Equivalent uniform RBE-weighted dose in eye plaque brachytherapy

Area of Science:

• Radiation Oncology
• Medical Physics
• Ophthalmology

Background:

• Ocular melanoma brachytherapy utilizes eye plaques with varying dose distributions, rates, and relative biological effectiveness (RBE).
• Accurate dosimetry is crucial for optimizing treatment outcomes and minimizing side effects.

Purpose of the Study:

• To introduce the equivalent uniform RBE-weighted dose (EUD_RBE) for quantitative assessment of cell survival.
• To apply EUD_RBE to ¹²⁵I eye plaque brachytherapy for ocular melanoma, considering variable RBE, dose, and dose rate.

Main Methods:

• Defined EUD_RBE as a uniform dose (RBE=1) yielding equal cell survival to a non-uniform dose (RBE>1).
• Modeled ¹²⁵I COMS eye plaque brachytherapy, incorporating steep dose gradients, protracted irradiation (3-8 days), and dose-rate dependent RBE (max 1.4).
• Converted physical dose distributions to biologically effective dose (BED) to account for sublethal repair and protracted irradiation.

Main Results:

• EUD_RBE increased with prescription depth (up to 6% for 12mm tumor height) due to steep dose gradients.
• Increasing dose rate (decreasing implant duration) increased EUD_RBE by up to 12%.
• Combined effects of dose non-uniformity and dose rate altered EUD_RBE by up to 18% for an 85 Gy prescription.

Conclusions:

• The calculated EUD_RBE range for ¹²⁵I COMS eye plaque brachytherapy validates the chosen models and hypotheses.
• EUD_RBE offers a valuable tool for analyzing treatment outcomes and comparing dose regimens in ocular brachytherapy.