Biological damage from radioactive nuclei incorporated into DNA of cells; implications for radiation biology and radiation protection

Radionuclides with short or very short range emission cause biological effects the severity of which largely depends on the degree of distribution of the radionuclide between various organs, cells, and subcellular structures. Decisive here is the spatial congruence between site of decay and of energy absorption and radiosensitive structure. Amongst the many biological entities the most sensitive structure and therefore the critical structure appears to be the nucleus of the stem cell. Stem cells are distributed throughout the body; yet their concentration may vary from one organ tissue to another. Within the stem cell nucleus the critical molecule is the DNA. Radionuclides with very short range emissions and which exert any kind of transmutation effect are therefore most damaging when directly incorporated into the DNA of stem cell nuclei. In order to determine derived limits for radiation protection purpose, the following 4 factors need specific consideration in addition to the conventional radiation quality factor: cell distribution factor; metabolic rate factor; cell renewal factor; relative distribution effectiveness. The product of these 4 factors yields the total modifying factor that can be used for calculating dose equivalent. The specific case of tritium, 14-carbon, and 125-iodine will be considered to examplify the use of the total modifying factor in radiation protection