2686 - Stereotactic Body Radiation Therapy for Radioresistant Lumbosacral Spine Metastases: Clinical Outcomes and Predictors of Local Control

Purpose/Objective(s): Lumbosacral (L4-S3) spine metastases present unique anatomical and radiobiological challenges due to their non-mobile nature. This study aimed to evaluate the efficacy of established stereotactic body radiation therapy (SBRT) fractionation strategies in this population and identify predictors of local control (LC).

Materials/Methods: We conducted a single-institution retrospective review of patients with radioresistant lumbosacral spine metastases treated with SBRT. Patient characteristics, including demographics, spinal location, histology, Spinal Instability Neoplastic Score (SINS), extraosseous extension (soft tissue or epidural disease), dosimetry, and treatment-related toxicities, were collected under institutional review board approval. Clinical target volumes (CTV) encompassed entire involved sectors per consensus guidelines. The thecal sac was delineated with no PRV. Patients received either single-fraction SBRT (16–18 Gy) or fractionated SBRT (21–30 Gy in 3–5 fractions) based on tumor characteristics and adjacent organ-at-risk constraints. Dosimetric parameters and clinical outcomes were analyzed using t-tests and univariate logistic regression, with multivariate Cox regression used to identify predictors of local recurrence.

Results: A total of 40 patients (median age: 65 years, 55% male) with 74 treated spinal segments were included. Radioresistant histology was present in 90% of cases and moderately radiosensitive in four. Eighteen patients had a SINS = 7. Single-fraction SBRT was used in 21 patients and was associated with a significantly smaller planning target volume (PTV) than fractionated regimens (89.4 cc vs. 237.9 cc, p=0.001). Thecal sac Dmax ranged from 1.9–19.5 Gy (single fraction) and 22.3–33.9 Gy (3 fractions). With a mean follow-up of 20 months, the 2-year LC rate was 58%. Patients with extraosseous disease had significantly worse 2-year LC rates (26% vs. 61%, log-rank, p=0.02). On multivariate analysis, local recurrence was associated with extraosseous disease (HR=8.0, p=0.003) and radioresistant histology (HR=11.0, p=0.04), but not with age, gender, or fractionation (p=0.9, 0.1, and 0.5, respectively). Dosimetric factors (PTV volume, PTV min dose %, PTV max dose %, PTV mean dose %) were not predictive of local recurrence or toxicity (p>0.05). Thirteen patients experienced post-treatment pain flare, and six developed post-radiation fractures, three of whom required surgical intervention. Post-radiation neuropathy occurred in six patients.

Conclusion: Lumbosacral spine metastases treated with SBRT demonstrated lower LC rates than mobile spine metastases, reinforcing prior findings that this region presents unique challenges due to larger target volumes and higher rates of extraosseous extension. Further prospective studies are needed to refine dose selection and treatment planning strategies to optimize local control and reduce treatment-related toxicity in this high-risk population.