MO‐AB‐BRA‐03: Calorimetry‐Based Absorbed Dose to Water Measurements Using Interferometry

Abstract

Purpose:Interferometry‐based calorimetry is a novel technique to measure radiation‐induced temperature changes allowing the measurement of absorbed dose to water (ADW). There are no mechanical components in the field. This technique also has the possibility of obtaining 2D dose distributions. The goal of this investigation is to calorimetrically‐measure doses between 2.5 and 5 Gy over a single projection in a photon beam using interferometry and compare the results with doses calculated using the TG‐51 linac calibration.Methods:ADW was determined by measuring radiation‐induced phase shifts (PSs) of light passing through water irradiated with a 6 MV photon beam. A 9×9×9 cm3 glass phantom filled with water and placed in an arm of a Michelson interferometer was irradiated with 300, 400, 500 and 600 monitor units. The whole system was thermally insulated to achieve sufficient passive temperature control. The depth of measurement was 4.5 cm with a field size of 7×7 cm2. The intensity of the fringe pattern was monitored with a photodiode and used to calculate the time‐dependent PS curve. Data was acquired 60 s before and after the irradiation. The radiation‐induced PS was calculated by taking the difference in the pre‐ and post‐irradiation drifts extrapolated to the midpoint of the irradiation. Results were compared to computed doses.Results:Average comparison of calculated ADW values with interferometry‐measured values showed an agreement to within 9.5%. k=1 uncertainties were 4.3% for calculations and 14.7% for measurements. The dominant source of uncertainty for the measurements was a temperature drift of about 30 µK/s caused by heat conduction from the interferometer's surroundings.Conclusion:This work presented the first absolute ADW measurements using interferometry in the dose range of linac‐based radiotherapy. Future work to improve measurements’ reproducibility includes the implementation of active thermal control techniques.