SU‐E‐T‐379: Development of a Monte Carlo Based Correction Strategy for a TG‐43 Based Brachytherapy Treatment Planning System to Account for Applicator Inhomogeneities

Abstract

Purpose: 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: The American Association of Physicists in Medicine Task Group 43 (TG‐43) report was utilized to calculate absolute dose around an Ir‐192 source. MC was utilized to simulate dose distributions resulting from an Ir‐192 source at eight dwell‐positions within Nucletronˈs ICBT CT‐MR Fletcher applicator (with and without a shield surrogate). With these distributions, a library of ovoid‐ and source‐based 3D attenuation‐correction factor datasets characterizing the dosimetric effects of the ICBT applicator was developed. These factors were then applied to correct a brachytherapy TPS‐calculated plan. Plans with different maximum dwell‐time gradients 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: Plans generated by a brachytherapy TPS with a maximum dwell‐time gradient less than 10% can be corrected utilizing either correction method to agree with full simulated Monte Carlo datasets to within criteria. A dwell‐time gradient of 10% is a threshold for the ovoid‐ based correction scheme. The source‐based correction method consistently results in 100% agreement between a corrected plan and the equivalent MC generated plan. Conclusions: No correction scheme is necessary for the ICBT CT‐MR Fletcher compatible ovoid applicator. For a shielded applicator, source‐ based correction factors can be applied to correct a TG‐43 based treatment plan with any set of dwell‐time combinations to match a full MC simulation.