On The Impact of High-Density Plastics On The Dose Calculations for Vaginal Cylinder Brachytherapy: A Benchmark Study
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PO-GePV-T-52 (Sunday, 7/10/2022) [Eastern Time (GMT-4)]
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Purpose: Some of the commercial vaginal cylinder (VC) applicators are made of high-density plastic materials. Considering the lumen(s) for placement of the HDR source, these applicators create a heterogeneous environment around the sources that could affect the dose distribution, as opposed to the TG43 formalism. In addition, the normalization point, either the surface or 5mm beyond the surface, can intensify this effect. This study utilizes Monte Carlo (MC) simulations to assess this heterogeneity effect and benchmarks the accuracy of a model-based dosimetry calculation dosimetry algorithm (MBDCA) against the MC simulation results, considering the effect of normalization point.
Methods: we used GEANT4 to simulate the 192Ir HDR source and a commercial VC, made of PEEK and PPSU plastics with the density of about 1.3 g/cc and diameters ranging from 20-35 mm inside a water phantom. Standard plans were fed from a commercial MBDCA, BrachyVision ACUROS (BVA), considering two different models: The TG43 model assuming all the environment as water and the MC/ACUROS models accounting for the heterogeneity of VCs. The dose and/or energy deposited profiles were extracted.
Results: the MC simulation results showed that the VC inhomogeneity can cause shrinkage of the isodose lines up to 1 millimeter at the peripheral sides mainly inside the VCs compared to TG43 models, leading to the reduction of the dose when prescribing to the surface. In addition, ACUROS overestimated the dose on the surface about 5.0% and 8.5% at the periphery and the apex, when prescribing to the surface. However, the difference between ACUROS and MC simulations were negligible at prescription point when prescribing to 5mm beyond the surface.
Conclusion: High density plastic materials used for fabrication of commercial VC increase the uncertainty on dose calculation. This effect is more notable when prescribing dose to the surface of the applicator.
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