Single- vs. Two-Fraction Spine Stereotactic Radiosurgery for the Treatment of Vertebral Metastases

Inclusion Criteria: * Karnofsky Performance Status (KPS) ≥ 70 * RPA class 1 (KPS \>70 AND controlled systemic disease) or RPA Class 2 (KPS \>70, uncontrolled systemic disease OR KPS ≤70, age ≥54, no visceral metastases)44 (see Appendix II) * Vertebral metastases from C3 to L5 based on bone scan, CT, PET, or MRI. * Vertebral metastases must be (1) solitary, (2) at two contiguous levels, or (3) a maximum of three separate sites, with a maximum of two contiguous levels. * Radioresistant metastases are permitted (including sarcomas, melanomas, and renal cell carcinomas). * Patients with epidural disease are permitted so long as there is no cord compression. * Paraspinal extension is permitted, so long as the paraspinal component is ≤5 cm * Multiple small metastatic lesions (\<20% vertebral body involvement) of no clinical correlate are permitted, and not included in the irradiated segments as per RTOG 0631 * History and physical within four weeks of registration. * Negative pregnancy test within four weeks of registration for women of childbearing potential. * Diagnostic spine MRI with and without contrast within four weeks of registration * Neurological exam within four weeks of registration to rule out rapid neurological decline. Mild to moderate neurological deficits are acceptable, as long as distance between lesion and spinal cord is ≥3 mm * Patients may have prior EBRT at the index site. * Informed consent of the participant. Exclusion Criteria: * Lesions at C1-2 or S1-Coccyx. * Hematologic malignancies including lymphoma and myeloma. * Multiple primary cancers. * Primary neoplasms of the spine * Prior corpectomy, kyphoplasty/vertebroplasty, or instrumentation at the site of planned sSRS. * Spinal cord compression. * Paraspinal mass \>5 cm. * Patients with rapid neurologic decline. * Bony retropulsion resulting in neurologic deficit. * Patients with contraindications to MRI. * Patients allergic to intravenous contrast for MRI or CT. * Patients with emergent spinal cord compression. * Patients with mechanical instability of the spine. * Patients with active connective tissue disease. * Patients who previously underwent sSRS to the vertebrae of interest. * Patients with diffuse or multilevel metastatic spinal disease with \>20% involvement of vertebral bodies, defined as involvement of \>5 vertebral levels. * Inability to participate in study activities due to physical or mental limitations. * Inability or unwillingness to return for all required follow-up visits and imaging. * Inability to deliver sSRS, either 18 Gy in one fraction, or 24 Gy in two fractions.