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Top Line: This may be the top line, but at QuadShot, we’re all about TBL.
The Study: This paper on spinal cord dose constraints for spine stereotactic body radiation (SBRT) gets straight to it. But it’s also a great philosophical read on spine SBRT. It exclusively includes published series where the method of spinal cord volume delineation and dosimetry was explicitly defined, and it specifically focuses only on levels where there is spinal cord, not cauda. While the authors point out that the “near-max” dose to 0.03 cc is a more reliable way of estimating cord dose, almost all the series used “absolute” max dose to a point and thus that’s what they have to base their recs on. The first takeaway is that the incidence of myelitis after SBRT is extremely rare with the dose constraints used in prior studies. Second, the authors recommend using the thecal sac (or a spinal cord PRV) as the structure for defining max dose. For de novo spine SBRT, the thecal sac max doses with a 1-5% risk of radiation myelitis are: 12-14 Gy in 1 fraction, 17 Gy in 2 fractions, 20.3 Gy in 3 fractions, 23 Gy in 4 fractions, and 25.3 Gy in 5 fractions. Here are the safety pointers for reirradiation: the time since prior radiation should ideally be >5 months, and thecal sac cumulative EQD2 dose max should be ≤70 Gy, with the SBRT portion ≤25 Gy and ≤50% of the cumulative EQD2 dose.
TBL: Here’s your go-to paper for spinal cord max dose constraints when doing spine SBRT. | Saghal, Int J Radiat Oncol Biol Phys 2019