Backscatter measurements from a single seed of 125I for ophthalmic plaque dosimetry.

To determine the dosimetric effect of a gold plaque applicator used in 125I ophthalmic irradiation, relative dose rates at points 2-18 mm transverse to the axis of a single seed of 125I were measured in an acrylic phantom under three different measurement conditions. The detectors were 1-mm diameter X 3-mm length LiF thermoluminescent dosimeters (TLD's). Conditions corresponded to the following: (i) full scatter, (ii) the presence of an ophthalmic gold plaque, and (iii) no scatter material on the side of the seed opposite to the TLD's. The dose rate with the gold plaque is less than that with full scatter phantom. There is no significant decrease in dose rate at 2.2 mm from the seed. Dose rate is significantly reduced at greater distances. The does rate decrease ranges from 4% at 5 mm to 10% at 18 mm. The 125I seed in the gold plaque gives 3%-5% higher dose rate than in the absence of backscatter material.