SU‐E‐T‐377: Evaluation of Calculated and Measured HDR Brachytherapy Doses in the Presence of Heterogeneities

Purpose: With the recent introduction of the Grid‐Based Boltzmann Solver (GBBS) in commercial HDR brachytherapy treatment planning (Acuros™, Varian), there is now a rapid and accurate alternative to Monte Carlo calculations to attain improved dosimetric accuracy. The validation of Acuros™ has been performed by comparison to Monte Carlo calculations in water phantom. In this investigation, we compare calculated and measured doses in the presence of both high and low density heterogeneities. Methods: Phantoms were created incorporating both low and high density heterogeneities including representative densities of bone and air cavities. For the comparisons HDR plans were developed to deliver 100 cGy with TG‐43 formalism to optically stimulated luminescence (OSL) detectors placed in or near the heterogeneities. The detectors were calibrated by exposure to a known dose. The plans were then recalculated using the Acuros™ algorithm, and the dose to the detectors was determined both by calculation and measurement. Results: The OSL detectors measured an average dose of 105 cGy when placed after a low density insert in the phantom, as compared to a calculated average dose of 107 cGy. When the OSL detectors were placed after three different higher density inserts ranging from 1.3 to 1.9 g/cm3, the measured doses were 78, 77, and 75 cGy, compared to the calculated doses of 84, 82, and 75 cGy. Conclusions: The Acuros™ algorithm has been demonstrated to perform rapid and accurate dosimetry in homogenous water phantom. In this study we demonstrate that it performs accurately in the vicinity of heterogeneities expected to be encountered in the clinic. This work shows good qualitative agreement between measured and calculated high dose rate brachytherapy doses in the presence of both low and high density heterogeneities, and shows that the confidence‐building step of comparing calculated to measured dose can be readily made with simple phantom geometries.