Comprehensive Dosimetric Evaluation of Eye Physics I-125 Episcleral Plaque Brachytherapy Using DVH-Based and Geometric Quality Indices for Ocular Melanoma.
Abstract
Purpose
Episcleral plaque brachytherapy is the standard eye-preserving treatment for medium-sized ocular melanoma (OM), yet dosimetric evaluation remains challenging due to steep dose gradients, plaque curvature, and anisotropic source geometry. This study presents a comprehensive dosimetric analysis of I-125 Eye Plaque (EP) implants using dose–volume histogram (DVH) metrics and established geometric quality indices (QIs).
Methods
A retrospective analysis of 28 OM patients treated with EP over a two-year period was performed. Treatment planning incorporated high-resolution ultrasound and CT imaging(1-mm slice thickness) with Eye Physics Plaque Simulator software. All patients received 85Gy to the tumor apex with implant durations of 70–125hrs. DVH-based indices (D100%,D98%,D95%,D90%,D50%,D2%, Dmax,Dmean and D0.1cc/D0.2cc) were evaluated for the tumor, entire eye, and lens. Geometric Qis, including Target Coverage Index (TCI), Dose Homogeneity Index (DHI), Overdose Volume Index(ODI), Dose Non-uniformity Ratio(DNR), Conformity Index(COIN), Normal Tissue Conformity Index(TCINT), External Volume Index(EI), and Gradient Index(GI), were derived to quantify plan quality.
Results
Excellent tumor coverage was achieved across all cases, with a mean TCI of 0.99±0.02, indicating near-complete prescription dose coverage of the target. DVH analysis demonstrated substantial intra-tumoral dose heterogeneity, with mean tumor Dmean of 180.6Gy and Dmax up to 610Gy, reflecting the characteristic high-dose gradients of plaque brachytherapy. The mean DHI was 0.35, while ODI and DNR averaged 0.50, confirming expected hot-spot dominance near the scleral surface. Conformity indices (mean COIN and TCINT≈0.31) revealed moderate dose spill into surrounding ocular tissues. Mean whole-eye and lens doses remained within clinically acceptable ranges despite high focal maxima.
Conclusion
Eye Physics I-125 plaque brachytherapy provides highly reliable tumor coverage with predictable dose heterogeneity and steep dose fall-off. Comprehensive use of DVH-based and geometric QIs offers a robust framework for plan evaluation and supports ongoing optimization toward individualized prescription strategies that balance tumor control with ocular toxicity reduction.