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

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.

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.

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.

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.