Abstract
The effects of I-131 on the thyroid have been examined using the rat model. (1) Kinetic analysis of I-131 accumulation into and excretion from the thyroid indicated that proportion of iodine in the thyroid was highest in 1-week-ols rats. Yet, this could be because of the amount of blood circulation proportional to the volume of whole body. (2) Carcinogenesis experiments indicated the higher effectiveness of external exposure to internal exposure irradiated at 1 week old. The mechanism how I-131 cause strand breaks of DNA of the thyroid epithelium was speculated. (1) I-131 could exist as a chemical component, HI surrounding the gonome. (2) I-131 could be incorporated into T3, form hormone-receptor complex, bind to specific sites on DNA (miri sec/transcription). Considering the topology of I-131 in the nuclei, the estimation of the frequency of DNA breaks were available for SSB/cell/Gy or for DSB/cell/Gy. The energy of electrons from I-131 (800keV) is similar to that of second electrons from Co-60 (1.25MeV), therefore, the biological effectiveness of the two radiations is thought to be similar. The dose rate should be contributed for the thyroid tumor igenesis.
