Single Institution Preliminary Evaluation of a National Study for the Development of Daily Online Magnetic Resonance Image Guided Radiotherapy.

PURPOSE/OBJECTIVE(S): A 4-stage non-comparative prospective feasibility study to assess and develop imaging protocols for MRIgRT was opened at the first two centers using MR Linac technology in the UK. The primary aims of this study were to determine a) the acceptability of MR images for target and organ at risk delineation and registration; b) inter/intra observer registration and delineation variation. This work reports on the initial results from a single center.

MATERIALS/METHODS: In June 2019, following ethical and regulatory approvals the 2nd UK centre began study recruitment as follows: Stage A: non-patient volunteer imaging to determine sequence suitability for normal tissue in 6 anatomical sites (head & neck (H&N), chest wall/breast, lung/esophagus, abdomen, male and female pelvis). Volunteers were recruited in cohorts of 3 participants per region, and image quality was assessed by 3 independent observers using a visual guidance assessment tool (VGA). Stage B: the most suitable sequences defined in stage A used to assess the visibility of targets/normal tissues in patient volunteers using the same methods as in stage A. Stage C: patient volunteers were imaged using sequences selected from stage B to determine inter and intra observer segmentation and registration variation. Stage D recruitment of patient and non-patient volunteers for further image develop and refinement of MRIgRT workflows. All participants completed experience questionnaires to optimize workflows. Participants were asked to undergo 1-12 imaging sessions, lasting no more than 60.

RESULTS: To date 151 participants (61 non-patients; 90 patients) have undergone 231 imaging sessions. From stage B, vendor provided, in-workflow sequences have been agreed from 47 completed VGAs by prioritizing high scores in either the tumor (e.g., lung) or organs at risk (e.g., cervix). T2w 3D sequences scored best in cervix, pancreas, prostate, bladder, liver, soft-tissue metastases and rectal cancers; T1w 3D sequences for H&N, and patient a specific approach for lung. No suitable sequences have been selected for partial breast. Research sequences (e.g., diffusion weighted or motion corrected imaging) have been agreed or are in development in stages C & D for H&N, cervix, bladder and prostate cancers. The mean interobserver (n = 8) vector variation in 5 H&N patients was largest (3.6mm) using T1-CT boney registrations and smallest (2.1mm) using T1-T1 soft-tissue registrations (mean observer match confidence 3.7/5). Analyses using MR to CT, MR to MR and CT to CT registrations in lung, pancreas, cervix, bladder, and prostate have also been completed. Interobserver delineation studies are on-going.

CONCLUSION: Using a 4-stage non-comparative prospective feasibility study has facilitated clinical implementation MRIgRT of multiple treatment sites at our institution.