WE‐A‐108‐11: Patient Specific Quality Assurance Tool in Rectal Brachytherapy

Abstract

Purpose: To design and evaluate a patient‐specific quality assurance program for HDR Brachytherapy of rectal cancer. Methods: A solid water phantom with a rectal applicator positioned in the center was designed. Dose distributions were measured with two sheets of EBT3 model GAFCHROMIC™ film placed at 2 cm above and 5 cm below the center of the applicator. Treatment plans of 12 patients were studied, with each plan recalculated for the phantom geometry. FilmQA software was used to compare calculated and measured doses using the Gamma function. The region of interest (ROI) for analysis was defined as doses in excess of 20%, 30% or 40% of the maximum film dose. The average percentage of passing pixels (APP) over the 12 patients was scored for analysis. Sensitivity to source positional errors was evaluated by introducing errors of 1, 3, and 5 mm in one channel and quantifying the resultant change in APP. Results: The patient‐averaged data showed that an APP greater than 95% was achieved for the 2 cm film using criteria of 2%/3 mm with ROI=30%. For the 5 cm film an APP greater than 95% was achieved with criteria of 3%/3 mm with ROI =40%. Introducing positional errors greater than 1 mm resulted in drastic fall off in APP for all chosen ROIs and all DD/DTA criteria. For film at 2 cm and 2%/3 mm criteria, 2 mm shift error resulted in as much as 50% decrease in APP. Conclusion: The patient plans in this study were used to establish the use of 2%/3mm for gamma analysis in the 2 cm film plane on an ROI that incorporates doses in excess of 30% of the max film dose. In the 5 cm film plane, 3%/3mm for doses in excess of 40% of the max film dose is appropriate. Natural Sciences and Engineering; Research Council of Canada Contract No. 386009.