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

Abstract

<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.