2619 - Long-Term Outcomes of Single-Fraction Stereotactic Ablative Radiotherapy (SAbR) Using a Simultaneous-Integrated Boost (SIB) Approach for the Treatment of de Novo Spinal Metastases

B. Tortelli¹, R. Khandekar², A. Simmons¹, S. Brocklehurst¹, S. Gottumukkala¹, J. Trivedi³, Z. Wardak¹, T. Dan¹, R. D. Timmerman¹, and K. A. Kumar Jr¹; ¹Department of Radiation Oncology, University of Texas Southwestern Medical Center, Dallas, TX, ²UT Health San Antonio, San Antonio, TX, ³Sidney Kimmel Medical College, Philadelphia, PA

Purpose/Objective(s): SAbR for spinal metastases is a widely accepted treatment with 1-year rates of local tumor failure (LF) of 10-20% and vertebral compression fracture (VCF) of 10-15% (Glickman RM et.al. JAMA Onc 2020). International Spine Radiosurgery Consortium Consensus Guidelines recommend targeting both the gross tumor volume (GTV) and clinical target volume (CTV) to a single total dose of 16-24 Gy (Cox et al. IJROBP 2012). Such high dose to the CTV may be unnecessary to control microscopic disease and could increase risk of toxicity, such as VCF. Our institution has established a novel treatment protocol designed to minimize risk of toxicities in a single fraction by applying a lower dose to the defined CTV and a higher ablative therapeutic dose to GTV through a simultaneous-integrated boost (SIB) technique. This study reports the long-term outcomes of patients with de novo spinal metastases treated with our novel SIB, single-fraction SAbR technique. Materials/

Methods: We performed an IRB-approved, retrospective review of all de novo solid tumor spinal metastases treated with single-fraction SAbR using a SIB approach between 2014 and 2021. Radiation treatment was performed as previously described in a phase II clinical trial of prophylactic vertebroplasty immediately after spine SAbR (Wardak Z, et al. IJROBP 2019). Primary endpoint was cumulative incidences of image-based LF with secondary endpoints of VCF and overall survival (OS).
Results: We identified and reviewed 245 solid tumor spinal metastases from 182 patients, of which 194 metastases from 156 patients had evaluable follow-up imaging. The median radiographic follow-up per metastasis was 18.0 months (IQR 7.7-45.1). The 1-, 3- and 5-year cumulative incidences of LF were 12.2%, 17.2% and 18.8% respectively, with a 7.2-month median time to failure (IQR 3.2-12.0). Of the patients who failed locally, 16 (57%) occurred in the high dose SIB region, 8 (29%) occurred only in the lower dose region and 4 (14%) were out of the treatment field. Prophylactic vertebroplasty was performed in 34% of cases. The 1-year and 3-year cumulative incidences of VCF were 13.3% and 16.8%, with a 5.2-month median time to VCF (range 0.5-37.7). Overall survival at 1-, 3- and 5-years after treatment were 71.6% (95% CI 65.6-78.3), 42.1% (95% CI 35.5-49.6) and 34.3% (95% CI 22.6-36.2). Median OS was 23 months (95% CI 19.3-35.6).
Conclusion: Long-term follow up indicates that single-fraction spine SAbR with SIB approach offers excellent long-term local tumor control with low rates of VCF. LF was rare, with less than a third of failures occurring within the low dose region. This suggests that a lower dose of 14 Gy is sufficient to treat microscopic disease in most cases. This technique may provide a reasonable balance between efficacy and safety in the treatment of spinal oligometastases.