Medulla Oblongata OAR Tolerances in SBRT
The medulla oblongata should be treated as spinal cord tissue with a maximum dose constraint of 20 Gy in 3 fractions or 13 Gy in a single fraction to maintain myelopathy risk below 1%, and cumulative dose should not exceed 33 Gy across all treatments. 1, 2
Specific Dose Constraints
The medulla oblongata, as part of the brainstem and continuous with the spinal cord, follows spinal cord dose constraints in SBRT planning:
- Single fraction SBRT: Maximum point dose ≤13 Gy 2
- 3-fraction SBRT: Maximum point dose ≤20 Gy 2
- Cumulative lifetime dose: Maximum ≤33 Gy across all treatments 1
- Risk threshold: These constraints maintain radiation myelopathy risk below 1% 2
Clinical Context for Spine SBRT
When treating spinal metastases near the medulla (cervical spine), the following fractionation schemes achieve approximately 90% local control at 1 year while respecting spinal cord tolerance 1:
- 16-24 Gy in 1 fraction
- 24 Gy in 2 fractions
- 24-27 Gy in 3 fractions
- 30-35 Gy in 5 fractions
Careful delineation of the spinal cord/medulla and treating to maximal dose limits are critical strategies to reduce both toxicity risk and treatment failure. 3
Where to Find Guidelines
The most authoritative sources for these constraints include:
- American Society for Radiation Oncology (ASTRO) recommendations for SBRT dose constraints 1
- International Stereotactic Radiosurgery Society (ISRS) practice guidelines for spine SBRT 3
- NRG-BR001 protocol dose constraints, which have been validated in prospective trials combining SBRT with immunotherapy 4
Critical Planning Considerations
Use planning organ at risk volumes (PRV) with appropriate margins around the medulla/spinal cord, especially for centrally located tumors. 1 This accounts for setup uncertainty and organ motion during treatment delivery.
The crude risk of radiation myelopathy in spine SBRT when respecting these constraints is approximately 0.2-1.2%, demonstrating that adherence to these dose limits provides excellent safety 3
Common Pitfalls to Avoid
- Never exceed cumulative spinal cord/medulla dose of 33 Gy across all treatments, including any prior conventional radiotherapy 1
- Do not use conventional low-dose palliative radiation (8 Gy in 1 fraction) for patients with reasonable life expectancy, as this achieves suboptimal local control and increases risk of subsequent spinal cord compression 1, 2
- Ensure advanced dose calculation algorithms (type B) are used for treatment planning to accurately account for tissue heterogeneity near the medulla 1