SU‐E‐T‐459: Impact of Source Position and Traveling Time On HDR Skin Surface Applicator Dosimetry

Abstract

Purpose:Observed dosimetric discrepancy between measured and treatment planning system (TPS) predicted values, during applicator commissioning, were traced to source position uncertainty in the applicator. We quantify the dosimetric impact of this geometric uncertainty, and of the source traveling time inside the applicator, and propose corrections for clinical use.Methods:We measured the dose profiles from the Varian Leipzig‐style (horizontal) HDR skin applicator, using EBT3 film, photon diode, and optically stimulated luminescence dosimeter (OSLD) and three different GammaMed HDR afterloders. The dose profiles and depth dose of each aperture were measured at several depths (up to about 10 mm, depending on the dosimeter). The measured dose profiles were compared with Acuros calculated profiles in BrachyVision TPS. For the impact of the source position, EBT3 film measurements were performed with applicator, facing‐down and facing‐up orientations. The dose with and without source traveling was measured with diode detector using HDR timer and electrometer timer, respectively.Results:Depth doses measured using the three dosimeters were in good agreement, but were consistently higher than the Acuros dose calculations. Measurements with the applicator facing‐up were significantly lower than those in the facing‐down position with maximum difference of about 18% at the surface, due to source sag inside the applicator. Based on the inverse‐square law, the effective source sag was evaluated to be about 0.5 mm from the planned position. The additional dose from the source traveling was about 2.8% for 30 seconds with 10 Ci source, decreasing with increased dwelling time and decreased source activity.Conclusion:Due to the short source‐to‐surface distance of the applicator, the small source sag inside the applicator has significant dosimetric impact, which should be considered before the clinical use of the applicator. Investigation of the effect for other applicators that have relatively large source lumen inner diameter may be warranted.Christopher Barker and Gil'ad Cohen are receiving research support for a study of skin surface brachytherapy from Elekta.