The impact of prescription depth, dose rate, plaque size, and source loading on the central axis using ¹⁰³Pd, ¹²⁵I, and ¹³¹Cs•

Abstract 

Purpose

Modern dosimetry data are not available for Collaborative Ocular Melanoma Study-based eye plaques. This report aims to provide these data for eye plaques ranging from 10 to 22mm, and for three different low-energy, photon-emitting radionuclides.

Methods and Materials

Recent publications on brachytherapy dosimetry parameters for ¹⁰³Pd, ¹²⁵I, and ¹³¹Cs were evaluated for use as eye plaque reference data. These data were entered into the Pinnacle treatment planning system for 3D calculations of brachytherapy dose distributions along the central axis for depths ranging from −1 to 10mm based on the origin positioned at the inner sclera. In accordance with the original Collaborative Ocular Melanoma Study protocol and in the absence of radionuclide-specific heterogeneity factors, inhomogeneity corrections were not applied.

Results

As expected due to the mean photon energies, ¹⁰³Pd, ¹²⁵I, and ¹³¹Cs provided increasingly penetrating dose distributions. Dose distribution tables were prepared for fully loaded plaques and for plaques with the central source(s) removed. Over the entire range of central axis depths, and for all plaque sizes and loadings, ¹³¹Cs produced minimal outer scleral doses. Similarly, ¹⁰³Pd generally produced more favorable dose distributions than ¹²⁵I for depths less than 4mm.

Conclusions

A modern analysis of eye plaque dosimetry evaluated dose as a function of lesion height and applicator size, and showed dependence on radionuclide selection and implant duration. For a fixed dose at the prescription point, we observed higher scleral dose corresponded with lower photon energy for a variety of plaque sizes and lesion heights.

Keywords: Eye plaque, Brachytherapy dosimetry, Brachytherapy, Radionuclides, Ophthalmic applicator