LAUSR

Purpose The introduction of MR-guided radiotherapy (MRgRT) into clinical practice opens a lot of new possibilities and at the same time presents novel challenges. Funded by the German Research Foundation, a Viewray MRIdian Linac was installed in our department in 2017 with the first patient treatments in April 2018. Methods The Viewray MRIdian Linac combines a 0.345T magnetic resonance imaging system with a 6 MV FFF linac. Fields are shaped by a double-stack, double-focus MLC, enabling the delivery of static conformal beams as well as step-and-shoot IMRT. 3D-MRI scans performed prior to treatment are used for patient positioning as well as adaption of treatment plans. Additionally, single slices of cine MRI can be continuously acquired during treatment and used for gating. Results The commissioning tests can be divided into RT-tests, MR-tests and tests for the combination of both. RT-related tests in general followed existing recommendations for commissioning and quality assurance (QA), but needed to be adapted due to the presence of the magnetic field. For the MR-related tests of the MRIdian system, the customer is presented with a set of MR-phantoms and test recommendations by the vendor. Those verifications comprised tests of field homogeneity, spatial integrity, uniformity and signal-to-noise ratio. Other MR-related tests as the ACR-test may be adapted from diagnostic MRI. Regarding the combination of MR and RT, one important test was for example the coincidence of both isocenters. Furthermore, phantoms for RT-QA with adequate MR imaging contrast were developed and evaluated. Regarding the clinical integration, we follow a step-by-step concept. In the first step, patients with not too complicated, i.e. 3D conformal treatment plans are treated, in order to enable the whole team to get acquainted with the workflow and all novel concepts and processes coming along with daily MR imaging. For the second step, inclusion of IMRT treatments as well as gating are planned, which both should involve additional QA tests. Thirdly, the goal is to implement on-table adaption of treatments, which will also need additional verification. Conclusions Commissioning tests and workflow considerations were presented for the clinical deployment of an MR-Linac.