Effect of microdistribution of radionuclides on recommended limits in radiation protection, a model.

The heterogeneous distribution and accumulation of radionuclides in discrete areas of cellular and subcellular dimensions is called microdistribution. The biological effect of microdistributed radionuclides with low-range emissions is determined by the degree of irradiation of the radiosensitive microareas of the body. The critical microareas of the body are nuclei of such cells which (1) are radiosensitive, (2) are essential to maintaining life, (3) are irreplaceable, (4) have a long life span and/or renew themselves. In this sense, the stem cell nuclei are considered critical microareas of the body. Stem cells constitute only a small fraction of the total body's cellularity. In case of concentration of radionuclides in stem cell nuclei, such as from incorporated labeled DNA precursors, there is a total congruence of the radionuclide microdistribution with the radiosensitive microarea, and the biological effect is expected to be enhanced over that from a homogeneous distribution of the same amount of radionuclides. This situation is discussed for 3H, 14C and 125I incorporate into mice as tracers of DNA precursors. The average labeling intensity of the bone marrow cell nucleus was taken to represent the average labeling intensity of the stem cell nucleus. The dose to the stem cell nucleus, then, is derived from the number and energy of decays originating in the nuclear mass of 270 X 10(-12) g. The transmutation effect from isotopic decay in DNA is considered in order to arrive at dose equivalents. On the basis of known data on labeling efficiency of bone marrow and on stem cell proliferation kinetics in the mouse, the infinite accumulation of decays in and the total expected dose to the stem cell nucleus was calculated for intravenous injection or ingestion of 1 muCi 3H-TdR per g body weight. The distribution factor and an annual limit on intake for the mouse model was suggested. Corresponding data are presented for 14C-TdR and 125I-UdR. A special situation is given for the case of hot particles where there is a random relationship between microdistributed radionuclides and critical microareas of the body. In this instance, theory predicts a decreased biological effect in comparison to the situation where the same amount of radionuclides is homogeneously distributed. There is experimental evidence that supports the theoretical predictions particularly for the case of 236Pu dioxide in the human lung.