Selection of Iodine 125 for the Collaborative Ocular Melanoma Study

Abstract

Concern has been expressed regarding the use of "state-of-the-art" radioisotopes in the brachytherapy of choroidal melanoma.¹The radioisotope iodine 125 was selected for use in the Collaborative Ocular Melanoma Study (COMS)²only after careful consideration of available isotopes. Plaque therapy is an available option at any center associated with a well-staffed radiotherapy facility. Radiation sources for plaques have included radon 222 seeds, gold 198 seeds, cobalt 60, iridium 192, tantalum 182, iodine 125 seeds, and ruthenium 106. All of these radioisotopes are photon emitters (gamma rays or x-rays), except ruthenium 106, which is primarily a beta emitter. Because these radioisotopes differ from one another in the energy of the emitted radiation, their relative penetration in tissue, as well as the thickness of material required for shielding, also differs. The amount of radioisotope used in a particular procedure is specified in terms of activity expressed in millicuries (or in