Early Clinical Outcomes and Local Control Following Spine Stereotactic Proton Ablative Radiation Therapy.

Purpose: To evaluate the local control (LC) outcomes and safety profile of stereotactic proton ablative radiation therapy (SPAR) for spine metastases in a highly selected patient population, with emphasis on reirradiation cases. Methods and Materials: We retrospectively reviewed all patients who received SPAR between June 2015 and October 2019. Treatments were delivered in 1 to 5 fractions with a median of 3 fractions. The median prescription dose was 39 Gy (range, 20-60 Gy) to the high-dose clinical target volume (CTV) and 24 Gy (range, 16-50 Gy) to the low-dose CTV. All treatments were delivered using pencil-beam scanning proton therapy with single-field or multifield optimization. LC was assessed per spine response assessment in neuro-oncology (SPINO) criteria using follow-up imaging. Survival and recurrence outcomes were estimated by the Kaplan-Meier method. Acute and late adverse events (AEs) were graded using the National Cancer Institute Common Terminology Criteria for Adverse Events v5.0. Results: Forty-two patients (47 lesions) were treated with SPAR, with 66% treated in the reirradiation setting. With a median imaging follow-up of 13 months, 1- and 2-year LC rates were both 74% (95% CI, 58%-91%). Ten lesions progressed locally (8 in-field, 2 marginal), most commonly within the vertebral body. No significant predictors of progression were identified aside from low-dose CTV and D95%. Acute AEs included 9 pain flares and one grade 3 dermatitis. Late AEs included 3 vertebral compression fractures and one grade 3 peripheral neuropathy. No cases of radiation myelopathy were observed. Conclusions: SPAR provides durable LC with acceptable toxicity in a highly selected patient population. These findings support the feasibility of SPAR in complex or reirradiation cases, though broader applicability may be limited by access to high-precision proton therapy systems.