SU-GG-T-50: Evaluating Source Constancy from the Xoft Electronic Brachytherapy System

Abstract

Purpose: The Xoft Axxent 50 kVp electronic brachytherapy source is currently used to treat a variety of clinical sites including endometrial carcinoma (single‐channel HDR cylinder), accelerated partial breast irradiation (single‐channel HDR balloon), and surface lesions (external cone applicator). Unlike conventional linac‐based dosimetrycontrol systems, the 50 kVp x‐ray source is not monitored during treatment, raising the question of source output stability as a function of absolute dose rate and the spatial constancy of the x‐ray emission. This study evaluates the constancy of the Xoft Axxent 50 kVp electronic brachytherapy source using a pinpoint ion chamber and EBT2 GafChromic film. Materials and Methods: A 0.015 cm3 pinpoint ion chamber was used to verify the constancy of source output during pre‐treatment quality assurance checks using a lucite phantom. The phantom is designed to accept a single channel Xoft applicator and a chamber verification plug. A standard treatment plan was developed to verify source output from a single 50 second dwell position. EBT2 GafChromic film was also used to verify the spatial constancy of a coronal dose plane anterior to the source channel. Results: A total of 37 independent quality assurance measurements were performed using 7 different Xoft sources. The mean pinpoint chamber reading was 10.34 nC with a relative standard error of 2.5%. The ratio of the maximum and minimum chamber reading versus the mean was 1.06 and 0.96, respectively. In comparison, the Xoft on‐board well chamber measures a relative standard error of <1% for the pre‐treatment air‐kerma strength. Conclusions: The output from the Xoft electronic brachytherapy source was monitored using a pinpoint ion chamber during pre‐treatment quality assurance checks. The relative standard error of 37 independent ion chamber readings was 2.5% indicating a relatively stable output across multiple x‐ray tubes in a clinical setting. Research sponsored by the Xoft Corporation.