Dose-Escalated Two-Fraction Spine Stereotactic Body Radiotherapy: 28 Gy vs. 24 Gy in 2 Daily Fractions

Abstract

<h3>Purpose/Objective(s)</h3> Stereotactic body radiotherapy (SBRT) for spine metastases improves pain response rates and local control compared to conventional external beam radiotherapy, however, the optimal fractionation schedule is unclear. We report local control and toxicity outcomes after dose-escalated two-fraction spine SBRT. <h3>Materials/Methods</h3> A prospectively maintained institutional database of over 600 patients and 1400 vertebral segments treated with spine SBRT, using an established treatment technique, was reviewed to identify those treated with 28 Gy or 24 Gy in two daily fractions. The primary endpoint was MRI-based local failure (LF), and secondary endpoints included overall survival (OS) and vertebral compression fracture (VCF). <h3>Results</h3> A total of 947 treated vertebral segments in 482 patients were identified, of which 159 patients (301 segments, 31.8%) received 28 Gy and 323 patients (646 segments, 68.2%) received 24 Gy in 2 fractions. Median follow-up was 23.5 months and median OS was 49.1 months. In the 28 Gy cohort, the 6-, 12-, and 24-month cumulative incidences of local failure were 3.5%, 5.4% and 11.1%, respectively, vs. 6.0%, 12.5% and 17.6% in the 24 Gy cohort, respectively (p=0.0075). On multivariable analysis, 24 Gy (HR: 1.572, 95%CI: 1.08-2.30, p=0.0196), epidural disease (HR: 1.522, 95%CI: 1.10-2.10, p=0.0108), and paraspinal extension (HR: 1.517, 95%CI: 1.08-2.12, p=0.0151) predicted for greater rates of local failure. Risk of VCF was 5.5%, 7.6% and 10.7% at 6-, 12- and 24-months, respectively, and similar between the two cohorts (p=0.573). Spinal malalignment (p<0.001), baseline vertebral body collapse (p=0.0027), junctional spine location (p=0.0296), and a greater PTVD90 predicted for greater rates of VCF. <h3>Conclusion</h3> 28 Gy in 2 daily fractions was associated with improved local control without increasing the risk of VCF, and the 2-year local control rates fit the HYTEC spine tumor control probability model.