Objectives:

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 which is further complicated by the accurate visualization of spinal cord in CT-based treatment plans and linear accelerators. We hypothesized that lower spinal cord doses as well as better target coverage could be yielded with Stereotactic MR-Guided Adaptive Radiotherapy (SMART which has the advantages of MRI visualization and online adaptive radiotherapy. In this study we have documented our institutional results of spine SMART with the ViewRay MRIdian®.

Methods:

Institutional records were reviewed to retrieve the patients who received spinal SBRT in the ViewRay MRIdian® between 2018-2023. Patients’ characteristics were collected and lesion-based analyzes were performed to reach the intent of treatment, dose and fractionation, spinal cord dose and local control. Afterwards, all fractions were individually reviewed to generate the information regarding whether adaptive planning was needed and the violation that necessitated adaptive planning as well as spinal cord tolerance dose for each fraction. The impact of adaptive planning on spinal cord dose reduction was looked over.

Results:

A total of 61 lesions were irradiated in 34 patients with a median of 1 lesion (range 1-5 lesions) per patient. Six lesions were cervical, 32 lesions were thoracal, 22 lesions were lumbar and one lesion was sacral vertebral origin. Median age was 61.5 years (range: 40-83 years). Forty-seven (77.1%) of the lesions were oligometastases and 7 of them were reirradiation; 12 (19.7%) lesions were irradiated palliatively and 2 (3.3%) lesions were irradiated postoperatively. Seventeen of the lesions were symptomatic (pain:17, neurological symptom:2). Median BED10 of the prescribed doses were 51.3 Gy (range: 28-100 Gy). Median spinal cord Dmax in original plans were 12.02 Gy (10.29-13.66 Gy) for 1 fraction (n=4); 16.37 Gy (1.7-18.06 Gy) for 2 fractions (n=4), 16.2 Gy (0-21.03 Gy) for 3 fractions (n=45); 10.65 Gy for 4 fractions (n=1) and 14.16 Gy (0-21.24 Gy) for 5 fractions (n=7) treatments. In 153 of the 187 fractions (81.8%), treatments were performed with re-optimized plans. PTV coverage in the adapted plans improved compared to predicted plans (median PTVreopt 95% vs. median PTVpredict 92.75%, p< 0.001). In 24 (12.83%) of the predicted plans, spinal cord doses were violated with a median spinal cord Dmax constraint of 7.3 Gy; median predicted spinal cord Dmax of 7.76 and median adaptive spinal cord Dmax of 6.18 (p< 0.001). No grade >2 acute side effect were observed. Lesion-based median follow-up from the irradiation was 7.5 months (range: 1-46 months). Symptom relief was achieved in all but 2 of the lesions. Median SBRT dose was 24 Gy (range 20-35 Gy) given in median 3 fractions (range 1-5 fractions). Metabolic complete and partial response were achieved in 40 (65.6%) and 11(18.0%) lesions respectively whereas three (4.9%) lesions progressed and seven (11.48%) lesions were not evaluated. One-year local progression free survival was 94.3%. Local recurrence occurred in five lesions (8.2%) and all received doses below the median dose (24 Gy).

Conclusion(s):

SMART to spinal bone metastases is feasible and adaptive planning yields improved target coverage and reduced spinal cord doses which translates to an efficient and safe method to deliver SBRT.