An accurate and simple method for megavoltage radiation therapy of retinoblastoma

A simple but highly accurate irradiation method for the "conservative" treatment of retinoblastoma has been developed at the Utrecht Retinoblastoma Centre. Treatment is carried out with a 6 MV or 8 MV linear accelerator using a lateral D-shaped field of 26 X 32 mm2. This D-shaped field is specially contoured to include the entire retinal surface or vitreous and 10 mm of the anterior optic nerve and to protect the sensitive anterior structures of the eye, particularly the lens, as much as possible. Accurate positioning of the sharply collimated treatment field is easily done by magnetic fixation of the eye to the beam defining collimator by use of a low vacuum contact lens. As the fixed eye is located in the isocentre of the accelerator, the radiation beam can be directed at any angle by simple rotation of the gantry or table, without alteration of the position of the eye within the treatment field. Simultaneous irradiation of bilateral retinoblastoma is just as easily possible and can be done with the same accuracy by fixation of both eyes with contact lenses and alternately applying two opposing lateral fields. Prior to treatment, the axial dimensions of each eye are determined by ultrasonic biometry to ascertain the exact volume to be irradiated.