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

Parameters influencing the accuracy of absorbed dose measurements for HDR192 Ir 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‐HDR192 Ir 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.