Room: Track 3
Purpose: The National Research Council of Canada has, in recent years, been developing a capability for using alanine dosimeters to measure absorbed dose to water at therapy dose levels in Co-60 and MV photon beams. This procedure has been successfully trialled in standard clinical photon beams (McEwen, COMP ASM, 2019). This system has now been applied to Elekta Unity MR-linacs (MRLs) installed at two Canadian cancer centres to verify the output calibration, validating in the process the correction factors for ionization chambers in magnetic fields, and investigate the impact of using solid phantoms.
Methods: A calibration curve for a single batch of alanine dosimeters was established through reference 6 MV irradiations using the NRC Elekta Precise linear accelerator over the range 10-50 Gy. For measurements at the cancer centres, the dosimeters were loaded into POM holders with the same external dimensions as a Farmer-type ionization chamber. Each dosimeter was irradiated to a dose in the range 15 Gy to 25 Gy, as determined using the centre’s standard calibration method, and then returned to the NRC for read-out and analysis.
Results: A correction is required for the effect of the magnetic field on the alanine signal. Data from Billas et al (ESTRO37 S461, 2018) was analyzed to give a correction factor for a 1.5 T field of 0.994. The standard uncertainty in the dose measured using the alanine was 0.9%. The difference between the delivered and measured dose was less than 1% in all cases, consistent with the measurement uncertainties, and there was no significant difference between alanine irradiations in water or in solid phantom.
Conclusion: Alanine can be used for the accurate determination of dose in MR-linac fields with an uncertainty consistent with AAPM TG-51. Measurements can be made in solid phantom, significantly simplifying this QA procedure.