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BLOOD RADIOIODINE CONCENTRATION AND BLOOD RADIATION DOSAGE DURING I¹³¹ THERAPY FOR METASTATIC THYROID CARCINOMA^*

Medical Physics Research Laboratory, Medical Division, Montefiore Hospital New York, N.Y.

TO DETERMINE the radiation dosage delivered to the blood as well as to the whole body during radioiodine therapy, studies were made of the blood I¹³¹ concentration following 53 oral therapeutic doses to 23 patients with metastatic thyroid carcinoma. Data were obtained for four days or more after ingestion of the isotope.

As a result of the administration of I¹³¹, radiation is delivered not only to the thyroid tissue, which concentrates radioiodine, but also, in smaller amounts, to the blood and to the whole body. Radioiodine therapy is clinically expedient only when the radiation dosage to the thyroid gland or to the thyroid tumor greatly exceeds that to the blood. The radiation dosage rate to the blood may be calculated, as shown subsequently, from its I¹³¹ concentration and from the disintegration scheme and physical halflife of the isotope. The ratio between the generalized body radiation and that delivered to the blood will approximately equal the ratio of the average concentration of radioiodine in the body to its concentration in whole blood.

^* Presented by title at the Thirty-third Annual Meeting of the Association for the Study of Internal Secretions, Atlantic City, N. J.June 7–9, 1951.

Received February 1, 1952.