Multimodal Nuclear Imaging Response as a Prognostic Indicator Following Spine Stereotactic Body Radiation Therapy

Magnetic resonance imaging (MRI) is the primary modality for diagnosing and monitoring spinal metastases, but its interpretation can be complicated by nontumor-related changes like fibrosis or osteoradionecrosis. Nuclear imaging can complement MRI in evaluating treatment response. Yet, the prognostic significance of nuclear metabolic response after spine stereotactic body radiation therapy (SBRT) remains underexplored. This study evaluates the utility of nuclear imaging in assessing post-SBRT treatment response and its association with local control (LC) and survival outcomes.

This retrospective study evaluated spine metastases treated with SBRT focusing on pre- and first available posttreatment positron emission tomography (PET) or bone scans to assess nuclear metabolic response. PET responses were categorized as complete response (CR), partial response, stable disease, or disease progression, while bone scan interpretations were based on nuclear medicine reports. MRI was used for LC assessment. Predictors of LC and overall survival (OS) were identified via Cox regression analyses. Additionally, metabolic responses within the first 3 months and between 4 and 6 months post-SBRT were correlated with 2-year outcomes.

Of the 53 patients, 66% underwent PET imaging and 34% underwent bone scans. Nuclear imaging revealed a metabolic CR in 74%, partial response in 13%, stable disease in 5%, and disease progression in 8%. Patients achieving metabolic CR (mCR) had significantly better 2-year LC (97% vs 60%, P < .001) and OS (76% vs 36%, P < .001). In multivariable analysis, mCR was independently predictive of improved LC (hazard ratio [HR]: 12.76; P = .005) and OS (HR: 4.03; P = .003). Systemic disease stability was also significantly associated with OS (HR: 5.27; P = .001). Early (≤3 months) and intermediate (4-6 months) mCR correlated with 100% LC, while non-mCR was associated with only 40% LC (P = .04 and P = .018, respectively). Among 10 patients who had local recurrence, one of them had mCR and the other one near mCR on concurrent PET scan suggesting pseudoprogression.

Multimodal nuclear imaging could provide a valuable functional insight in evaluating response after spine SBRT and may help overcome limitations of MRI. A metabolic CR is a strong independent predictor of LC and survival. Prospective studies with standardized imaging protocols are warranted to guide adaptive treatment strategies.