MR-Linac Radiotherapy in Year One: Experience's in Imaging, Patient Treatment and Data Collection

Abstract

To present our institutions initial clinical experience with a high field MR-Linac with regards to patients treated, data collected, functional imaging performed and general performance of the machine. From Aug 15, 2019 to Feb 7, 2020, thirty-three patients were treated on our 1.5 T MR-Linac with all patients enrolled in the MOMENTUM study (ClinicalTrials.gov, identifier: NCT04075305). Two types of workflows were used: one accounting for daily patient shifts only (adapt to position or ATP) and the other involving online plan adaptation through re-contouring and re-optimization based on daily MRI images (adapt to shape or ATS). Of the thirty-three patients, thirty were CNS brain patients (15 GBM and 15 other) treated using ATP and three were prostate cancer patients treated using ATS and SBRT to a dose of 40 Gy in 5 fractions. Imaging involved T1 (brain ATP) or T2 (prostate ATS) weighted scans captured online for registration/planning. Functional imaging sequences for brain ATP (DWI, MT, BOLD, CEST) were taken before, during and post irradiation in addition to T2-FLAIR. DWI was done for the prostate cases before, during and post irradiation. Timing was recorded for the various stages of the workflows. Machine performance was characterized in terms of unscheduled downtime events. A total of 540 fractions were delivered on the MR-Linac over its initial 25 weeks of use. The average session time (n = 200 fractions) for the brain ATP workflow (excluding post imaging) was 26.9 minutes (3.7 min setup, 5.5 min pre-tx imaging, 5.1 min image registration, 4.5 min plan optimization, 2.0 min physics QA, and 6.1 min beam on). The minimum session time was 24 minutes for the brain ATP. The average session time for the prostate ATS workflow (n = 10 fractions) was 47.7 minutes (6.6 min setup, 3.2 min pre-tx imaging, 3.5 min image registration, 15.1 min contouring, 5.6 min plan optimization, 1.5 min physics QA, 2.9 min verification imaging and 9.3 min beam on). Additional post beam research scans took, on average, 12.5 minutes for the prostate cases. The following multi-parametric imaging maps were generated for the brain patients: T1 and T2, ADC, T2-FLAIR, CEST asymmetry and Amide MTR. The brain T2-FLAIR signal, in some cases, reduced over the treatment course. Changes in ADC maps were observed for both brain and prostate over the treatment. In some instances, brain re-planning was done offline based on daily online T2-FLAIR imaging. In terms of downtime, patients were transferred to a standard Linac for 7.1 % of fractions due to either a magnetron change, modulator tank replacement, or a cooling issue. An MR-Linac program has been implemented at our institution involving a multi-disciplinary group with both machine data and patient data being captured. The current status, clinical and technical considerations, and most recent image-based findings will be presented.