DOSIMETRY FOR I-125 SEED (MODEL-6711) IN EYE PLAQUES

The effect of eye plaque materials (gold backing and silastic seed-car rier insert) on the dose distribution around a single I-125 seed has b een measured, using cubic lithium fluoride thermoluminescent dosimeter s (TLDs) 1 mm on an edge, in a solid water eye phantom embedded in a s olid water head phantom. With an I-125 seed (model 6711) positioned in the center slot of the silastic insert for a 20-mm plaque of the desi gn used in the collaborative ocular melanoma study (COMS), dose was me asured at 2-mm intervals along the plaque central axis (the seed's tra nsverse axis) and at various off-axis points, both with and without th e COMS gold backing placed over the insert. Monte Carlo calculations ( MORSE code) were performed, as well, for these configurations and clos ely the same geometry but assuming a large natural water phantom. Addi tional Monte Carlo calculations treated the case, both for 20- and 12- mm gold plaques, where the silastic insert is replaced by natural wate r. Relative to previous measurements taken in homogeneous medium of th e same material (without the eye plaque), the dose reduction found by both Monte Carlo and TLD methods was greater at points farther from th e seed along the central axis and, for a given central-axis depth, at larger off-axis distances. Removal of the gold backing from the plaque did not make measurable difference in the dose reduction results (10% at 1 cm).