Estimation of Radiation Dosage Delivered by a Single Massive Injection of I 131 in the Rat

Abstract

Young adult female Sprague-Dawley rats were injected with I/sup 131/ at levels of 1, 10, 50, and 90 mu c/g body weight. Retention of I/sup 131/ was determined by in vivo counting for 2 weeks after injection. Fifty rats injected with 50 mu c/g of I/sup 131/ were serially sacrificed at intervals from 4 hr to 6 days later. Tissue weights and I/sup 131/ concentrations were determined, and beta - and gamma -ray doses were calculated for the whole body and several tissues. The internal radiation significantly altered bcth release of I/sup 131/ from the thyroid gland and gastrointestinal metabolism of radioiodine. The thyroid glands of the high-dose rats accumulated I/sup 131/ normally for the first 12 hr. after the twelfth hour the heavily irradiated thyroid tissue lost I/ sup 131/ rapidly, so that within a short time all the accumulated I/sup 131/ had been returned to the circulation. When the rapid release of I/sup 131/ from the irradiated thyroid was taken into account, the calculated beta -ray dose to the gland was 25% of that which would be calculated from tracer studies. Gastric motility was reduced during the first 36 hr after the I/sup 131/ injection, causing a significantmore » delay in reabsorption of I/sup 131/ from the intestine. Prolonged whole-body retention of I/sup 131/ and elevated tissue I/sup 131/ concentration s could be accounted for by delayed reabsorption of I/sup 131/ secreted by the stomach, and by the release of I/sup 131/ from the thyroid. The gamma -ray contribution to the total dose was significant, 15 to 30% of the total dose in the whole body and in all tissues except the thyroid and gastric mucosa. The total radiation dose in the whole body and the doses calculated for the individual tissues from a single high-level injection of I/sup 131/ were generally consistent with previous observations on lethality and pathology of long-term survivors. It is suggested that the pathologic changes previously observed in long-term surwivors of a single massive injection of I/sup 13/1 are not due to either radiation or endocrine deficiency alone but are the consequences of both radiation damage and profound and prolonged endocrine deficiency. (auth)« less