A device for precision X-irradiation of the macular region of the retina of a patient's eye has a mounting plate mounted to a linear accelerator collimator. A housing extends from the mounting plate, and a secondary collimator is positioned in the housing. An elongate aperture extends through the secondary collimator and is in linear alignment with an axis of an X-ray beam emanating from the linear accelerator collimator. A support member extends outwardly from a side of the housing, and an elongate, hollow, sight tube is mounted on the support member. The longitudinal axis of the sight tube aligns with the axis of the X-ray beam so that the axis of the sight tube and the axis of the X-ray beam intersect each other. A light beam is transmitted down the sight tube. The light beam impinges on a cornea of a patient's eye. Light reflected from the cornea is transmitted up the sight tube so that impingement of the light beam on the cornea can be observed through an upper end of the sight tube. The patient is positioned so that the macular region of the retina of the patient's eye lies at the intersection of the longitudinal axis of the sight tube and the axis of the X-ray beam.