is still relatively new. The main challenge of this approach is the generation of diffusion coefficients. A common approach adopted by many MC codes is the flux separability approximation (FSA). However, diffusion coefficients generated with FSA have trouble matching the heterogenous solution. This is due to a strong coupling between the energetic, spatial, and angular distributions of neutrons. This is a well-documented challenge, with some literature proposing consistent methods to handle energy-angle dependence and others proposing consistent methods to handle energy-space-angle dependence. Recently, the Cumulative Migration Method has been used to generate consistent diffusion coefficients using MC. However, these diffusion coefficients are valid only for infinite single region problems. As such, it is important to revisit the consistent group constant generation method to better handle the energy-space-angle dependence using MC. This paper will lay out the method to generate these consistent group constants using MC and then verify that they can then be used for a two-stage approach using a steady-state nodal expansion method.

Purpose:To describe a new infrastructure and process that will enable end‐users to commission model‐based dose calculation (MBDC) software for brachytherapy planning, and to invite end‐users to participate in the beta testing phase of this process.Methods:The AAPM working groups on model‐based dose calculation algorithms in brachytherapy (WGDCAB) and brachytherapy source registry (WGBSR) collaboratively implemented the basic MBDC software commissioning workflow described in the AAPM+ESTRO TG‐186 report. For a small number of initial test cases virtual CT image, anatomical structure set, treatment plan, and associated 3D dose data for a single dwell position occupied by a WGDCAB generic high‐dose‐rate Ir‐192 virtual source, with or without a generic shielded applicator, were generated. Dose distributions were calculated using Monte Carlo (MC) code MCNP6, Elekta's Advanced Collapsed cone Algorithm (ACE™), and Varian's Acuros™ BV module. All data were saved in DICOM RT format and uploaded to a provisional, web‐accessible repository in the joint AAPM—IROC Houston brachytherapy source registry. It is envisioned that end‐users will download data for a test case from the repository, verify its correctness, calculate 3D dose locally, and then compare with MCNP6 and MBDC doses obtained from the repository using TPS‐based comparison tools. User guides in preparation for each TPS will facilitate the process.Results:Initial evaluations of DICOM data download, TPS import, MBDC dose calculation, and reference dose comparison indicate that the infrastructure and process described above are both viable and practicable. Ancillary data including generic source and shielded applicator specifications, CAD drawings, and MC code definitions are also available in the repository.Conclusion:With further refinement and beta‐test input from end‐users, a practical MBDC software commissioning process for brachytherapy can be established. A procedure for adding new test cases submitted by end‐users to the repository will be put in place by WGDCAB.

Purpose:The aim of this study was to investigate the impacts of tissue inhomogeneity on dose distributions using a three‐dimensional (3D) gamma analysis in cervical intracavitary brachytherapy using Monte Carlo (MC) simulations.Methods:MC simulations for comparison of dose calculations were performed in a water phantom and a series of CT images of a cervical cancer patient (stage: Ib; age: 27) by employing a MC code, Particle and Heavy Ion Transport Code System (PHIT) version 2.73. The 192Ir source was set at fifteen dwell positions, according to clinical practice, in an applicator consisting of a tandem and two ovoids. Dosimetric comparisons were performed for the dose distributions in the water phantom and CT images by using gamma index image and gamma pass rate (%). The gamma index is the minimum Euclidean distance between two 3D spatial dose distributions of the water phantom and CT images in a same space. The gamma pass rates (%) indicate the percentage of agreement points, which mean that two dose distributions are similar, within an acceptance criteria (3 mm/3%). The volumes of physical and clinical interests for the gamma analysis were a whole calculated volume and a region larger than t% of a dose (close to a target), respectively.Results:The gamma pass rates were 77.1% for a whole calculated volume and 92.1% for a region within 1% dose region. The differences of 7.7% to 22.9 % between two dose distributions in the water phantom and CT images were found around the applicator region and near the target.Conclusion:This work revealed the large difference on the dose distributions near the target in the presence of the tissue inhomogeneity. Therefore, the tissue inhomogeneity should be corrected in the dose calculation for clinical treatment.

