Physical Aspects of Eye Plaque Brachytherapy Using Photon Emitters

Abstract

The current trend toward the use of ¹²⁵I seed rather than ⁶⁰Co applicators in the treatment of ocular tumors stems largely from the extent to which photons from the two radionuclides are attenuated differently by plaque material on the one hand and by intraocular tissue on the other. The much lower energies of ¹²⁵I photons from seeds in a rimmed gold plaque not only permit near-total elimination of radiation dose in orbital tissue adjacent to the plaque but also allow significant dose reduction to critical structures within the eye, relative to either ⁶⁰Co plaque or proton beam reatment (Fairchild 1984). Although other photon-emitting radionuclides have been used in ophthalmic applicators (Luxton et al. 1988a), we will focus attention here on the physical characteristics of ⁶⁰Co and ¹²⁵I applicators because they have been the most widely used and because they illustrate well the pertinent differences between “high energy” and “low energy” photons for this type of brachytherapy.