Challenges and opportunities in calorimetry for clinical radiation dosimetry

Abstract

Primary Standard Dosimetry Laboratories establish the standards of absorbed dose to water (ADW) using calorimetry. This is because calorimetry is the most direct method to account for the energy imparted to matter by radiation. The traditional calorimetry framework is based on the use of thermistors to measure temperature changes. There are some drawbacks for the use of thermistors in radiation calorimetry and different approaches for temperature measurement are under investigation. One of these techniques uses the temperature dependence of the refractive index of water to measure with an interferometer the radiation-induced temperature changes. This work presents a comparison in calorimetric dose measurements using thermistors and interferometry. A circuit for thermistor-based measurements was designed and characterized. For interferometer-based measurements, a Michelson interferometer was built. Phase shifts in one of the interferometer arms were calculated from the intensity changes measured with an unbiased photodiode placed at the center of the interference fringe. Radiation-induced temperature rises were measured simultaneously with the interferometer and thermistor in a glass phantom filled with water. The water phantom was irradiated with 200, 300, 400 and 500 monitor units from a 6 MV photon beam using a linear accelerator Varian 21 EX. Results. The thermistor circuit built was able to achieve micro-kelvin resolution. Absorbed dose to water measurements performed with the interferometer agreed within the uncertainty with thermistor-based measurements. Time dependent temperature curves showed that heat transfer after irradiations was higher for thermistor than for the interformetric technique due to the thermistor overheating. The equivalent type-A uncertainty was 0.32 Gy for the interferometric measurements and 0.05 Gy for thermistor-based measurements. Thermal fluctuations in the water phantom have a higher contribution to measurement uncertainty for interferometer-based measurements. Improved precision in interferometer-based measurements will require increasing the thermal insulation or the water phantom and using a stabilized laser. Up to date, interferometer-based measurements are feasible in the dose range of interest for external beam therapy, but further investigation is necessary to achieve a precision comparable with the thermistor framework.