Impact of Dose Hot Spots on Spinal Cord Tolerance following Stereotactic Body Radiotherapy: A Generalized Biological Effective Dose Analysis

The purpose of this study was to investigate the effects of high-dose inhomogeneous irradiation to small volumes of spinal cord with a new generalized biological effective dose (gBED) analysis for spine stereotactic body radiotherapy (SBRT). The gBED was applied to spinal cord dosimetric data (contoured per the thecal sac) at specified volumes for a cohort of five patients with radiation-induced myelopathy (RM) and compared to nineteen patients without RM post-SBRT. The spinal cord gBED was calculated and normalized to a conventional 2-Gy equivalent dose fraction scheme (α/β = 2 Gy for late toxicity). Differences between the conventional BED and those gBED calculations by accounting for small-volume dosing within the spinal cord was observed. Statistically significant differences in the mean gBED between the RM group and the non-RM group was observed both at the maximum point volume (gBED of 66 Gy vs. 37 Gy(p = 0.01), respectively) and at the 0.1 cm3 volume (gBED of 53 Gy vs. 28 Gy (p = 0.01), respectively). No significant difference at the 0.1 cm3 volume was observed based on the mean BED comparisons. No significant differences were observed at the larger 1 cm3, 2 cm3 or 5 cm3 volumes for either BED or gBED comparisons. We conclude that differences in dose hot spots characteristics within small inhomogenously irradiated volumes of spinal cord can affect spinal cord tolerance following SBRT treatments.