Dosimetric considerations relative to radionuclides for thyroid diagnosis and therapy

Recent changes have occurred in the radionuclidic approach to the diagnosis and therapy of thyroid diseases. These changes have been directed toward reduction of radiation dose by the use of short-lived radionuclides for imaging and toward better control of late effects of therapy by substituting iodine-125 for iodine-131. Imaging of the thyroid is now widely performed following the administration of technetium-99m or iodine-123. Some problems exist relative to the use of technetium-99m in that the distribution of radioactivity in the thyroid is not always identical to the distribution of radioiodine. Iodine-123 represents a substantial advance over iodine-131 despite contamination of the former with small amounts of other radioiodines. The high incidence of induced hypothyroidism following iodine-131 therapy of thyrotoxicosis has spurred interest in the use of iodine-125. The biological effectiveness of iodine-125 is believed to be greater because of the high abundance of Auger electrons and possibly because of chemical effects following molecular disruption. Clinical results to date are discussed