Spinal cord dose tolerance to stereotactic body radiotherapy

Abstract Spinal cord tolerance data for stereotactic body radiotherapy (SBRT) were extracted from published reports, reviewed and modelled. For de novo SBRT delivered in 1 to 5 fractions, the following spinal cord point maximum doses (Dmax) are estimated to be associated with a 1 – 5% risk of radiation myelopathy (RM): 12.4 to 14.0 Gy in 1 fraction, 17.0 Gy in 2 fractions, 20.3 Gy in 3 fractions, 23.0 Gy in 4 fractions and 25.3 Gy in 5 fractions. For re-irradiation SBRT delivered in 1 to 5 fractions, reported factors associated with a lower risk of RM include: cumulative thecal sac equivalent dose in 2 Gy fractions with an alpha/beta of 2 (EQD22) Dmax ≤ 70 Gy; SBRT thecal sac EQD22 Dmax ≤ 25 Gy, thecal sac SBRT EQD22 Dmax to cumulative EQD22 Dmax ratio ≤ 0.5, and a minimum time interval to re-irradiation of ≥ 5 months. Larger studies containing complete institutional cohorts with dosimetric data of patients treated with spine SBRT, with and without RM, are required to refine RM risk estimates.