Predictors of Vertebral Compression Fracture Following Stereotactic Body Radiation Therapy in SINS Potentially Unstable Spinal Metastases.

Abstract

The Spinal Instability in Neoplasia Score (SINS) is the gold standard to determine if the metastatically involved spine is stable, potentially unstable, or frankly unstable. In potentially unstable spines, clarity is needed about the risk of post-stereotactic body radiation therapy (SBRT) vertebral compression fracture (VCF) and which patients may benefit from early stabilization. We aimed to identify predictors of VCF following spine SBRT in patients with potentially unstable SINS spinal metastases.. A retrospective review of a prospectively maintained database of patients treated with SBRT for spinal metastases from January 2008 to December 2022 was performed. This analysis included only spine segments categorized as potentially unstable (SINS 7-12). The primary outcome was the rate of VCF. The cumulative incidence of VCF and the impact of covariates were estimated. Five hundred twenty-four patients with 976 treated spinal segments were SINS potentially unstable. Out of 976, 168 patients (17.2%) experienced a VCF after SBRT. Out of 168, 107 patients (63.7%) were iatrogenic and 61 (36.3%) concurrent with tumor progression. The 12-month incidence of iatrogenic VCF was 9.3% (95% CI, 7.4%-11.5%) as opposed to 23.4% (95% CI, 17.4%-29.9%) when concurrent with tumor progression (P < .0001). Multivariable analysis confirmed iatrogenic VCF associated with pre-existing VCF (hazard ratios [HR] = 1.83; 95% CI, 1.235-2.714; P = .003), no previous spine surgery (HR = 1.67; 95% CI, 1.024-2.710; P = .040), SINS total ≥10 (HR = 1.68; 95% CI, 1.122-2.512; P = . 012), and an increasing D90 clinical target volume in equivalent dose in 2 Gy (HR = 1.03; 95% CI, 1.010-1.055; P = .004). In the setting of concurrent tumor progression, only an increasing D90 to the clinical target volume in equivalent dose in 2 Gy fractions (HR = 1.04; 95% CI, 1.013-1.076; P = .005) predicted for VCF. Tumor control outweighs the risk of VCF associated with spine SBRT in potentially unstable metastases. Prophylactic stabilization could be considered in segments with a total SINS ranging from 10 to 12, a pre-existing VCF, and when treating with high doses.