STEREOTACTIC BODY RADIOTHERAPY (SBRT) FOR SACRAL METASTASES: DEVIATION FROM RECOMMENDED TARGET VOLUME DELINEATION PREDICTS HIGHER RISK OF LOCAL FAILURE

Report outcomes after sacrum SBRT, focusing on the impact of contouring deviation on local failure (LF) risk, with an aim to validate the contouring consensus guidelines. METHODS/MATERIALS: Patients who underwent sacral SBRT from 2010- 2021 were identified from a prospectively maintained institutional database. Primary outcomes were magnetic resonance-based LF and vertebral compression fracture (VCF). RESULTS: A total of 215 treated sacral segments in 112 patients were reviewed. Most segments were treated with 30 Gy/4 fractions (51%), 24 Gy/2 fractions (31%), or 30 Gy/5 fractions (10%). Thirty-one percent of segments were of radioresistant histology (gastrointestinal, kidney, melanoma, sarcoma, or thyroid primary), and 51% had extraosseous disease. Sixteen percent of segments were under-contoured per consensus guidelines. The cumulative incidence of LF was 18.4% (95% CI 13.5-24.0) at 12-months and 23.1% (95% CI 17.6-29.0) at 24-months. On multivariate analysis (MVA), under- contouring (HR 2.4, 95% CI 1.3-4.7, p=0.008), radioresistant histology (HR 2.4, 95% CI 1,4-4.1, p=0.001), and extraosseous extension (HR 2.5, 95% CI 1.3-4.7, p=0.005) were predictors of increased risk of LF. The LF rates at 12/24-months were 15.1%/18.8% for segments contoured per guideline versus 31.4%/40.0% for those under-contoured. The cumulative incidence of VCF was 7.1% (95% CI 4.1-11.1) at 12-months and 12.3% (95% CI 8.2-17.2) at 24-months. On MVA, female gender predicted increased VCF risk (HR 2.3, 95% CI 1.1-5.2, p=0.04). CONCLUSIONS: Sacral SBRT is associated with high rates of efficacy and an acceptable VCF risk. Adherence to target volume delineation consensus guidelines reduces the risk of LF.