Author: Mandar S. Bhagwat, Casey Y. Lee 👨🔬
Affiliation: Duke University, Massachusetts General Hospital and Harvard Medical School 🌍
Purpose: TG43-based dose calculation leads to inaccurate field-size dependent dose calculation in superficial HDR brachytherapy, due to full-scatter condition accounted in TG43, but not during treatment delivery. Prior Oncentra TPS TG186-based calculations showed that dose deviations relate to backscatter thickness and field size (FS) (Lee and Bhagwat, 2024). Here, this relationship was validated using Monte Carlo (MC) simulations and corroborate the applicability of a previously proposed correction equation for clinical use.
Methods: MC simulation was conducted using a Geant4-based tool ‘TOPAS’. Water-equivalent contours for 9 cm tissue, Freiburg Flap (FF, 1 cm thick), and additional backscatter (aBS, 0 - 10 cm) above the FF were created. Field sizes ranged from 4x4 cm² to 16x16 cm², matching dwell times from TG186-based Oncentra TPS plans optimized to a uniform depth of 10 mm to a reference point in the center of the dose plane for each FS. Relative dose (RD) was defined as the ratio of dose for a given aBS to that with no aBS for each FS, averaged over 5 mm³ voxels at 1 cm below the center of FS. MC-derived RD values were compared to TG186 results.
Results: MC simulations confirmed a linear correlation with TG186 results (R2 > 0.96), confirming FS-dependent backscatter effect. RD increased with both FS and aBS thickness, from 2.5% for 4x4 cm² to 7.7% for 16x16 cm² at 10 cm aBS. This increase was well-characterized (R² > 0.98) by the previously reported 2-parameter model: RD=[1-β/(1+α*BS)]/(1-β).
Conclusion: MC simulation validated the FS-dependent backscatter effect previously identified with TG186 calculations. The 2-parameter model effectively characterizes these dose deviations, supporting its potential clinical application in superficial HDR brachytherapy to improve dose calculation accuracy.