Stereotactic Radiosurgery Dose for 2.5 cc Brain Metastasis
For a 2.5 cc brain metastasis, the recommended single-fraction SRS dose is 18 Gy, based on established dose-volume relationships that balance local control with toxicity risk. 1
Dose Selection Algorithm
The appropriate SRS dose for your 2.5 cc lesion follows a volume-based approach:
- Lesions <2 cm diameter (typically <4 cc): 18-24 Gy single fraction is recommended 1
- Your 2.5 cc lesion falls within this range, making 18 Gy the standard dose 2, 1
- Lesions 2.5 cm or less: Historical data supports 16 Gy as effective, though modern practice favors 18 Gy for optimal control 2
Evidence Supporting This Dose
The dose recommendation is derived from RTOG 90-05 protocol, which established maximum marginal doses based on tumor volume: 24 Gy for lesions <2 cm, 18 Gy for lesions 2-3 cm, and 15 Gy for lesions 3-4 cm 2. Your 2.5 cc lesion (approximately 1.7 cm diameter if spherical) positions it in the 18-24 Gy range, with 18 Gy being the conservative and widely accepted choice 1.
Multiple guidelines confirm that minimum doses of ≥18 Gy provide optimal local control, with reported control rates of 81-98% for lung cancer metastases and 90-94% for breast cancer metastases 3. The International Stereotactic Radiosurgery Society practice guideline specifically notes that lesions ≤2.5 cm can be treated with single-fraction doses in this range 2.
Critical Considerations That May Modify Dose
Location relative to critical structures is the primary factor that could necessitate dose reduction or fractionation:
- If your lesion is near the optic apparatus, brainstem, or other radiosensitive structures, consider fractionated SRS (25 Gy in 5 fractions or 27 Gy in 3 fractions) instead of single-fraction treatment 1
- For lesions in non-eloquent locations away from critical structures, proceed with 18 Gy single fraction 2, 1
Prescription isodose line should typically be 50% (range 20-90%) for optimal dose conformality 4. This means the maximum dose at the center of the lesion will be approximately 36 Gy when prescribing 18 Gy to the margin.
Alternative Fractionation Approach
If single-fraction SRS is contraindicated due to location or patient factors, fractionated stereotactic radiosurgery delivers 25 Gy in 5 fractions as the most common alternative schedule 1. This approach:
- Reduces the risk of radiation necrosis for larger volumes 2
- Allows safer treatment near critical structures 1
- Provides equivalent local control with potentially lower toxicity 2
Common Pitfalls to Avoid
Do not reduce the dose below 18 Gy for a 2.5 cc lesion in a non-critical location, as this compromises local control 3. Studies demonstrate that doses <18 Gy result in inferior outcomes, with local control rates dropping significantly 2.
Do not automatically add whole brain radiation therapy (WBRT) to SRS for this single lesion, as WBRT causes significant neurocognitive decline without survival benefit 2. SRS alone is the preferred approach for limited metastases 2.
Ensure adequate margins: Use 0-2 mm PTV margins for intact metastases to minimize normal brain exposure while accounting for setup uncertainty 4. Larger margins are unnecessary and increase toxicity risk.
Expected Outcomes
With 18 Gy single-fraction SRS to a 2.5 cc brain metastasis, you can expect: