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 ≤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 ≥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 = -1.9 mm) and decreased during treatment (M = -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.

MRI-guided mid-position liver radiotherapy: Validation of image processing and registration steps.

BACKGROUND & PURPOSE: To propose a novel mid-position (midP) workflow for MRI-guided liver SBRT and provide a validation of the required midP-MRI generation and registration steps. MATERIALS & METHODS: The first step of the midP workflow is the generation of a simulation midP-MRI from a 4D-MRI scan using deformable image registration. Next, a planning midP-CT is warped to the midP-MRI to enable planning in the midP-MRI anatomy. For daily MRI-guidance, three different registration methods to the simulation midP-MRI are proposed; (1) 4D rigid registration of all phases of the daily 4D-MRI, (2) 3D rigid registration of the daily midP-MRI, and (3) 3D deformable registration of the daily midP-MRI. The midP-MRI image quality was assessed with respect to 4D-MRI acquisition time, which is related to over-sampling of the data acquisition (i.e. number of dynamics). The deformable registration precision for the midP-MRI generation was validated using the distance discordance metric (DDM). The deformable CT-MRI and daily MRI-MRI registration accuracies were quantified using the 'full circle method'. RESULTS: The DDM was 1.5 mm (median) within the liver, independent of the number of dynamics. The root-mean-squared difference between midP-MRIs based on 10 and 60 dynamics was only 5.2%. The full circle CT-MRI deformable registration error had a median 3D vector length of 1.8 mm in the liver. The daily MRI-MRI registration error was submillimeter for all three evaluated methods. CONCLUSION: The feasibility of an MRI-guided mid-position workflow for liver SBRT is supported by the demonstrated high precision of all image processing and registration steps.