Commissioning Measurements On An Elekta Unity MR-Linac

Abstract

MR-guided radiotherapy technology is relatively new and commissioning publications, QA protocols and commercial products are limited. This work provides guidance for implementation measurements that may be performed on the Elekta Unity MR-Linac (Elekta, Stockholm, Sweden). Adaptions of vendor supplied phantoms facilitated determination of gantry angle accuracy and MV isocentre, whereas in-house developed phantoms were used for End-to-End (E2E) testing and anterior coil attenuation measurements. Third-party devices were used for measuring beam quality, reference dosimetry and during IMRT commissioning; however, due to several challenges, variations on standard techniques were required. Gantry angle accuracy was within 0.1°, confirmed with pixel intensity profiles, and MV isocentre diameter was < 0.5 mm. Anterior coil attenuation was < 0.6 %. Beam quality as determined by TPR20,10 was 0.704 ± 0.002, in agreement with treatment planning system (TPS) calculations, and gamma comparison against the TPS for a 22.0 × 22.0 cm2 field was above 95.0 % (2.0 %, 2.0 mm). G90 output was 1.000 ± 0.002 Gy per 100 MU, depth 5.0 cm. During IMRT commissioning, sub-standard results indicated issues with machine behaviour. Once rectified, gamma comparisons were above 95.0 % (2.0 %, 2.0 mm). Centres which may not have access to specialized equipment can use in-house developed phantoms, or adapt those supplied by the vendor, to perform commissioning work and confirm operation of the MRL within published tolerances. The IMRT QA devices and techniques used in this work highlight issues with machine behaviour when appropriate gamma criteria are set.