Margin Expansion for Hypofractionated Stereotactic Radiation Therapy (HSRT)
For post-operative HSRT to resection cavities, use the same margin expansion (0.3-0.5 cm added to the clinical target volume to create the planning target volume) as recommended for single-fraction post-operative SRS. 1
Margin Recommendations Across Treatment Modalities
The available evidence does not differentiate margin expansion requirements between single-fraction SRS and HSRT for post-operative treatment of brain metastases or other intracranial lesions. The standard approach applies uniformly:
Add 0.3-0.5 cm margin to the clinical target volume (CTV) to account for setup uncertainties and treatment delivery variations, creating the planning target volume (PTV). 1
This margin recommendation comes from the American Society of Clinical Oncology guidelines and applies to post-operative stereotactic radiation regardless of fractionation scheme. 1
Evidence Supporting Margin Use in HSRT
Research demonstrates that margin expansion improves local control without significantly increasing toxicity:
A 2-mm margin around the resection cavity improved 12-month local control from 16% to 3% (p=0.042) without increasing toxicity rates (3% vs 8%, p=0.27). 2
Studies using HSRT with 5-fraction regimens (typically 30 Gy in 5 fractions) have employed 2-mm margins with excellent outcomes, achieving 84% one-year local control with only 6% symptomatic radiation necrosis. 3
Multi-institutional data comparing pre-operative and post-operative SRS showed that post-operative treatment used a median 2-mm planning target volume margin, which is consistent across both single-fraction and fractionated approaches. 4
Fractionation-Specific Considerations
HSRT may actually provide superior local control compared to single-fraction SRS, making the standard margin even more effective:
Meta-analysis data shows fractionated SRS achieved better local control than single-fraction SRS (87.3% vs 80.0% at 12 months, p=0.021). 5
For larger cavities (>3 cm), HSRT regimens like 30 Gy in 5 fractions or 27 Gy in 3 fractions achieve 91% one-year local control with only 8% radiation necrosis. 1
Multidose SRS (9 Gy × 3) to large resection cavities using a 2-mm margin achieved 93% one-year and 84% two-year local control rates with 9% radionecrosis (5% symptomatic). 6
Critical Implementation Points
The margin expansion does not change based on fractionation scheme—the same 0.3-0.5 cm (typically 2 mm in practice) applies to both single-fraction SRS and HSRT. 1, 2
Key technical considerations that remain constant:
Setup margins should be minimized through accurate patient positioning with reproducible immobilization and digital imaging during treatment. 1
The target volume is defined as the resection cavity with contrast enhancement on post-operative imaging, plus the standardized margin. 2, 3
Post-operative radiation (whether single-fraction or fractionated) should ideally be delivered within 4 weeks of surgery to optimize outcomes. 1
Common Pitfall to Avoid
Do not eliminate the margin expansion when using HSRT thinking that fractionation compensates for geographic miss. The meta-analysis found no improved local control with margin addition (84.3% vs 83.1%, p=0.71), but this reflects that margins are already standard practice—not that they are unnecessary. 5 Individual studies clearly demonstrate that omitting margins worsens local control. 2