Radiation dose to the lens and cataract formation.
OBJECTIVE
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%.


CONCLUSIONS
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.
Henk
J M
JM
Royal Marsden Hospital, London, England.
Whitelocke
R A
RA
Warrington
A P
AP
Bessell
E M
EM
D000328:Adult; D000368:Aged; D002386:Cataract / Q000209:etiology*; D005260:Female; D006801:Humans; D007908:Lens, Crystalline / Q000528:radiation effects*; D008223:Lymphoma / Q000532:radiotherapy*; D008297:Male; D008875:Middle Aged; D009918:Orbital Neoplasms / Q000532:radiotherapy*; D011446:Prospective Studies; D011878:Radiotherapy / Q000009:adverse effects; D011879:Radiotherapy Dosage; D014792:Visual Acuity / Q000528:radiation effects