Abstract
Cyclotron-produced 123I has short half-life (13.3 h) and an optimum gamma ray energy for scintigraphy (159 keV) . It makes radiation dose to the thyroid as low as nearly one hundredth of 131I. The purpose of this study is to evaluate 123I for clinical use in place of 131I. Thyroid uptake measurements were performed with the standard techniques at 3 and 24 hours after oral administration of radioiodine, and the thyroid scintigraphy were made by a rectilinear scanner or a scintillation camera. Thyroid uptakes of 123I did not coincide so well with 131I when the measurements were performed one after another, but they coincided very well when patients were given both radioiodine simultaneously and measured by the double tracer technique. Thyroid scintigrams of 123I were as excellent as those of 131I except a few cases showed slight increment of the background density. The radioactivity of 123I in the thyroid at 3 hours decays one third in 24 hours. Of 64 patients studied, 59 or 92% showed higher count rate in the thyroid region at 3 hours than 24 hours. The thyroid scintigram using 123I, therefore, is better to be made in 3 to 6 hours while the radioactivity well remains in the thyroid tissue. Iodine-123, instead of I-131, will soon be in use for the thyroid studies in our country.
