2492 - Predictors of Vertebral Compression Fracture Following Spine Stereotactic Body Radiotherapy Vary with Cause: Iatrogenic or in Conjunction with Local Failure

L. Burgess¹, E. Atenafu², B. Zhang¹, L. Zeng¹, D. Dinakaran¹, C. L. Tseng¹, J. Detsky¹, S. D. Myrehaug¹, H. Soliman¹, J. Larouche³, A. Sahgal¹, and H. Chen¹; 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). Iatrogenic VCF has been widely reported, but it has not yet been reported in the context of tumor progression. We hypothesized that predictors of VCF vary with cause, whether iatrogenic or in conjunction with local failure. Materials/

Methods: A prospectively maintained institutional database of patients treated with SBRT for spinal metastases was retrospectively reviewed. The primary outcome was VCF, with or without local failure. Clinical, dosimetric and radiographic factors were reported with descriptive statistics. The cumulative incidence of iatrogenic VCF and VCF with tumor progression 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 median follow up was 19.9 months. Of the 254 VCF events (14%), 179 (70.5%) were iatrogenic and 75 (29.5%) associated with concurrent tumor progression. Median time to iatrogenic VCF was 10.1 months and 9.0 months in those with tumor progression. The majority of segments that had an iatrogenic VCF or with tumor progression were treated with 24Gy in 2 fractions (41.8% and 41.3%, respectively) and the median dose per fraction was 12Gy in both (range 5-24). The 1- and 2-year iatrogenic VCF rates were 6.8% and 9.9%, respectively, and 15.6% and 22.0% in those concurrent with tumor progression, respectively. On multivariable analysis (MVA), spinal level and increasing dose to 90% (D90) of the clinical target volume (CTV) as equivalent dose in 2-Gy fractions (EQD2) were the only predictors common to both iatrogenic VCF and those secondary to tumor progression. Risk factors that uniquely increased the risk of iatrogenic VCF were presence of a baseline VCF (HR 1.85, 95% CI 1.25-2.74, p=0.002), increasing consecutive segments treated (HR 0.76, 95% CI 0.64-0.90, p<0.01), and older age (HR 1.02, 95% CI 1.002-1.03, p=0.03). Meanwhile, epidural disease (HR 2.84, 95% CI 1.68-4.79, p<0.001), mechanical pain (HR 3.33, 95% CI 1.53-7.25, p<0.01) and fewer SBRT fractions (HR 0.65, 95% CI 0.45-0.93, p<0.02) were predictive of VCF in those concurrent with tumor recurrence.
Conclusion: This first report of predictive factors for VCF following spine SBRT, according to an iatrogenic cause or concurrent with tumour progression, may improve clinical decision making in VCF management.