A constraint-normalized robustness index for HDR brachytherapy.

Area of Science:

• Medical Physics
• Radiation Oncology
• Brachytherapy

Background:

• Geometric uncertainties in high-dose-rate brachytherapy (HDR-BT) lead to patient-specific dose deviations.
• Current HDR-BT lacks standardized metrics for quantifying plan sensitivity to geometric perturbations, unlike external beam radiotherapy.

Purpose of the Study:

• To define a dimensionless robustness index for evaluating HDR-BT plans per fraction.
• The index is normalized to dose-limit constraints and incorporates dose-dependent penalties.
• It offers both a full-scenario severity measure and a dose-threshold-filtered variant.

Main Methods:

• Geometric perturbations (translations, rotations, deflections) were applied to source dwell positions.
• A convex-transformed, constraint-normalized penalty was used for organs at risk.
• A prescription-normalized penalty assessed target volume coverage and dose escalation.

Main Results:

• The framework yields interpretable, structure-independent indices.
• A Robustness Index of ≈0.90 signifies that scenarios consume only 10% of the normalized margin to dose limits.
• A threshold-filtered variant assesses robustness for scenarios exceeding clinical dose levels.

Conclusions:

• The proposed index offers a standardized, clinically anchored metric for HDR-BT plan evaluation under geometric uncertainty.
• It complements existing dose-volume histogram reporting with a single, interpretable number.
• This enhances the assessment of HDR-BT plans for improved patient outcomes.