TH‐C‐M100E‐01: Numerical Feasibility Study of a Novel Absorbed Dose to Water Calorimeter‐Based Standard for 192Ir HDR Brachytherapy

Purpose: To study the feasibility of developing a new standard for absorbed dose to water based on water calorimetry for high dose rate iridium‐192 brachytherapy sources. Method and Materials: The heat conduction pattern generated in water by the Nucleotron microSelectron‐HDR brachytherapy source was simulated using Comsol Multiphysics™ software. Source self‐heating due to self‐attenuation of photons was calculated with GEANT4. A smooth, well‐behaved three‐dimensional function was fit to the entire dose distribution data using TableCurve3D™. The heat‐loss correction K c was calculated as the ratio of the temperature in the calorimeter under ideal conditions to realistic conditions. Results: The feasibility of a water calorimeter based absorbed dose standard is determined by a balance between the requirements to obtain sufficient signal to perform a reproducible measurement, the effects of heat loss on the measured signal, and the positioning uncertainties. Due to self‐absorption, the source equilibrium temperature was found to be above the ambient temperature by a constant amount that depends only on setup conditions and source activity. For the source inside its nylon‐12 catheter inserted into water, the steady state excess temperature per unit source activity was found to be 0.5671 K/Ci. The source temperature reached 96% of its steady state temperature after 60 s. Conduction correction factors K c were calculated for several exposure times and at various measurement points away from the source inside the calorimeter. A total exposure time between 140 s and 240 s at a distance that receives a minimum of 1 Gy of dose was found to allow reduction of K c to below 0.1% of unity. Conclusions: Water calorimetry for HDR brachytherapy is feasible and total uncertainties of significantly better than 5% on the dose can be achieved with current water calorimetry techniques and instruments.