Sacral fracture risk after stereotactic spinal radiosurgery: a multi-institution, retrospective analysis.

Objective: Spinal stereotactic radiosurgery (SSRS) is an established treatment modality for spinal metastatic disease. A common complication of SSRS is postradiation pathologic fracture. The authors evaluated the risk of pathologic fracture in sacral SSRS.

Methods: All patients receiving SSRS in 1 to 5 fractions to 1 or more sacral levels between 2011 and 2020 at participating institutions were included. Demographic, histological, anatomical, and outcome data were obtained from electronic medical records. Data were analyzed using SPSS version 28.

Results: Of 67 total patients, 50 (74.6%) were men and 17 (25.4%) were women. The most common tumor histologies were prostate adenocarcinoma (n = 16 [23.9%]) and renal cell carcinoma (n = 12 [17.9%]). Median (range) age at treatment was 63 (8-87) years. The mean ± SD (range) number of levels treated was 1.5 ± 0.7 (1-4) levels. Mean ± SD (range) bone density at S1 was 208.0 ± 74.4 (-3.0 to 443.3) Hounsfield units (HUs). Median (range) follow-up was 16.6 (1.5-132) months, median ± SD (range) overall survival was 24 ± 21.0 (1.7-83.3) months, and the 1-year local control rate was 83.6%. Fourteen patients (20.9%) experienced fracture within the radiation field. There was no association between fracture and patient sex (p = 0.32), age (p = 0.40), S1 HU (p = 0.28), tumor location according to Denis classification (p = 0.87), presence of pain (p = 0.66), single versus multifraction radiation (p = 0.44), or number of levels treated (p = 0.52). Patients experiencing in-field progression were more likely to experience fracture (61.5% vs 10.2%, p < 0.001), and patients experiencing fracture had a higher average Spine Instability Neoplastic Score than patients without fracture (5.7 vs 4.3, p = 0.049). Only local control remained as a factor associated with sacral fracture in the multivariate analysis. Of the patients experiencing fracture, 10 (71.4%) were symptomatic and 3 of those (30.0%) underwent post-SSRS surgical intervention.

Conclusions: Pathologic fracture may occur after sacral SSRS in a significant number of patients and is significantly associated with failure of local control.