Sci-Fri AM(2): Brachy-01: Commissioning of Brachytherapy TPS Using EBT Model GafChromic Film

Abstract

Method for quality assurance (QA) of conformal CT‐based high dose rate (HDR) brachytherapy is described, based on comparing dose distributions generated by treatment planning system and measured by EBT model GAFCHROMIC film within dedicated brachy‐therapy QA phantom. Phantom consists of 3 catheters embedded in surface plane of 2 cm thick slab of solid water (SW) mimicking the Fletcher GYN applicator set. Additional slabs of solid water were added above and below this slab to provide full scattering conditions. Single sheet of film was placed 2 cm below the plane containing catheters. Dose distribution was calculated with OnCentra Brachy treatment planning software and exported in DICOM format. Same plan was delivered to the phantom containing the film, and dose distribution was measured using our novel extended range radiochromic film dosimetry protocol, employing all three channels of the film absorptionspectrum allowing dose measurements in the range 0.02–100 Gy. Calculated and measured dose distributions were compared with FilmQA software in terms of gamma function, over region of interest (ROI) defined by region containing isodose ≥ 5% of prescription dose. Based on recommendations given in IAEA TRS‐430 report, we established a 5% dose/2 mm distance criterion for gamma function. We found that 95% of points within ROI were passing this criterion. QA method described provides a comprehensive verification of all the steps involved in CT‐based HDR brachytherapy. We recommend the method to be used during commissioning process and once a year as part of brachytherapy QA program.