To ensure accurate reference dosimetry with ionization chambers in magnetic resonance linear accelerators (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 primary standard. At PTB, a new water calorimeter has been designed which is capable to determine D_w,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 can be determined which replace the current radiation-quality dependent correction factors for dosimetry in the presence of magnetic fields. In cooperation with Heidelberg University Hospital, 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.

为了确保在磁共振线性加速器（MR-linacs）中使用电离室的准确参考剂量，必须考虑磁场对电离室响应的影响。考虑剂量学中磁场影响的最直接方法是应用适当的吸收剂量-水-初级标准。在PTB，已经设计了一种新的水量计，该水量计能够确定MR直线加速器中的D _w，Q 。新设备可以根据MR直线加速器照射条件对水的吸收剂量直接校准电离室。因此，可以确定替代当前辐射质量相关的校正因子的校正因子。在有磁场的情况下进行剂量测定。与海德堡大学医院合作，在6 MV 0.35 T Viewray MR-直线加速器上测量了敏感体积范围从0.015 cm ³到0.65 cm ^3的不同圆柱电离室的因子。相对于磁场垂直和平行放置室。达到约0.5％的标准不确定度。