Table 5. Common spinal cord constraints that are applied to either true cord or a surrogate of the true cord (cord PRV or thecal sac) according to no prior and common prior radiation dose exposure.

Prior Conventional RT dose	1 Fraction	2 Fractions	3 Fractions	4 Fractions	5 Fractions
No prior RT & no cord compromise	10–14 Gy Dmax 10 Gy to <10% cord*	17 Gy Dmax	18–21 Gy Dmax	23–26 Gy Dmax	25–30 Gy Dmax
No prior RT but cord compromise	8–14 Gy Dmax 10 Gy to <10% cord*	17 Gy Dmax	18–21 Gy Dmax	23–26 Gy Dmax	25–28 Gy Dmax
800 cGy in 1 fraction	9 Gy Dmax	12.2 Gy Dmax	14–21 Gy Dmax	16.2 Gy Dmax	17. 5 –27. 5 Dmax
2000 cGy in 5 fractions	9–12 Gy Dmax	12.2 Gy Dmax	14–21 Gy Dmax	16.2 Gy Dmax	15–27.5 Gy Dmax
3000 cGy in 10 fractions	9–12 Gy Dmax	12.2 Gy Dmax	14–21 Gy Dmax	16.2 Gy–24 Gy Dmax	17.5–26 Gy Dmax
4000 cGy in 20 fractions	9–12 Gy Dmax	12.2 Gy Dmax	14–21 Gy Dmax	16.2 Gy Dmax	12–25 Gy Dmax
4500 cGy in 25 fractions	9–12 Gy Dmax	12.2 Gy Dmax	14–21 Gy Dmax	16.2 Gy Dmax	12–18 Gy Dmax

D_max = maximum point dose.

^*The 10% criterion uses the spinal cord volume 5–6 mm above and below the target volume. Note that these constraints are intended as a summary of practice patterns of experienced spine specialists. However, these constraints are not data driven. They should be utilized with caution and may not be applicable to all clinical scenarios. Evidence-based constraints have been previously published by Sahgal et al.^31,32 and are cited here as a reference for the interested reader.