Radiation Dose to the Lens in Treatment of Tumors of the Eye and Adjacent Structures

Abstract

When therapy is being planned for tumors of the eye, orbit, or adjacent structures, the question arises frequently as to the amount of radiation that will be delivered to the lens. The dose to this structure must be considered in view of the possibility of a radiation cataract. In a recent study (1) of 100 radiation cataracts and 73 cases without cataracts following irradiation, phantom measurements showed a few minimum, stationary lens opacities at an estimated dose of 200 r at the lens delivered in a single treatment. A few were also found with 400 r at the lens if this dose were delivered in three weeks to three months. When the treatment time was over three months, the minimum dose that produced a cataract was 550 r. In this same study the probability of cataracts at various dosage levels was estimated. With a dose range of 40 to 250 r to the lens in three weeks to three months there were no lens opacities in 20 cases. With doses of 350 to 550 r, cataracts developed in 4 of 9 patients. With 550 to 750 r, 6 of 10 patients showed lens changes; with 750 to 950 r, 15 of 25 patients, and with 950 to 1,150 r, 3 of 4 patients. With higher doses to the lens, the incidence of cataracts was 100 per cent. A dose to the lens of approximately 700 r has thus about a 50 per cent chance of producing a cataract. Although the number of cases in some of these ranges was small, the figures give some indication of the expected incidence of radiation cataracts for a given dose delivered in three weeks to three months. In this same study the effect of dose on the incidence of stationary and progressive lens opacities was investigated for the cases measured. As might be expected, the higher the dose the greater the number of progressive cataracts with resulting loss of vision. Thus, in the group with a treatment time of three weeks to three months, with an average dose of 750 r to the lens, progressive cataracts developed in only 2 of 14 cases. In a group of 20 patients with a dose of 1,450 to 6,000 r all had cataracts and all of these were progressive except 1, which was indeterminate. In the published paper 1 of these 20 cases was listed as stationary, but a subsequent ex-amination showed the cataract to have become progressive. Superimposed senile changes, however, could not be excluded. It is useful, therefore, to be able to estimate the dose that the lens will receive from a particular treatment. Attempts at calculation are difficult. References to standard isodose curves may give an inadequate estimate. Isodose charts are not available for every quality, field size, distance, and diaphragm system. Moreover, the usual depth dose tables start from a flat surface and are not exactly applicable to the contour of the face, especially the region of the orbit. Within the direct beam, shielding arrangements of various shapes, over part of the surface, further complicate the determination of the dose to the lens.