Sci-Sat AM(2): Brachy-01: A novel HDR Ir-192 brachytherapy water calorimeter standard

Parameters influencing the accuracy of absorbed dose measurements for HDR 192Ir brachytherapy using water calorimetry were investigated with the goal to develop a novel primary absorbed dose to water standard. To provide greater stability, flexibility, and accuracy in the source-detector distance d_src-det positioning and measurement, a new spring-loaded catheter holder composed of two concentric cylindrical sleeves with multiple orthogonal adjusting screws was developed. The absorbed dose from Nucletron microSelectron-HDR 192Ir brachytherapy sources with air kerma strengths ranging between 21000-38000 U was studied. d_src-det is optimized so as to balance signal-to-noise ratio (decreasing with increasing d_src-det ) and temperature drift effects resulting from source self-heating. The irradiation times were adjusted to yield a minimum 1 Gy of dose at the measurement point. Successful measurements at d_src-det ranging between 25-50 mm were performed. COMSOL MULTIPHYSICS™ software was used to determine the heat loss correction due to conduction defined as the ratio between temperature rise at a point under ideal conditions to realistic conditions (i.e., no conduction). An agreement of better than 6.5% was observed between TG-43 calculated and calorimetrically measured absorbed dose rates. The effects of convection where calculated to be negligible as the glass vessel provides a convective barrier significantly decoupling the water velocity in the interior and exterior of the vessel (water velocities were 1-2 orders of magnitude different). Our work paves the way to successful primary absorbed dose determination for radioactive sources using calorimetric techniques.