Intranuclear 3H Thymidine

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Abstract
It appears from a study of available data that the degree of early somatic biological effect from intranuclear H3 is that anticipated on the basis of calculated average absorbed dose to the cell nucleus, at least down to doses of 5 rad or less. This result indicates: (a) the absorbed dose concept holds down to the order of 10-11 g or less, and somatic effects can be predicted on this basis; (b) the distribution of H3 atoms incorporated into DNA as H3TdR (and ion pairs from the beta particles) are randomly distributed as far as dose calculations for the purpose of predicting somatic effects are concerned; (c) every part of the cell nucleus lies within one H3 beta range of some part of a chromatid, and the nucleus contains no sizeable contiguous "insensitive" volume of a radius exceeding the effective range of H3 beta rays (1-2 [mu]); (d) the origin of H3 beta tracks in, or their close juxtaposition to the DNA molecule does not appear to enhance the degree of somatic effect. "Transmutation" effects of the disintegrating H3 nuclide in DNA appear to have been demonstrated to produce genetic mutations in microorganisms, and data suggestive of this mechanism in Drosophila are available. The degree to which this factor in addition to absorbed dose must be taken into account in estimating possible genetic effects in the mammal cannot be assessed adequately at this time, but appears to be small. From available data, it appears that the degree of genetic effect in mammalian cells can be approximated closely from the average absorbed dose to the nucleus. The "intrinsic" RBE [Relative Biological Efficiency] of H3 beta particles appears to be 1.0 (compared to 250 kVp X-rays), rather than the value of 1.7 currently used for radiation protection.