[Biological effects of the iodine 131 radionuclide].

Radionuclides released into the biosphere by local sources are added to artificial radionuclides introduced by atmospheric nuclear weapon tests or nuclear accidents. The main polluters are nuclear industry reprocessing plants and nuclear power stations. Other sources are hospitals using radionuclides for diagnostics and therapy. This article presents the effects of radioisotope iodine-131 on a living organism, on the cell and on the genetic material within the cell. Beside the accident iodine-131 contamination, its wide diagnostic and therapeutic application attributes to the total radiation dose. The biological distribution of this radiopharmaceutical in the body depends on the way of use and on the and physical and chemical shape of this radiopharmaceutical. A few minutes after the administration, all iodine ingested or inhaled is rapidly absorbed by blood. The irradiation dose in blood depends on the administered amount of 131I and the fraction passing into the plasma as protein-bound 131I. After entering the organism, iodine mostly accumulates in the thyroid gland. The effect of ionized radiation on living cells are proportional to the dosage of 131I. Intracellular radioactive decay occurs in cytoplasm, in karyoplasm, and in the DNA molecule incorporated fraction. Beside the damage resulting from the deposition of radiation energy, 131I causes biological damage when it decays while the isotope is incorporated into the DNA of human cells. Certain beta/gamma-emitting isotopes such as 131I may be particularly hazardous when incorporated into DNA. Incorporation of radioactively labeled compounds into the DNA leads to highly selective irradiation of the nucleus of the proliferating cells. This produces pronounced biological damage as the genetic material in the cell nucleus constitutes the most radiosensitive target within the cell.