A FIRST REPORT ON OUTCOMES OF STEREOTACTIC BODY RADIOTHERAPY FOR POSTERIOR ELEMENT ONLY SPINAL METASTASES AND VALIDATION OF RECOMMENDED CLINICAL TARGET VOLUME DELINEATION PRACTICE

Spine stereotactic body radiotherapy (SBRT) improves local control and pain response compared to conventional external beam radiotherapy. Consensus stipulates MRI-based delineation of the clinical target volume (CTV) and is based on spine segment sector involvement. The applicability to metastases limited to the posterior elements remains to be validated. We aimed 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. The primary outcome was local failure and secondary outcomes included patterns of failure, toxicities. RESULTS: 24/605 patients and 31/1412 segments were treated to the posterior elements only. Local failure occurred in 11/31 segments. The cumulative rate of local recurrence was 9.7% at 12 months and 30.8% at 24 months. Amongst 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. 6/11 (54.5%) failed exclusively within treated CTV sectors and 5/11 (45.5%) with both treated and adjacent untreated sectors. 4/5 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.