MO-G-BRA-06: A Primary Standard for HDR Brachytherapy Calibrations

Abstract

Purpose: A new Canadian primary standard for high dose rate (HDR) brachytherapycalibrations has been developed, offering improved accuracy through the use of a Pb wedge to determine the scatter component k sc at each measurement position. Methods: The new HDR standard calibrates ,192,Ir brachytherapy sources in terms of air‐kerma strength (S K ). An HDR brachytherapycalibration rig was manufactured and commissioned. This rig utilizes a low (0.5 m) aluminum table to which is mounted a motorized single‐axis linear stage. An afterloader feeds an ,192,Ir source vertically up through a fixed 1 m tall PMMA tube column attached to the table. A second 1 m PMMA tube column is attached to a movable carriage on the linear stage and supports a spherical graphiteionization chamber. A series of 10 measurements are made with the source‐to‐detector distance varying from 10 cm to 55 cm in 5 cm increments. The largest correction factor, k sc (scatter from the stage, walls, floor and ceiling) is determined using a Pb‐wedge technique to block the direct beam and allow a direct measurement of scatter at each distance. The small distance correction is then a simple matter to determine, allowing a more accurate determination of the source strength based on distance squared. Results: : Measurements with a Pb‐wedge technique for determining k sc demonstrated that the scatter component for each position varied, in contradiction to the constant‐scatter supposition of the alternative 7‐distance measurement calculation. These measurements were supported by EGSnrc Monte Carlo simulations. The disagreement between these two techniques for determining k sc represent an (S K ) difference of approximately 0.4%. Conclusions: The new HDR brachytherapycalibration capability demonstrates higher accuracy through a more accurate scatter correction. This standard will participate in a formal international comparison for HDR brachytherapy, tentatively scheduled for 2011.