What are the criteria for evaluating a stereotactic radiosurgery (SRS) plan?

SRS Plan Evaluation Criteria

An SRS plan should be evaluated using a combination of conformity index (CI), gradient measures (GM), dose-volume metrics (particularly V12Gy for brain), and organ-at-risk (OAR) dose constraints, with imaging quality and target delineation accuracy serving as foundational requirements.

Imaging and Target Delineation Requirements

The foundation of any quality SRS plan begins with precise imaging 1:

• High-resolution volumetric T1 post-gadolinium MRI (3DT1 sequence)
• High-resolution T2-weighted volumetric sequence (FIESTA or CISS) with and without gadolinium
• High-resolution CT scan (contrast optional) for fusion and dose calculation

This multimodal approach minimizes partial volume effects and distortions, allowing direct identification of critical structures. Targeting errors of even 1 mm can significantly reduce dose to small targets and increase dose to surrounding critical structures 1.

Core Dosimetric Quality Metrics

1. Conformity Index (CI)

The CI quantifies how well the prescription isodose conforms to the target volume 2:

• Target range: CI should be between 1.0-2.0
• Values closer to 1.0 indicate better conformity
• For benign meningiomas, acceptable CI values averaged 1.81-1.92 3
• Multi-target VMAT plans typically show slightly lower CI than single-target plans 4

2. Gradient Measures (GM)

GM evaluates dose falloff outside the target 5, 6:

• Optimal GM: Predictions accurate to within 0.2-1.1 mm for high-quality plans
• Steeper gradients (lower GM values) minimize dose to surrounding normal tissue
• For small-to-medium targets without critical OAR involvement, GM should be predictable within 0.2 ± 0.3 mm 6

3. Normal Brain Dose Metrics

V12Gy (volume receiving ≥12 Gy) is the critical metric for brain metastases 7, 6, 4:

• Should be minimized to reduce radiation necrosis risk
• For small-medium targets: fractional brain V12Gy should be predictable to 0.04 ± 0.12 6
• Plans with V12Gy deviations >25% from expected values should be re-optimized 6
• Multi-target plans show larger V12Gy variation and require careful review 4

4. Organ-at-Risk Constraints

Cochlear dose (for vestibular schwannomas) 1:

• Maximum or mean dose <4 Gy to optimize hearing preservation
• Hearing preservation achieved in ~50-67% when this constraint is met

Spinal cord (for spine SRS) 8:

• Dose limits depend on prior radiation and fractionation scheme
• Cumulative EQD2 >120-130 Gy significantly increases radiation necrosis risk 9

Prescription Dose Evaluation

Single Fraction SRS

• Brain metastases: 18-24 Gy 7
• Vestibular schwannomas: 11-13 Gy (strong recommendation) 1
• Benign meningiomas: 12-15 Gy 2

Hypofractionated SRS (2-5 fractions)

• Brain metastases: 20 Gy in 2 fractions, 24-27 Gy in 3 fractions, or 25-40 Gy in 5 fractions 7
• Recurrent HGG: 24-25 Gy in 3-5 fractions 9

Advanced Quality Indices

Efficiency Index (η50%)

This novel metric combines conformity, gradient, and mean dose 10:

• Formula: η50% = (integral dose to target) / (integral dose to 50% prescription isodose volume)
• Target range: 37.7%-58.0%, with mean of 49.0%
• Values closer to 1.0 (100%) indicate better efficiency
• Quantifies the ratio of dose "doing good" versus dose "doing harm"

CGI (Conformity/Gradient Index)

A simplified scoring system requiring only three data points 5:

1. Total volume irradiated to prescription isodose
2. Target volume
3. Total volume irradiated at half the prescription isodose

Plan Optimization Red Flags

A plan should be flagged for re-optimization if 6, 4:

• GM deviation >1.35 mm from expected values
• Fractional brain V12Gy deviation >0.20 from predicted
• CI >2.0 for single targets
• V12Gy reduction of >33% is achievable through re-optimization

Multi-Target Considerations

For single-isocenter multi-target plans 4:

• V12Gy shows larger variation compared to single-target plans
• Number of targets and inter-target distance have minimal impact on V12Gy
• Benchmark data from single-target plans can guide optimization
• Re-optimization of outliers can reduce V12Gy by ~34%

Common Pitfalls to Avoid

1. Inadequate imaging resolution: Using standard diagnostic MRI sequences instead of high-resolution volumetric sequences leads to targeting errors 1

2. Ignoring cumulative dose: For re-irradiation cases, failing to account for cumulative EQD2 >120-130 Gy significantly increases toxicity risk 9

3. Accepting suboptimal plans: Plans with GM >1.35 mm deviation or V12Gy >25% above expected should trigger re-optimization 6

4. Overlooking OAR constraints: Particularly cochlear dose in vestibular schwannomas and brainstem dose in skull base lesions 1

5. Poor immobilization: Patient comfort and precision of immobilization directly impact targeting accuracy, especially for small targets 1