Proton SBRT for Spinal Metastasis

Spine metastases are common with a prevalence of up to 30-50% among patients with a cancer diagnosis. As cancer survivorship improves, durable local control is essential to decrease the risk of pain and neurologic deficits that are associated with spinal metastasis. SBRT for these tumors has demonstrated overall local control rates of approximately 80-90% in the largest published series. Emerging data highlight the importance of dose escalation for local control in SBRT for spinal metastases. The spinal cord is often the dose limiting structure in patients with spinal metastasis; however, achieving sufficiently high dose and acceptable target coverage while meeting dose constraints of other organs at risk (OARs) such as the small bowel or kidneys also poses a significant technical challenge when using current SBRT modalities for complex cases (e.g., extensive paraspinal disease, epidural disease, disease abutting the kidneys, re-irradiation setting). Maintaining safety is the first priority such that complex cases in current practice often require significant target volume under-coverage in order to meet organ at risk dose constraints, increasing the risk of progression and its associated neurologic morbidity and mortality. To attempt to improve OAR sparing, proton SBRT has been utilized in other disease sites including prostate, non-small cell lung cancer, and liver. The unique qualities of proton beam therapy may improve target coverage while maintaining OAR sparing in cases where current practice with photon SBRT would require a limited dose and/or under-coverage of the target, both of which are associated with an increased risk of tumor progression.