Room: Karl Dean Ballroom C
Purpose: To build and evaluate the use a probe-format calorimeter â€“ herein referred to as the Aerrow â€“ for absolute dosimetry of MRI-guided radiotherapy modalities. Similar in size to a Farmer-type ion chamber, the Aerrow is the first calorimeter intended for use by clinical physicists, and has been developed to help meet the clinical need for accurate dosimetry in non-standard fields without the need for calibration.
Methods: Based on a numerically-optimized design obtained in previous work, an Aerrow prototype capable of isothermal operation was constructed in-house. In isothermal mode, the sensitive volume is subject to thermal control and the quantity of interest is the electrical power required to maintain a stable temperature during irradiation. The ratio of the power and absorber mass is a direct measure of the dose rate. The Aerrow was used to perform reference dosimetry in the 7-MV FFF photon beam of an Elekta MR-linac in both the presence and absence of the 1.5 T B-field. The measurements were carried out in water, at an SSD of 133.5 cm, and otherwise standard conditions for two detector orientations (parallel and perpendicular to the B-field). Aerrow-derived absorbed dose to water measurements were compared to results obtained using a reference-class ion chamber (Exradin A19) with a calibration traceable to national standards, as well as a cross-calibrated thimble chamber (Exradin A1SL).
Results: Compared to the Exradin A19 (0.836 Â± 0.008 cGy/MU) and A1SL (0.833 Â± 0.008 cGy/MU), the average doses measured using the Aerrow in the absence of a B-field were 0.841 Â± 0.007 and 0.837 Â± 0.007 cGy/MU for the parallel and perpendicular orientations, respectively.
Conclusion: Within combined standard uncertainty, all dose to water measurements using the Aerrow agreed with chamber-based clinical reference dosimetry. Analysis of correction factors and results in the presence of the 1.5 T B-field are underway.
Funding Support, Disclosures, and Conflict of Interest: Calorimetry research at McGill University has been funded in part by Sun Nuclear Corporation, Melbourne FL.