Investigation of TG-43 dose calculation algorithm for volumetric HDR Brachytherapy using Monte Carlo simulation in TOPAS.
Abstract
The accuracy of dose calculation algorithms significantly influences brachytherapy treatment outcomes. TG-43 algorithm has been widely adopted due to its simplicity, which has standardized contemporary brachytherapy dose calculations worldwide. However, TG-43's homogeneity assumption potentially compromises clinical treatment accuracy. The present study aimed to investigate TG-43 accuracy against Monte Carlo (MC) simulation. Varian GammaMed Plus 192Ir High-dose-rate (HDR) brachytherapy source was modeled using TOPAS3.9 MC simulation-framework. Three well-defined test scenarios were examined: Homogeneous water phantom, TG-186 generic-applicator, and computed-tomography (CT) based intracavitary brachytherapy (ICBT) patient plan. Corresponding TG-43 based treatment plans were generated using Eclipse BrachyVision and imported into TOPAS for MC-simulation. Both TG-43 and MC dose distributions were evaluated using point-dose assessment, dose-volume metrics, dose-volume histogram, and γ-analysis with 2% Dose-difference and 1 mm Distance-to-agreement. In a homogeneous water phantom, TG-43 and MC exhibited close agreement, with dose-difference (Δ%) at the reference point and for High-risk clinical target volume (HRCTV) found to be below 1-3% and γ-passing rates ≥95%. However, in the presence of a TG-186 applicator, TG-43 substantially overestimated doses by 60-93% on shielded side and 4-13% on unshielded side. For CT-based ICBT patient plan, TG-43 overestimated HRCTV and organ-at-risk (OAR) doses relative to MC. The mean γ-passing rate in clinical ICBT plan was 90%, indicating moderate spatial agreement. Present study validated the accuracy of TG-43 algorithm against MC simulation confirming its adequacy in homogeneous phantom. However, significant deviations were observed in the presence of an applicator, and tissue heterogeneity revealed TG-43 limitations. TG-43 consistently overestimates dose owing to the homogeneity assumption. These findings emphasize the necessity of adopting Model-based calculation algorithms such as MC to improve the dosimetric precision and treatment accuracy in HDR brachytherapy.