SU‐E‐T‐366: Dosimetric Characterization of Brachytherapy Surface Applicators

Purpose: Conical surface applicators with diameters ranging from 10mm to 45mm have been developed by Varian Medical Systems, Inc. These applicators are designed to be used with the GammaMedplus iX and VariSource iX high‐dose rate Ir‐192 afterloaders, allowing for conformal dose delivery for the treatment of surface lesions. Clinical use of these applicators has become widespread, yet no peer‐reviewed method of output verification exists. Traditional brachytherapydosimetry protocols (AAPM TG‐43U1) cannot be applied directly, creating a gap in the chain of calibration traceability. The ultimate goal of this work is the creation of a dosimetry protocol for the calibration of applicators like those under investigation. An initial step towards this includes Monte Carlo and experimental assessment of the dose‐rate and distributions for each applicator. Methods: Each applicator was modeled in MCNP5 based on specifications provided by the manufacturer. A collision kerma tally was used to determine the kerma distributions at the surface and at depth in water. Surfacedose profiles and fractional depth dose curves in water were generated for each applicator. Results: Initial assessment of the surfacedose distributions show a peak at the center of the applicator with a rapid fall off to the edges. A prominent dip in the surfacedose profile is observed with the vertical entry applicators due to the shorter source‐to‐surface (SSD) distance. The penumbra for both the horizontal and vertical applicators was 1.04cm, despite differences in the SSD. Conclusions: The surfacedose distributions and fractional depth dose curves generated from this initial Monte Carlo investigation show characteristics typical for applicators of this type. This work expands the knowledge about output characteristics of these kinds of applicators, leading to the ultimate goal of a clinically relevant dosimetry protocol for clinicians. Conflict of Interest: Varian Medical Systems, Inc. provided sources and applicators.