Local Control and Outcomes Associated With Proton Spine Hypofractionated Radiation Therapy

Little is known about the long-term outcomes of patients with spinal metastases treated with proton hypofractionated radiation therapy (RT), particularly in the setting of re-irradiation (re-RT).

This is a retrospective cohort study of patients treated between 2015 and 2023 using proton hypofractionated RT (≤10 fractions). All patients received follow-up magnetic resonance imaging at least 1 month after treatment. Outcomes of interest included local failure, defined radiographically, and long-term adverse events.

A total of 52 patients treated for 58 lesions met the inclusion criteria. Median follow-up after RT was 14 months (IQR, 8-25). The most common dose-fractionation scheme was 40 Gy in 5 fractions (n = 47; 81%). There were 10 cases of first-time RT using protons (17%), whereas 25 courses (43%) were second-time re-RT, 19 courses (33%) were third-time re-RT, and 4 courses (6.9%) were fourth-time re-RT. Median cumulative equivalent dose in 2-Gy fractions was 115 Gy (IQR, 91-153). The 1-year local failure rate was 10% (95% CI, 4%-20%), and the 2-year local failure rate was 22% (95% CI, 12%-35%). There were 5 grade (G) 3 adverse events, including radiation myelitis (1), esophageal fistula (1), esophageal stricture (1), wound dehiscence (1), and bowel necrosis (1). All G3 adverse events occurred in patients who received 3-4 overlapping courses of RT. There were no G4-5 adverse events.

Proton hypofractionated RT is associated with excellent local control in this large cohort of heavily re-irradiated patients (81% re-RT). G3 adverse events were limited to patients who received 3-4 overlapping courses of RT.