Dosimetry and treatment planning of Occu-Prosta I-125 seeds for intraocular lesions

Abstract

Intraocular malignant lesions are frequently encountered in clinical practice. Plaque brachytherapy represents an effective means of treatment for intraocular lesions. Recently Radiopharmaceutical Division, BARC, Mumbai, has indigenously fabricated reasonable-cost I-125 sources. Here we are presenting the preliminary experience of dosimetry of sources, configuration of treatment planning system (TPS) and quality assurance (QA) for eye plaque therapy with Occu-Prosta I-125 seeds, treated in our hospital, for a patient with ocular lesions. I-125 seeds were calibrated using well-type chamber. BrachyVision TPS was configured with Monte Carlo computed radial dose functions and anisotropy functions for I-125 sources. Dose calculated by TPS at different points in central axis and off axis was compared with manually calculated dose. Eye plaque was fabricated of 17 karat pure gold, locally. The seeds were arranged in an outer ring near the edge of the plaque and in concentric rings throughout the plaque. The sources were manually digitized on the TPS, and dose distribution was calculated in three dimensions. Measured activity using cross-calibrated well-type chamber was within +/-10% of the activity specified by the supplier. Difference in TPS-calculated dose and manually calculated dose was within 5%. Treatment time calculated by TPS was in concordance with published data for similar plaque arrangement.

Keywords: I-125; dosimetry and planning; eye plaque; intraocular lesions.

Figure 1

Sectional diagram of Occu-Prosta seed

Figure 2

Anatomy of Eye and eye plaque placement

Figure 3

Magnified view of eye plaque placed over scleral surface

Figure 4

A & B shows dose distribution around the plaque in frontal at 5mm depth & side view

Figure 5

Radial dose fall off on the plaque central axis