Chronic ocular lesions associated with bi-directional microbeam radiation therapy in an experimental rat study for therapy of C6 and F98 gliomas

Objectives To describe the chronic ocular lesions associated with microbeam radiation therapy (MRT) in an experimental rat study. Procedures MRT was administered bi-directionally with a skin entry dose of 350 Gy. During laterally directed irradiation, the beam entered the head on the right with the center of the beam array 3 mm posterior to the center of the right eye. During irradiation in anterior-posterior direction, the right eye was almost completely in the path of the beam array. Twelve months after MRT ophthalmic examinations were completed on 37 treated (MRT+) and 16 control (MRT-) rats. Electroretinography (ERG) was completed in two MRT+ and one MRT- rat. Histopathology was performed on eyes of 16 MRT+ and 9 MRT- rats, and retinal and choroidal thicknesses were measured. Results Biomicroscopic and indirect ophthalmoscopic examinations revealed fundus pallor and retinal vascular attenuation in 33 of 37 right and 2 of 37 left eyes of MRT+ rats. Cataracts were present in the right eyes of 12 of 37 MRT+ rats. ERG amplitudes were reduced in the eyes of MRT+ rats. Light microscopy revealed retinal lesions ranging in severity with loss of outer to inner retinal cell layers, in 16 of 16 right and of 8 of 16 left eyes MRT+ rats. The mean right retinal thickness of MRT+ rats was reduced. Conclusions Eyes within the field treated with MRT at a dose of 350 Gy develop retinal degeneration and occasionally, cataract.