Towards rapid and efficient simulation-free radiotherapy: MR guided adaptive prostate radiotherapy on the MR-Linac...

Simulation-free radiotherapy offers improved efficiency for adaptive treatments. This planning study presents a simulation-free pre-treatment workflow for prostate cancer online adaptive radiotherapy on a MR-Linac. Previously acquired diagnostic MR images are used to create a reference treatment plan without clinician input, and adapted plans are then simulated, and compared with those from the traditional workflow.

All patients treated with 36.25 Gy in 5-fractions within the HERMES trial were retrospectively assessed for eligibility of simulation-free reference planning. If eligible, reference images were created from existing diagnostic MRI (T1w and T2w) to enable MR-only reference treatment planning. Target and OAR reference structures were autosegmented without clinician input. Online plan adaptation was simulated using existing clinical treatment images and structure sets. Adapted plans were compared with existing clinical plans, and synthetic CT accuracy assessed.

87.5 % of patients had suitable diagnostic scans. Reference images and treatment plans were successfully created. Online treatment plans were simulated and were clinically acceptable for target dose and conformality, meeting all mandatory clinical goals with no detriment to OAR dose or plan deliverability. Accuracy of the synthetic CT approach was high with gamma results at 2mm/2% all above 98.9 %.

This study has shown that non radiotherapy-dedicated diagnostic MRI can be used for reference prostate planning on the MR-Linac, generating clinically equivalent adapted plans when compared to those originating from radiotherapy-simulation reference plans. This potentially saves multiple weeks in the pathway, improving radiotherapy efficiency and patient experience.