Initial experience with MR-guided adaptive spinal stereotactic radiotherapy: a new indication for the MR-linac

Background and purpose Stereotactic body radiotherapy (SBRT) is associated with good local control and symptom relief in the management of spinal metastases. Delivery of ablative doses and re-irradiation is challenged by spinal cord toxicity. We hypothesized that lower spinal cord doses as well as better target coverage could be yielded with stereotactic magnetic resonance-guided adaptive radiotherapy (SMART). Materials and methods Institutional records were reviewed to retrieve patients who received online MR-guided SBRT for spinal metastases. Each fraction was reviewed to determine the necessity of adaptive planning, to identify reasons for violations that required adaptive planning, and to assess the spinal cord dose. The study also evaluated how adaptive planning contributed to reducing spinal cord doses. Results A total of 34 patients with 61 lesions were included. The treatment intent was definitive for 47 (77.1%), palliative for 12 (19.7%), and postoperative for two (3.3%) lesions. The median prescribed Biological Equivalent Dose (BED)10 was 51.3 Gy. Treatment plans often required adaptive adjustments (81.8%). Adaptive planning significantly improved target coverage (median PTV coverage 92.75% vs. 95%; p < 0.001) and reduced spinal cord Dmax (median spinal cord Dmax constraint: 7.3 Gy, median predicted spinal cord Dmax: 7.76, and median adaptive spinal cord Dmax 6.18; p < 0.001). Lesion-based median follow-up from irradiation was 7.5 months (range: 1-46 months). One-year LPFS was 94.3%. Six lesions progressed and none of the progressed lesions received a dose above the median BED10 of 51.3 Gy. Conclusion Herein we present our institutional experience with SMART for spinal bone metastases. According to our results, adaptive planning yields better target coverage as well as lower spinal cord doses compared to the predicted plan, which translates into a feasible method for delivering SBRT. Future prospective studies evaluating spinal SMART are awaited.