Room: Karl Dean Ballroom A1
Purpose: Water calorimetry is one of the major techniques for absolute absorbed dose to water measurement. This is done through analyzing radiation induced temperature rise. With the advent of MRI guided radiotherapy, the establishment of absolute dose measurement techniques in this setting is important. The purpose of this project is to design, construct, and show feasibility of water calorimetry in MR-linac.
Methods: Several designs of a fully MRI compatible water calorimeter were analyzed through FEM analysis. The final optimized design (based on thermal stability and performance) was constructed. The calorimeter was made of only plastic/ceramic components, can be hydraulically stirred (for thermal stability), and is fully imageable with MRI, CT, and CBCT for accurate positioning. Measurements were conducted in the Elekta MR-linac (Elekta AB, Stockholm, Sweden) in the presence and absence of the 1.5T magnetic field (B-field). The portable water tank containing the glass vessel (which housed the thermistors) was kept at 4Â°C and was positioned using kV imaging. The hydrogen saturated vessel was pre-irradiated to avoid heat defect. Following calorimetry, the thermistors inside vessel were removed and replaced by an A1SL chamber with NRC-traceable calibration. Hence, water calorimetry was compared against chamber dosimetry. Further comparisons against in-house probe-format graphite calorimeter was also performed.
Results: Through use of imaging, thermistors can be positioned to within 0.3mm. Preliminary uncorrected results showed an absolute dose of 0.841cGy/MU at the point of thermistors in the absence of B-field. A standard error of 1.70% on the measurements was reached on 31 measured caloric runs. Uncorrected water calorimetry results agreed with chamber dosimetry, and graphite calorimetry to within 2.1%.
Conclusion: This work presents a novel calorimeter design for measurement of absolute dose to water under the presence of a B-field. Analysis of correction factors and results in the presence of the 1.5T B-field are underway.
Funding Support, Disclosures, and Conflict of Interest: NSERC; Grant No. RGPIN-435608.