Author(s): Henk JM, Whitelocke RA, Warrington AP, Bessell EM
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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 len 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.
This article was published in Int J Radiat Oncol Biol Phys
and referenced in Oncology & Cancer Case Reports