BackgroundSeveral research groups have explored the potential of scandium radionuclides for theragnostic applications due to their longer half-lives and equal or similar coordination chemistry between their diagnostic and therapeutic counterparts, as well as lutetium-177 and terbium-161, respectively. Unlike the gallium-68/lutetium-177 pair, which may show different in-vivo uptake patterns, the use of scandium radioisotopes promises consistent behaviour between diagnostic and therapeutic radiopeptides. An advantage of scandium’s longer half-life over gallium-68 is the ability to study radiopeptide uptake over extended periods and its suitability for centralized production and distribution. However, concerns arise from scandium-44’s decay characteristics and scandium-43’s high production costs. This study aimed to evaluate the dosimetric implications of using scandium radioisotopes with somatostatin analogues against gallium-68 for PET imaging of neuroendocrine tumours.MethodsAbsorbed dose per injected activity (AD/IA) from the generated time-integrated activity curve (TIAC) were estimated using the radiopeptides [43/44/44mSc]Sc- and [68Ga]Ga-DOTATATE. The kidneys, liver, spleen, and red bone marrow (RBM) were selected for dose estimation studies. The EGSnrc and MCNP6.1 Monte Carlo (MC) codes were used with female (AF) and male (AM) ICRP phantoms. The results were compared to Olinda/EXM software, and the effective dose concentrations assessed, varying composition between the scandium radioisotopes.ResultsOur findings showed good agreement between the MC codes, with − 3 ± 8% mean difference. Kidneys, liver, and spleen showed differences between the MC codes (min and max) in a range of − 4% to 8%. This was observed for both phantoms for all radiopeptides used in the study. Compared to Olinda/EXM the largest observed difference was for the RBM, of 21% for the AF and 16% for the AM for scandium- and gallium-based radiopeptides. Despite the differences, our findings showed a higher absorbed dose on [43/44Sc]Sc-DOTATATE compared to its 68Ga-based counterpart.ConclusionThis study found that [43/44Sc]Sc-DOTATATE delivers a higher absorbed dose to organs at risk compared to [68Ga]Ga-DOTATATE, assuming equal distribution. This is due to the longer half-life of scandium radioisotopes compared to gallium-68. However, calculated doses are within acceptable ranges, making scandium radioisotopes a feasible replacement for gallium-68 in PET imaging, potentially offering enhanced diagnostic potential with later timepoint imaging.

This paper presents static and transient solutions for the pressurized water reactor (PWR) MOX/UO2 transient benchmark by Serpent 2 Monte Carlo (MC) code and open nodal core simulator called Abu Dhabi Polytechnic Reactor Simulator (ADPRES). The presence of mixed oxide (MOX) fuels and burn-up variation in the benchmark pose challenges for reactor simulators due to severe flux gradient across fuel assemblies. In this work, the two-step method was used, in which the assembly level two-group constants were generated from single assembly calculations with zero net current boundary conditions using Serpent 2 Monte Carlo code, and later the core calculation was performed using ADPRES open nodal core simulator. Two types of diffusion coefficients were generated: the conventional B1 leakage corrected and the cumulative migration method (CMM). Finally, the solutions of Serpent 2/ADPRES, including multiplication factor, power distribution, control rod (CR) worth, and critical boron concentration (CBC) using both diffusion coefficients, were compared against solutions from heterogeneous Serpent 2 calculations where the fuel and cladding are explicitly modeled. The reactor power during transients was also compared qualitatively against other nodal core simulators. The results showed that Serpent 2/ADPRES were able to predict the heterogeneous Monte Carlo solutions very well with reasonable differences. The transient solutions were also quite accurate compared to other nodal core simulators. For the diffusion coefficients comparison, it was found that the CMM diffusion coefficient provides more accurate solutions for the benchmark compared to the B1 leakage corrected diffusion coefficients.

Optimization treatment of point-wise nuclear data in Monte Carlo criticality and burnup calculations

The ARPANSA calibration service for (60)Co gamma rays is based on a primary standard graphite calorimeter that measures absorbed dose to graphite. Measurements with the calorimeter are converted to the absorbed dose to water using the calculation of the ratio of the absorbed dose in the calorimeter to the absorbed dose in a water phantom. ARPANSA has recently changed the basis of this calculation from a photon fluence scaling method to a direct Monte Carlo (MC) calculation. The MC conversion uses an EGSnrc model of the cobalt source that has been validated against water tank and graphite phantom measurements, a step that is required to quantify uncertainties in the underlying interaction coefficients in the MC code. A comparison with the Bureau International des Poids et Mesures (BIPM) as part of the key comparison BIPM.RI(I)-K4 showed an agreement of 0.9973 (53).