Validation of a 4D-MRI guided liver SBRT strategy for implementation on the MR-linac.

Accurate tumor localization for image-guided liver SBRT is challenging due to respiratory motion and poor tumor visibility on conventional X-ray based images. Novel integrated MRI and radiotherapy systems enable direct in-room tumor visualization, potentially increasing treatment accuracy. As these systems currently do not provide a 4D image-guided radiotherapy strategy, we developed a 4D-MRI guided liver SBRT workflow and validated all steps for implementation on the Unity MR-linac. The proposed workflow consists of five steps: 1) acquisition of a daily 4D-MRI scan, 2) 4D-MRI to mid-position planning-CT rigid tumor registration, 3) calculation of daily tumor midP misalignment, 4) plan adaptation using adapt-to-position with segment-weights optimization and 5) adapted plan delivery. The workflow was first validated in a motion phantom, performing regular motion at different baselines (±5 to ±10 mm) and patient-derived respiratory signals with varying degrees of irregularity. 4D-MRI derived respiratory signals and 4D-MRI to planning CT registrations were compared to the phantom input, and gamma and dose-area-histogram analyses were performed on the delivered dose distributions on film. Additionally, 4D-MRI to CT registration performance was evaluated in patient images using the full-circle method (transitivity analysis). Plan adaption was further analyzed in-silico by creating adapted treatment plans for 15 patients with oligometastatic liver disease. Phantom trajectories could be reliably extracted from 4D-MRI scans and 4D-MRI to CT registration showed submillimeter accuracy. The DAH-analysis demonstrated excellent coverage of the dose evaluation structures GTV and GTVTD. The median daily rigid 4D-MRI to midP-CT registration precision in patient images was <2 mm. The adapt-to-position strategy restored the target dose without increased exposure to the OARs and plan quality was independent from 3D shift distance in the range of 1 - 26 mm. The proposed 4D-MRI guided strategy showed excellent performance in all workflow tests in preparation of the clinical introduction on the Unity MR-linac.