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.