Reirradiation Human Spinal Cord Tolerance for Stereotactic Body Radiotherapy

Purpose We reviewed the treatment for patients with spine metastases who initially received conventional external beam radiation (EBRT) and were reirradiated with 1–5 fractions of stereotactic body radiotherapy (SBRT) who did or did not subsequently develop radiation myelopathy (RM). Methods and Materials Spinal cord dose–volume histograms (DVHs) for 5 RM patients (5 spinal segments) and 14 no-RM patients (16 spine segments) were based on thecal sac contours at retreatment. Dose to a point within the thecal sac that receives the maximum dose (P max ), and doses to 0.1-, 1.0-, and 2.0-cc volumes within the thecal sac were reviewed. The biologically effective doses (BED) using α/β = 2 Gy for late spinal cord toxicity were calculated and normalized to a 2-Gy equivalent dose (nBED = Gy 2/2 ). Results The initial conventional radiotherapy nBED ranged from ∼30 to 50 Gy 2/2 (median ∼40 Gy 2/2 ). The SBRT reirradiation thecal sac mean P max nBED in the no-RM group was 20.0 Gy 2/2 (95% confidence interval [CI], 10.8–29.2), which was significantly lower than the corresponding 67.4 Gy 2/2 (95% CI, 51.0–83.9) in the RM group. The mean total P max nBED in the no-RM group was 62.3 Gy 2/2 (95% CI, 50.3–74.3), which was significantly lower than the corresponding 105.8 Gy 2/2 (95% CI, 84.3–127.4) in the RM group. The fraction of the total P max nBED accounted for by the SBRT P max nBED for the RM patients ranged from 0.54 to 0.78 and that for the no-RM patients ranged from 0.04 to 0.53. Conclusions SBRT given at least 5 months after conventional palliative radiotherapy with a reirradiation thecal sac P max nBED of 20–25 Gy 2/2 appears to be safe provided the total P max nBED does not exceed approximately 70 Gy 2/2 , and the SBRT thecal sac P max nBED comprises no more than approximately 50% of the total nBED.