(OA26) Significance of Hormone Therapy & Bisphosphonate Use on Vertebral Compression Fracture (VCF) Incidence Following Spine Stereotactic Body Radiation Therapy (SBRT) for Breast Cancer Metastases

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Spinal metastases are increasingly being treated with SBRT. Though effective, a potential toxicity of SBRT is VCF. Breast cancer (BC) is one of the most common primary sites to metastasize to the spine, & due to factors such as HT use, these patients may be at higher risk for VCF following SBRT. Our study aimed to determine the rate of SBRT induced VCF (S-VCF) in BC patients, & to understand if there is a significant relationship between hormone therapy (HT) & bisphosphonate (Bp) use on this risk. 335 vertebral bodies (VB) (n=131 patients) were treated with spine SBRT for BC metastases at a single institution between 06/2001 & 12/2014. EMRs were retrospectively reviewed in this IRB approved study, & data on the use of Bp, Selective Estrogen Receptor Modulators (SERMs), & Aromatase Inhibitors (AIs) were recorded. Development of a new VCF, progression of an existing VCF & requirement of stabilization surgery after SBRT were the primary endpoints of this study. These were evaluated using CT & MRI. Only VCF development/progression occurring in VB treated with SBRT were considered. VBs with concurrent tumor progression, or surgery prior to SBRT were not included. Statistical significance of VCF incidence, & the use of Bp & HT concurrent with, within 6 m, 1 yr, & 2 yrs of SBRT, was evaluated using the Chi-Square Test for Independence, & Fisher’s Exact Test. Follow up was available for 91 patients (69%) & 233 VB. Median survival post SBRT was 21 m. 58.8% of patients were Caucasian, 33.6% African American, & 7.6% were of other ethnicities. Bone density was low in 59% of patients. Median SBRT dose was 18 Gy/1 fx. Median tumor volume was 35.83 cc. Median follow-up was 15.6 m. 33 VCFs in 20 patients were observed. 48% were new, 24% progressed, & 27% were VB that required surgical stabilization after SBRT. The overall S-VCF rate was 14%. 70 patients (192 VB), 50 patients (146 VB), & 78 (221 VB) were exposed to AIs, SERMs, & Bp, with S-VCF rates of 14.6%, 14%, & 13% respectively. The following were found to be statistically significant in relation to S-VCF: SERM use concurrent (p= .003), within 6 m of (p= .028), & within 2 yrs of (p= .021) SBRT. Also significant was Bp use within 1 yr of SBRT (p< .0001). AI use was not found to be statistically significant for any of the time frames evaluated in our study. Rate of developing a VCF in our cohort was 14%. The overall rate of S-VCF in BC patients is not considerably higher than rates previously reported in non-histology specific series. We will further analyze our cohort to better understand the protective or adverse effects of HT & Bp use in BC patients receiving spine SBRT. To the best of our knowledge, this is the only reported series analyzing S-VCF in BC with an emphasis on the relation of Bp & HT use.