Evolution of dose distribution for a ¹²⁵I eye plaque using MCNPX and Plaque Simulator software

Area of Science:

• Medical Physics
• Radiation Oncology
• Ophthalmology

Background:

• Eye plaque radiation therapy, particularly using Iodine-125 (¹²⁵I) seeds, is a primary treatment for choroidal melanomas.
• Accurate dosimetry is crucial for effective treatment and minimizing side effects.
• Comparing different treatment planning software is essential for understanding dose distribution variations.

Purpose of the Study:

• To investigate and compare the dose distribution around ¹²⁵I eye plaques using Monte Carlo N-Particle eXtended (MCNPX) and Plaque Simulator (PS) software.
• To evaluate the accuracy of dosimetry calculations for ocular melanoma treatment planning.

Main Methods:

• Utilized MCNPX code for dosimetry evaluation and comparison of isodose curves for ¹²⁵I COMS plaques.
• Calculated isodose curves and dose distributions using PS treatment planning software for a ¹²⁵I COMS plaque.
• Validated results against existing literature, noting a maximum relative difference of 9%-10% for COMS plaques.

Main Results:

• MCNPX calculations yielded dose distributions approximately 27.7%-35.4% lower than those calculated by PS.
• Discrepancies may stem from differences in ¹²⁵I source spectra, seed design, and the dose algorithm employed by PS.
• PS demonstrated limitations in providing accurate dose distribution details near the plaque insert surface.

Conclusions:

• Significant differences exist in dose distribution calculations between MCNPX and PS for ¹²⁵I eye plaques.
• PS may not accurately represent dose distributions near the plaque surface.
• Further investigation into source parameters and algorithms used in both MCNPX and PS is recommended to resolve discrepancies.