Spinal Cord Tolerance After High Dose, De Novo, Single Fraction Spine Stereotactic Radiosurgery

Abstract

Spine stereotactic radiosurgery (SSRS) offers high rates of local control (LC) in a critical anatomic area by delivering precise, ablative doses of radiation. However, there remains uncertainty regarding the single fraction dose tolerance of the spinal cord (SC) after SSRS. This is of considerable importance as limiting dose to the SC to prevent radiation myelopathy (RM) may lead to underdosing of tumor and ultimately, treatment failure. Here, we report on our experience with relaxed SC dose constraints during SSRS for spinal metastases. We reviewed patients who underwent single fraction, de novo SSRS from 2012 - 2017 at our institution and received >12 Gy Dmax to the SC, which was defined using MRI-CT fusion without PRV expansion. CT myelogram was only used if instrumentation was present. The standard SC constraint was D0.01cc ≤12 Gy, but could be exceeded on protocol for high-risk, inoperable patients. Patients with radioresistant and radiosensitive histologies were treated with 16 Gy to the CTV and a SIB to the GTV of 24 Gy and 18 Gy, respectively, without PTV margin. Patients were rigidly immobilized and aligned with stereoscopic imaging, cone-beam CT, and kV/mV imaging. LC and overall survival (OS) were analyzed with the Kaplan-Meier method. Non-informative beta priors of RM from published studies were used to compute posterior probabilities for the true rate of RM. A total of 132 patients and 146 unique SSRS treatments were analyzed. The median age was 63 years, 70% were male, and 90% had ECOG PS of 0-1. Most patients had renal cell carcinoma (42%), were treated at the thoracic level (86%), and had epidural disease (55%). A minority (16%) had high grade (Bilsky 1c or 2) epidural disease. The median SC Dmax was 12.6 Gy (range, 12.1–17.1 Gy). The SC Dmax was >12 and <13 Gy for 109 (75%) treatments, ≥13 and <14 Gy for 26 (18%) treatments, and ≥14 Gy for 11 (7%) treatments. The median OS was 2.2 years and the 1- and 2-year LC rates were 94% and 92%, respectively. With a median follow up time of 3.5 years, there were zero (0) RM events observed in the cohort. Assuming a conservative prior 4.3% risk of RM, we calculated the true rate of RM for SC Dmax of ≤14 Gy to be <1% with 98% confidence at 24 months. Similarly, we calculated the true rate of RM for SC D0.1cc ≤10 Gy and D1cc ≤8 Gy to be <1% at 24 months with high confidence (Table 1). In one of the largest series of patients treated with single fraction, de novo SSRS to date, there were no cases of RM observed with a median follow up of 3.5 years. These data provide rationale for the safe relaxation of MRI-defined spinal cord dose up to D0.01cc ≤12 Gy and with careful selection, Dmax ≤14 Gy, which correspond to a <1% risk of RM and may allow for durable local control even in those patients with high-risk disease.Abstract 3740; TableSpinal Cord Dose ThresholdPrior Probability of RMConfidence that True Rate of RM < rr = 0.1%r = 0.5%r = 1%r = 2%r = 3%Dmax ≤ 14 Gy4.3%90.5%96.1%98.0%99.3%99.7%D0.1cc ≤ 10 Gy0.53%97.8%98.6%99.0%99.3%99.5%D1cc ≤ 8 Gy0.53%98.1%99.0%99.3%99.6%99.7% Open table in a new tab