The eye in acute leukaemia. 1. Dosimetric analysis in cranial radiation prophylaxis

The dose to the ocular lens during standard cranial irradiation prophylaxis in childhood acute lymphoblastic leukaemia (ALL) has been studied both in patients and in an anthropomorphic phantom. Doses to the lens depend on patient set-up and in order that this is minimised, a simple immobilisation technique is recommended. Surface thermoluminescent dosimeter (TLD) measurements seriously underestimate the dose received by the ocular lens. Previous measurements made in a phantom have used a large volume ionisation chamber and therefore the minimum cataractogenic dose of 400 cGy for a fractionated treatment is an underestimate. The exact position of the anterior orbital field margin and thus its distance behind the surface of the eye is also important as regards lens dose. Data from the phantom demonstrate accurately the dose gradient through the eye during standard cranial prophylaxis and may explain the lower incidence of leukaemic relapse in the posterior segment of the eye, and yet explain the persistence of isolated anterior chamber relapses.