In the UMC Utrecht a prototype MR-linac has been installed. The system consists of a 6 MV Elekta (Crawley, UK) linear accelerator and a 1.5 T Philips (Best, The Netherlands) Achieva MRI system. This paper investigates the feasibility to correct the ionization chamber reading for the magnetic field within the dosimetry calibration method described by Almond et al (1999 Med. Phys. 26 1847-70). Firstly, the feasibility of using an ionization chamber in an MR-linac was assessed by investigating possible influences of the magnetic field on NE2571 Farmer-type ionization chamber characteristics: linearity, repeatability, orientation in the magnetic field; and AAPM TG51 correction factor for voltage polarity and ion recombination. We found that these AAPM correction factors for the NE2571 chamber were not influenced by the magnetic field. Secondly, the influence of the permanent 1.5 T magnetic field on the NE2571 chamber reading was quantified. The reading is influenced by the magnetic field; therefore, a correction factor has been added. For the standardized setup used in this paper, the NE2571 chamber reading increases by 4.9% (± 0.2%) due to the transverse 1.5 T magnetic field. Dosimetry measurements in an MR-linac are feasible, if a setup-specific magnetic field correction factor (P1.5 T) for the charge reading is introduced. For the setup investigated in this paper, the P1.5 T has a value of 0.953.

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AbslrscL An air kerma rste calibration method far low dose rate '921r brachytherapy sources was elaborated using three different large wlume spherical ionization chambers (PnV U-10, NE 2551, Exradin Ah) in a mllaborative series d aperimenls between the University of Ghent (UG) and the Nethedands Measurements Institute (NMI). B this end these chambers were calibrated for x-ray qualities in the energy range 35 keV to 305 keV and for 137cF and 60Co gamma ray. The results of here measurements are used to derive mean weighted calibration factors for the photon radiation of 1g21r brachytherapy sources. Furthermore, the effect of the finite chamber size on the effective measuring p i n t and the correction far the contribution of scattered radiation were studied. Measurements on the same sefs of s1u~'es show that the ionization chambers and the methods used yield results for the reference air kema rates of 1921r sources which agree within 0.42%.