Room: AAPM ePoster Library
Purpose: In water calorimetry (WC), absorbed dose-to-water is determined by measuring radiation-induced temperature rises. The heat transfer correction (k(ht)) accounts for additional heat loss/gain at the point of measurement due to convection/conduction. The point of measurement is surrounded by a glass vessel filled with pure water to prevent non-radiation induced temperature change. In this study, we aimed to optimize vessel/calorimeter design for photons/electrons prior to constructing/testing an MR-compatible WC.
Methods: Finite element method (FEM) software was used to optimize k(ht) for different designs: A cylindrical glass vessel was simulated, with varying vessel dimensions (front/back glass thickness) in various energies (6 MV, 7 MV-MRL, 6 MeV, 9 MeV, 18 MeV). Position of the vessel with respect to the measurement point was also varied. Subsequently, a WC and vessel were built using results of this study, and successful measurements performed in conventional 6 MV beam and MR-linac.
Results: The body of the calorimeter consists of a 3-shell acrylic system with Aerogel acting as insulation between shells. The calorimeter design allows for full imaging of the tank for positioning with MRI, CT, and CBCT, enabling very rapid setup in =1h. k(ht) was most sensitive to top vessel thickness, varying by as much as 8.9 % with a 1 mm change in thickness at 6 MeV. When the point of measurement is more than 6 mm away from the top glass surface, k(ht) converges to unity for all designs. Absolute dose during WC measurements in 6 MV and MR-linac agreed with reference dosimetry using a traceable chamber to within 0.25 %.
Conclusion: Our calorimeter and vessel were successfully used in a conventional linac and MR-Linac. Based on FEM analysis, it should perform well in electron beams provided the temperature detectors are 6 mm away from the top glass surface in low energy electron beams.
Funding Support, Disclosures, and Conflict of Interest: Funding Support, Disclosures, and Conflict of Interest: NSERC; Grant No. RGPIN-435608.