LAUSR

To ensure accurate reference dosimetry with ionization chambers in MR-linacs, the influence of the magnetic field on the response of the ionization chambers must be considered. The most direct method considering the influence of magnetic fields in dosimetry is to apply an appropriate absorbed-dose-to-water standard. At PTB, a new water calorimeter has been designed which is capable to determine Dw,Q in an MR-linac. The new device allows the direct calibration of ionization chambers in terms of absorbed dose to water for MR-linac irradiation conditions. Hence, the correction factors k_(B ⃗,Q,Q_0) can be determined which replace the current radiation-quality dependent correction factors k_(Q,Q_0) for dosimetry in the presence of magnetic fields. In cooperation with Heidelberg University Hospital, k_(B ⃗,Q,Q_0) factors were measured at the 6 MV 0.35 T Viewray MR-linac for different cylindrical ionization chambers with sensitive volumes ranging from 0.015 cm3 to 0.65 cm3. The chambers were placed both perpendicular and parallel in respect to the magnetic field. Standard uncertainties of about 0.5 % were achieved.