Technical feasibility and clinical evaluation of 4D-MRI guided liver SBRT on the MR-linac

Purpose: Image-guided stereotactic body radiation therapy (SBRT) is an important local treatment for liver metastases. MRI-guidance enables direct tumor visualization, eliminating fiducial marker implantation. The purpose of this study was to test technical feasibility of our 4D-MRI guided liver SBRT workflow. Additionally, intra-fraction target motion and consequent target-coverage were studied. Materials & methods: Patients with liver metastases were included in this sub-study of the prospective UMBRELLA-II clinical trial. Patients received mid-position (midP) SBRT. The daily adapt-to-position workflow included localization, verification and intra-fraction tumor midP monitoring using 4D-MRI. Technical feasibility was established based on persistence of the treatment protocol, treatment time similar to 1 h, no geographical miss and no unexpected acute toxicity grade >3. All 4D-MRIs were registered to the planning midP-CT and tumor midP and amplitude were calculated. Additionally, delivered target dose was accumulated incorporating the 4D-MRI intra-fraction tumor motion and evaluated with Monte-Carlo error simulations. Results: 20 patients with liver metastases were included and treated with 4D-MRI guided SBRT. Feasibility criteria were met in all-but-one patient. No grade similar to 3 acute toxicity was observed. Group mean (M), systematic and random midP-drifts were 2.4 mm, 2.6 mm and 3.1 mm in CC-direction. 4D-MRI tumor CC-amplitudes were reduced compared to the simulation 4D-CT (M = similar to 1.9 mm) and decreased during treatment (M =similar to 1.4 mm). Dose accumulation showed adequate target-coverage on a population level. Conclusion: We successfully demonstrated technical feasibility of 4D-MRI guided SBRT in a cohort of 20 patients with liver metastases. However, substantial midposition drifts occurred which stress the need for intra-fraction motion management strategies to further increase the precision of treatment delivery. (C) 2022 Elsevier B.V. All rights reserved.

Automated summary

This study aimed to test the technical feasibility of 4D-MRI guided liver SBRT and investigate target motion and coverage during treatment. The results showed that 4D-MRI guided SBRT was feasible in treating liver metastases, but the occurrence of midposition drifts highlighted the need for motion management strategies to enhance treatment precision.