The aim of this study was to identify potential risk factors for and determine the rate of vertebral compression fracture (VCF) after intensity-modulated, near-simultaneous, CT image-guided stereotactic body radiotherapy (SBRT) for spinal metastases.

The study group consisted of 123 vertebral bodies (VBs) in 93 patients enrolled in prospective protocols for metastatic disease. Data from these patients were retrospectively analyzed. Stereotactic body radiotherapy consisted of 1, 3, or 5 fractions for overall median doses of 18, 27, and 30 Gy, respectively. Magnetic resonance imaging studies, obtained at baseline and at each follow-up, were evaluated for VCFs, tumor involvement, and radiographic progression. Self-reported average pain levels were scored based on the 11-point (0-10) Brief Pain Inventory both at baseline and at follow-up. Obesity was defined as a body mass index ≥ 30.

The median imaging follow-up was 14.9 months (range 1-71 months). Twenty-five new or progressing fractures (20%) were identified, and the median time to progression was 3 months after SBRT. The most common histologies included renal cancer (36 VBs, 10 fractures, 10 tumor progressions), breast cancer (20 VBs, 0 fractures, 5 tumor progressions), thyroid cancer (14 VBs, 1 fracture, 2 tumor progressions), non-small cell lung cancer (13 VBs, 3 fractures, 3 tumor progressions), and sarcoma (9 VBs, 2 fractures, 2 tumor progressions). Fifteen VBs were treated with kyphoplasty or vertebroplasty after SBRT, with 5 procedures done for preexisting VCFs. Tumor progression was noted in 32 locations (26%) with 5 months' median time to progression. At the time of noted fracture progression there was a trend toward higher average pain scores but no significant change in the median value. Univariate logistic regression showed that an age > 55 years (HR 6.05, 95% CI 2.1-17.47), a preexisting fracture (HR 5.05, 95% CI 1.94-13.16), baseline pain and narcotic use before SBRT (pain: HR 1.31, 95% CI 1.06-1.62; narcotic: HR 2.98, 95% CI 1.17-7.56) and after SBRT (pain: HR 1.34, 95% CI 1.06-1.70; narcotic: HR 3.63, 95% CI 1.41-9.29) were statistically significant predictors of fracture progression. On multivariate analysis an age > 55 years (HR 10.66, 95% CI 2.81-40.36), a preexisting fracture (HR 9.17, 95% CI 2.31-36.43), and baseline pain (HR 1.41, 95% CI 1.05-1.9) were found to be significant risks, whereas obesity (HR 0.02, 95% CI 0-0.2) was protective.

Stereotactic body radiotherapy is associated with a significant risk (20%) of VCF. Risk factors for VCF include an age > 55 years, a preexisting fracture, and baseline pain. These risk factors may aid in the selection of which spinal SBRT patients should be considered for prophylactic vertebral stabilization or augmentation procedures. Clinical trial registration no.: NCT00508443.