L. Burgess1, H. Chen1, E. Atenafu2, B. Zhang1, L. Zeng1, D. Dinakaran1, C. L. Tseng1, J. Detsky1, S. D. Myrehaug1, H. Soliman1, J. Larouche3, and A. Sahgal1; 1Department of Radiation Oncology, Odette Cancer Centre, Sunnybrook Health Sciences Centre, University of Toronto, Toronto, ON, Canada, 2Department of Biostatistics, University Health Network, University of Toronto, Toronto, ON, Canada, 3Division of Orthopedic Surgery, Sunnybrook Health Sciences Centre, Toronto, ON, Canada
Purpose/Objective(s): The most common complication following spine stereotactic body radiotherapy (SBRT) is vertebral compression fracture (VCF). The purpose of this analyses was to determine if predictors of VCF vary according to presence or absence of a pre-existing (baseline) VCF. Materials/
Methods: A retrospective review of a prospectively maintained institutional database of patients treated with SBRT for spinal metastases performed. The primary outcome was VCF. Clinical, dosimetric and radiographic factors were reported with descriptive statistics. The cumulative incidence of VCF was estimated using a competing risk analysis method. The impact of covariates was estimated with Cox proportional hazards model and hazard ratios (HR) generated. Results: From 2008 – 2022, 744 patients with 1813 spinal segments were treated with spine SBRT. The median age was 64.9 years, and 19.1% had a baseline VCF. The majority of segments were treated with 24Gy (42.8%) or 28Gy (27.4%) in 2 fractions, the median dose-per fraction was 12Gy (range 5-24), 68% were denovo (no prior radiation exposure) and 235 (13.0%) had prior surgical stabilization. 254 VCF event were observed (14%), 179 were iatrogenic and 75 associated with concurrent tumour progression. The 1-, 2- and 5-year VCF rates were 8.3%, 12%, and 15.2%, respectively. 85/254 (33.5%) occurred in those with a baseline VCF and 169/254 (66.5%) in those without. On multivariable analysis (MVA), tumor progression (HR 2.24, 95% CI 1.66-3.04, p<0.01), baseline VCF (HR 1.91, 95% CI 1.35-2.71, p<0.01), mechanical pain (HR 2.01, 1.31-3.08, p<0.01), increasing age (HR 1.015, 95% CI 1.002-1.03, p<0.02), fewer consecutive segments treated (HR 0.80, 95% CI 0.65-0.98, p<0.03), increasing dose to 90% (D90) of the clinical target volume (CTV) as equivalent dose in 2-Gy fractions (EQD2) (CTV D90 EQD2, HR 1.007, 95% CI 1.002-1.01, p<0.02) predicted for VCF. Predictors common to segments treated with or without a baseline VCF included concurrent tumour progression and an increasing CTVD90 EQD2. No prior stabilization surgery, involvement of the posterior elements, fewer consecutive segments treated were unique to the baseline VCF cohort, while histology, mechanical pain and higher dose per fraction were unique predictors in the no prior VCF cohort. Conclusion: We report predictive factors for VCF following spine SBRT and observe distinct risk factors according to presence or absence of a baseline VCF that can guide treatment decisions.
