Purpose: Historically, the appropriate target volume to be irradiated for spinal metastases is 1–2 vertebral bodies above and below the level of involvement for three reasons: (1) to avoid missing the correct level in the absence of simulation or (2) to account for the possibility of spread of disease to the adjacent level, and (3) to account for beam penumbra. In this study, we hypothesized that isolated failures occurring in the level adjacent to level treated with stereotactic body radiosurgery (SBRS) were infrequent and that with improved localization techniques with image-guided radiation therapy, treatment of only the involved level of spinal metastases may be more appropriate.Methods and Materials: Patients who had received SBRS treatments to only the involved level of the spine as part of a prospective trial for spinal metastases comprised the study population. Follow-up imaging with spine MRI was performed at 3-month intervals following initial treatment. Failures in the adjacent (V±1, V±2) and distant spine were identified and classified accordingly.Results: Fifty-eight patients met inclusion criteria for this study and harbored 65 distinct spinal metastases. At 18-month median follow-up, seven (10.7%) patients failed simultaneously at adjacent levels V±1 and at multiple sites throughout the spine. Only two (3%) patients experienced isolated, solitary adjacent failures at 9 and 11 months, respectively.Conclusion: Isolated local failures of the unirradiated adjacent vertebral bodies may occur in <5% of patients with isolated spinal metastasis. On the basis of the data, the current practice of irradiating one vertebral body above and below seems unnecessary and could be revised to irradiate only the involved level(s) of the spine metastasis.
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