1. A method of treating a lesion on or adjacent the retina of an eye of a patient with an external-beam radiation device, comprising: (a) placing the patient's eye in alignment with a known system axis in an external three-dimensional coordinate system, and measuring the eye's axial length; (b) from the known position of the system axis and from the measured axial length, determining the coordinates of the lesion to be treated in the external coordinate system; (c) directing a collimated x-ray radiation beam along a known beam axis in the external coordinate system at the lesion to be treated; (d) during said directing, tracking the position of the patient's eye with respect to the known system axis, thus to track the position of the lesion to be treated in the external coordinate system; and (e) based on the known beam axis of the collimated beam in the external coordinate system, and the instantaneous position of the lesion to be treated in the external coordinate system, as determined at least in part by the tracked position of the eye, calculating a total radiation equivalent received at the lesion to be treated during the treatment.