Spine Stereotactic Radiosurgery for Patients with Metastatic Thyroid Cancer: Secondary Analysis of Phase I/II Trials.

Abstract

Metastatic deposits to the spine in thyroid cancer patients represent the most common site of bone involvement and can contribute to pain, neurologic deficits, and death. This study sought to determine the efficacy and safety of spine stereotactic radiosurgery (SSRS) for thyroid cancer patients. Thyroid cancer patients with spine metastases were selected and analyzed from a cohort of patients who were prospectively enrolled in two single-institution Phase I/II studies. SSRS was delivered in single or multi-fraction schedules. Dose regimens ranged from 16-18 Gy in one fraction to 27-30 Gy in three to five fractions. Toxicity was graded according to the NCI-CTC toxicity scale. Local control was determined by serial post-treatment magnetic resonance imaging scans showing no evidence of progressive disease. Patients were followed until date of death or date of last known visit for survival analyses. Local control and overall survival rates were carried out using Kaplan-Meier estimates. The log-rank test was used to assess the equality of the survivor function across groups. A p-value of ≤0.05 was considered to be statistically significant. A total of 27 spine lesions were treated in 23 patients over a six-year period. Median follow-up was 28.9 months (range 5-93 months). Local control was 88% at two years and 79% at three years. In patients with progressive disease following conventional radiation therapy, local control for salvage SSRS remained at 88% at three years. Patients requiring upfront surgical intervention and treated with adjuvant SSRS achieved sustained control rates of 86% at three years. Overall survival rates were 85% and 67% at one and two years, respectively. In patients classified with oligoprogression and controlled extra-spinal disease, overall survival was significantly higher than those with evidence of systemic progression (81% vs. 45% at two years; p = 0.01). Univariate analysis did not show significant correlations between local control and age, systemic disease status, prior (131)I therapy, SSRS fraction regimen, spine location, histological subtype, or time from initial diagnosis to evidence of spinal metastasis. No patient experienced any grade 3-5 toxicity. Pain flare was reported in 30% of patients, with only three patients (13%) requiring narcotics or short-course steroids. There was no evidence of vertebral body fracture in any patient that achieved local control in the treated area. SSRS for thyroid metastases as a primary or adjuvant/salvage therapy is well tolerated and yields high rates of local control.