Abstract
Purpose: The purpose of this work was to investigate the use of pre-calculated Monte Carlo (MC) ovoid and source-based attenuation-correction factors that would correct a commercially-available brachytherapy treatment planning system (TPS) generated plan in order to account for any dosimetric effects due to the presence of intracavitary brachytherapy (ICBT) applicators during treatment delivery. Methods: A MC model of an ICBT CT-MR compatible ovoid applicator set was confirmed utilizing radiochromic film (RCF). MC was used to simulate dose distributions resulting from eight source-dwell-positions within the ICBT applicator. Also, the American Association of Physicist in Medicine Task Group 43 Report (AAPM TG-43) was utilized to calculate absolute dose rate around a microSelectron version 2 192Ir source contained in water. With these dose distributions, a library of ovoid and source-based 3D attenuation-correction factor datasets characterizing the dosimetric effects of the ICBT applicator was developed. Appropriate attenuation-correction factors were then applied to correct a brachytherapy TPS-calculated plan. Several plans with different maximum dwell-time gradients (Þdt) were compared to evaluate the effectiveness of both correction methods with respect to criteria of acceptability being within +/- 2% absolute dose or +/- 2 mm distance-to-agreement (DTA). Results: RCF confirmation measurements from 3 active dwell-positions in a single ovoid agreed with MC simulated planes with over 96% of points agreeing within 2% or 2 mm DTA. Plans generated by Oncentra TPS can be corrected utilizing either the ovoid-based or source-based correction methods to agree with full simulated Monte Carlo datasets to within +/- 2% or +/- 2mm DTA. Although, dwell-time combinations utilized in this study with a maximum dwell-time gradient above 10% is a threshold for the ovoid- based correction scheme to correct the TG-43 calculation. The source- based correction method consistently results in 100% agreement between a corrected plan and the equivalent MC generated plan. Conclusions: The MC model is sufficient to predict measured RCF dose distributions accurately. Source- based correction factors can be applied to correct a TG-43 based treatment plan to match a full MC simulation.
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