Radiation dose to the lens and cataract formation

Abstract

Purpose : To determine the radiation tolerance of the lens of the eye and the incidence of radiation-induced lens changes in patients treated by fractionated supervoltage radiation therapy for orbital tumors. Methods : Forty patients treated for orbital lymphoma and pseudotumour with tumour doses of 20–40 Gy were studied. The lens was partly shielded using lead cylinders in most cases. The dose to the germinative zone of the lens was estimated by measurements in a tissue equivalent phantom using both film densitometry and thermoluminescent dosimetry. Ophthalmological examination was performed at 6 monthly intervals after treatment. Results : The lead shield was found to reduce the dose to the germinative zone of the lens to between 36–50% of the tumor dose for Cobalt beam therapy, and to between 11–18% for 5 MeV x-rays. Consequently, the lens doses were in the range 4.5–30 Gy in 10–20 fractions. Lens opacities first appeared from between 3 and 9 years after irradiation. Impairment of visual acuity ensued in 74% of the patients who developed lens opacities. The incidence of lens changes was strongly dose-related. None was seen after doses of 5 Gy or lower, whereas doses of 16.5 Gy or higher were all followed by lens opacities which impaired visual acuity. The largest number of patients received a maximum lens dose of 15 Gy; in this group the actuarial incidence of lens opacities at 8 years was 57% with visual impairment in 38%. Conclusion : The adult lens can tolerate a total dose of 5 Gy during a fractionated course of supervoltage radiation therapy without showing any changes. Doses of 16.5 Gy or higher will almost invariably lead to visual impairment. The dose which causes a 50% probability of visual impairment is approximately 15 Gy.