125I eye plaque dose distribution including penumbra characteristics

Area of Science:

• Medical Physics
• Radiation Oncology
• Ophthalmology

Background:

• Accurate dose calculation is crucial for effective and safe ophthalmic radiotherapy using Iodine-125 (125I) eye plaques.
• Understanding the interplay between the plaque, the eye-air interface, and dose distribution is essential for treatment planning.

Purpose of the Study:

• To determine penumbra characteristics for 125I eye plaques.
• To assess the influence of the plaque and eye-air interface on dose distribution.
• To initiate the development of a treatment planning algorithm for clinical dose calculations.

Main Methods:

• Dose measurements were performed in a solid water eye phantom using 125I (6711) seeds in a 20 mm COMS eye plaque.
• Thermoluminescent dosimeters (TLD) cubes and minichips were used inside and outside the phantom in longitudinal and transverse planes.
• Analytic fitting of measured data (Dr2) was conducted using double exponential and modified Fermi-Dirac functions.

Main Results:

• The combined plaque and eye-air interface reduced central axis dose by up to 20% at 2.5 cm.
• Dose reduction was more pronounced at larger off-axis coordinates.
• The plaque alone caused a 7% dose reduction at 2.5 cm; the eye-air interface contributed an additional 13% reduction.

Conclusions:

• The eye-air interface significantly impacts dose distribution, primarily by reducing backscatter.
• Penumbra characteristics were quantified, showing depth-dependent variations.
• Developed analytic functions provide a basis for clinical treatment planning algorithms for 125I eye plaque brachytherapy.