TH-D-BRB-04: Towards An Absorbed Dose-Based Calibration for Ir-192 Brachytherapy Dosimetry-Development of a Primary Standard Water Calorimeter

Abstract

Purpose: To develop, validate and improve the accuracy of a standard for absorbed dose to water for high dose rate iridium‐192 (192Ir) brachytherapysources based on 4 °C stagnant watercalorimetry.Method and Materials: The absorbed dose rate of several Nucleotron microSelectron‐HDR 192Ir brachytherapysources with nominal air kerma strength S k,air of 31600‐36940 U at source‐detector separations d src‐det ranging between 51–67 mm was measured. The irradiation time was adjusted to deliver a minimum 1 Gy at the measurement position. The heat‐loss correction was calculated using COMSOL MULTIPHYSICS™ and is defined as the ratio of the temperature in the calorimeter under ideal conditions to realistic conditions. Results: A random uncertainty of 0.6% on the calorimetric measurements was achievable. A total uncertainty of 3.0% on the final absolute dose rate to water measurement using watercalorimetry is possible. Our calorimeter‐measured dose rates were lower than the dose rate estimated using TG‐43 based on an ADCL calibrated well chamber by 0.1–3.4%. The dose rate obtained from chamber and film reference dosimetry in water agreed with calorimetry to within one‐sigma uncertainty. A reproducibility of better than 0.25 mm was achieved in measurement of d src‐det. Despite the significant source self heating (source excess temperature in‐water), we found that the effects of convection can be neglected (as is usually done in 4 °C stagnant watercalorimetry) for d src‐det larger than roughly 45 mm and an irradiation time that results in about 1 Gy of dose at the measurement point. Conclusions: Absolute dose rate to water of HDR 192Ir sources can directly be measured using 4 °C stagnant watercalorimetry with absolute accuracy of better than 3.0%. Our work provides the framework necessary for a shift from indirect HDR 192Ir brachytherapydosimetry to a more accurate, direct and absolute measurement of absorbed dose to water.