Stereotactic Body Radiation Therapy for Posterior Element-Only Spinal Metastases: A First Report on Outcomes and Validation of Recommended Clinical Target Volume Delineation Practice

PURPOSE: Spine stereotactic body radiation therapy (SBRT) results in improved local control and pain response compared with conventional external beam radiation therapy. Consensus exists stipulating that magnetic resonance imaging-based delineation of the clinical target volume (CTV) is critical and based on spine segment sector involvement. The applicability of contouring guidelines to metastases involving the posterior elements alone remains to be validated, and the purpose of this report was to determine the patterns of failure and safety of treating posterior element metastases when the vertebral body (VB) was intentionally excluded from the CTV. METHODS AND MATERIALS: A retrospective review of a prospectively maintained database of 605 patients and 1412 spine segments treated with spine SBRT was performed. Only treated segments involving the posterior elements alone were included for the analyses. The primary outcome was local failure, as per SPINO recommendations, and secondary outcomes included patterns of failure and toxicities. RESULTS: In total, 24 of 605 patients and 31 of 1412 segments were treated to the posterior elements only. Local failure occurred in 11 of 31 segments. The cumulative rate of local recurrence was 9.7% at 12 months and 30.8% at 24 months. Among local failures, the most common histologies were renal cell carcinoma (36.4%) and non-small cell lung cancer (36.4%), and 73% had baseline paraspinal disease extension. A total of 6 of 11 (54.5%) failed exclusively within treated CTV sectors and 5 of 11 (45.5%) with both treated and adjacent untreated sectors. Four of these 5 cases had recurrent disease extending into the VB, but no failure was observed exclusively within the VB. CONCLUSIONS: Posterior element alone metastases are rare. Our analyses support SBRT consensus contouring guidelines such that the VB can be excluded from CTV in spinal metastases confined to the posterior elements.