SU‐E‐T‐701: Guidance for Early Adoption of Model‐Based Dose Calculation Algorithms in Brachytherapy: Progress Report from AAPM Task Group 186

Purpose: The charge of Task Group 186 is to provide guidance to early adopters of model‐based dose calculation algorithms (MBDCA) for brachytherapydose calculations. Methods: Contrary to external beam radiotherapy(EBRT), the brachytherapy (BT) community has not widely adopted heterogeneity‐correction algorithms. Yet, BT dose calculation accuracy is highly dependent on scatter conditions and photoelectric effect cross‐sections relative to water. In specific situations, differences between the current water‐based BT dose calculation formalism (TG‐43) and MBDCA can lead to differences in dose calculation exceeding a factor of ten. MBDCA raise three major issues that are not addressed by current guidance documents: 1) MBDCA doses are sensitive to specification of dose medium, resulting in energy‐dependent dose differences between computing dose to water in homogeneous water geometry (TG‐43), dose to the local medium in the inhomogeneous geometry, and the intermediate scenario of computing dose to water in the inhomogeneous geometry. 2) MBDCA doses are sensitive to voxel‐by‐voxel interaction probabilities. Neither conventional single‐energy CT nor ICRU/ICRP tissue composition compilations provide useful guidance for the task. 3) Since each patient‐source‐applicator combination is unique, having reference data for each possible combination to benchmark MBDCA is an impractical strategy. Hence, a new commissioning process is required. Results: TG‐186 addresses in detail the above issues through literature review and provides explicit recommendations based on the current state of knowledge. These recommendations include voxel‐by‐voxel tissue/organ material assignments based on modern imaging devices, applicators and shields, transport medium definition, dose scoring medium effects, a proposed commissioning process, possible changes in BT dose prescriptions, and MBDCA limitations. When data are insufficient, interim recommendations are made and potential areas of research are identified. Conclusions: Application of TG‐186 guidance should retain practice uniformity in transitioning from the TG‐43 to the MBDCA approach.