Calorimetric determination of the response of ionization chambers in the 6 MV 0.35 Tesla MR-linac of Heidelberg University Hospital

MR image-guided radiation therapy, i.e. irradiation with high-energy photon radiation and simultaneous imaging by means of MR tomography, could turn into one of the standard methods in radiation therapy in the years to come. To ensure precise dosimetry in the presence of strong magnetic fields, suitable primary and secondary standards are required for measuring the absorbed dose to water in MR–linacs. At PTB, an MR–compatible water calorimeter has been developed for this purpose. In cooperation with the German Cancer Research Center (DKFZ) and Heidelberg University Hospital, the water calorimeter has recently been used in the 6 MV 0.35 Tesla MR–linac to experimentally determine the k_Q,B factors of different ionization chambers required for ionization–chamber–based absolute dosimetry. Standard measurement uncertainties for the measured k_Q,B factors of less than 0.7 % have been achieved.

An MR-linac combines magnetic resonance imaging with a medical accelerator for radiation therapy in a single instrument. As this offers the possibility of adaptive irradiation planning, MR–linacs could become one of the standard methods in radiation therapy in the years to come. In Germany, two of these instruments have been installed in hospitals, a measure funded by the German Research Foundation’s (DFG) Major Instrumentation Initiative in 2015. However, to ensure precise dosimetry with ionization chambers in the presence of strong magnetic fields, both the influence of the magnetic field on the response of the ionization chamber and on the dose distribution must be taken into account. The modification of the response in the case of cylindrical ionization chambers also depends on the orientation of the chamber axis relative to the magnetic field. The most direct method of taking these influences into account is the calibration of the ionization chambers under MR-linac irradiation conditions or the experimental determination of the so-called k_Q,B factors of the ionization chambers by means of a suitable primary standard. For this purpose, an MR-compatible water calorimeter was developed at PTB as a primary standard for the absorbed dose to water Dw.

In cooperation with the German Cancer Research Center (DKFZ) and Heidelberg University Hospital, the new water calorimeter has recently been used at the local Viewray 6 MV 0.3 Tesla MR-linac to determine the kQ,B factors for eight different cylindrical ionization chambers in perpendicular and parallel orientation to the magnetic field.

The respective measurements are carried out in a two-step procedure. First, the absorbed dose to water Dw is determined at the point of measurement at a water depth of 10 cm with the aid of the water calorimeter. For this purpose, approximately 80 consecutive measurements were performed, each with an irradiation time of 20 s at a dose rate of approximately 4.5 Gy/min. The field size of the 6 MV photon field was 10 cm x 10 cm at the point of measurement, with a distance of 100 cm between the source and the phantom surface. Detailed investigations of the stability of the MR-linac dose rate before and during the measurements with the water calorimeter (Figure 1) showed that the relative standard deviation of the MR-linac’s dose rate remained smaller than 0.25 %. Therefore, permanent external monitoring was dispensed with.

Fig. 1: Image of the MR-compatible water calorimeter (left) on the patient couch of the MR-linac at Heidelberg University Hospital. The calorimeter is moved into the MR tube for measurements. The 6 MV photon radiation of the linac enters the calorimeter horizontally through the calorimeter’s radiation entrance window. The water phantom set up at the right-hand side of the calorimeter was used for additional stability measurements of the linac’s dose rate.

In the second step of the determination of the k_Q,B factors, an ionization chamber is placed into the water phantom of the calorimeter after the calorimetric measurements have been completed, either perpendicular or parallel to the direction of the magnetic field. By measuring the charge generated in the ionization chamber per 20 s of irradiation and the absorbed dose to water Dw, which is known from the calorimeter measurement at the same measuring position, the calibration factor of the chamber can be determined under MR-linac irradiation conditions and consequently, the corresponding k_Q,B factor of the ionization chamber. For the present measurements, eight different cylindrical ionization chambers with measuring volumes between 0.015 cm³ and 0.15 cm³ were used. The following table lists the different ionization chambers and presents both the ratio of the k_Q,B factors between perpendicular and parallel orientation to the magnetic field and the k_Q,B factors for parallel orientation.

Table with experimentally determined k_Q,B factors for different cylindrical ionization chambers.

The analysis of all uncertainty contributions occurring during the measurements shows that the k_Q,B factors can be determined with a relative standard measurement uncertainty of less than 0.7 %. For the first time, experimentally determined k_Q,B factors for ionization chambers are thus available for irradiation conditions in a 6 MV 0.35 Tesla MR-linac. This allows traceable dosimetry with the aid of ionization chambers in magnetic fields.