Spinal Cord Tolerance to Radiotherapy
For conventional fractionated radiotherapy (1.8-2 Gy per fraction), the spinal cord can tolerate a maximum dose of 50 Gy to minimize the risk of radiation myelopathy to less than 1%, regardless of the length of cord irradiated.
Conventional Fractionation Dose Limits
The tolerance dose is not primarily determined by the length of spinal cord irradiated, but rather by the maximum dose delivered to any segment. The critical threshold is:
- 50 Gy in conventional fractionation (1.8-2 Gy/fraction): <1% risk of myelopathy 1
- 54 Gy: <1% risk of transverse myelitis 2
- 61 Gy: <10% risk of myelopathy 1
The guideline evidence from malignant spinal cord compression cases demonstrates that cumulative doses up to 54 Gy can be delivered safely, with one retrospective series showing patients received cumulative doses ranging from 36.5 to 81 Gy (median 54 Gy) over spinal cord lengths of 5 to 25 cm (median 10 cm) with only one episode of radiation myelopathy 3. Another series reported no myelopathy when spinal cord received less than 100 Gy total 3.
Length Considerations
The length of spinal cord irradiated is less critical than the maximum point dose. Evidence shows:
- Patients have been safely treated with RT fields covering 5 to 25 cm of spinal cord (median 10 cm) when dose constraints are respected 3
- Standard head and neck fields routinely deliver doses to extended cord segments (typically 52 Gy maximum, range 48.9-55.9 Gy) without causing myelopathy 4
Hypofractionated/SBRT Regimens
For stereotactic body radiation therapy (SBRT), different constraints apply based on partial volume irradiation:
Single Fraction SBRT:
- 12.4-14.0 Gy point maximum dose (Dmax): 1-5% risk of myelopathy 5
- 13 Gy maximum: <1% risk when treating focal lesions 1
Multi-Fraction SBRT (Most Recent High-Quality Evidence):
The 2025 JAMA Oncology study 6 provides the most robust data from 1,423 patients with 1,904 spinal metastases:
- D0.1cc (minimum dose to 0.1 cm³ receiving highest dose) of 19.1 Gy in 3 fractions: 1.0% risk of grade 3-4 myelopathy at 2 years
- Dmax of 20.8 Gy in 3 fractions: 1.0% risk of grade 3-4 myelopathy at 2 years
For other hypofractionated regimens:
Critical Caveats
Avoid hypofractionated regimens without careful dosimetry: The 17 Gy in 2 fractions protocol commonly used for palliative lung cancer can exceed safe spinal cord limits when 3D dosimetry accounts for patient separation and tissue inhomogeneity 7. Alternative doses below 15.5 Gy in 2 fractions (7 days apart) are safer 7.
For reirradiation, additional constraints apply:
- Cumulative EQD2 (α/β=2) Dmax ≤70 Gy to thecal sac
- SBRT EQD2 Dmax ≤25 Gy
- Minimum interval ≥5 months between treatments
- Ratio of SBRT to cumulative dose ≤0.5 5
The alpha/beta ratio for spinal cord myelopathy is very low (0.87-2 Gy), making the cord extremely sensitive to fraction size 1. This explains why even small increases in dose per fraction dramatically increase myelopathy risk.