Radiology now. Malignancies following low radiation exposures in man

Indications of the induction of cancer in man at doses of less than 10 rad do not themselves give any adequate basis for the various estimates required for protection purposes, of the risks of occupational exposures at corresponding dose levels of the whole body, or of various organs in the adult. Still less do they justify inferences as to the effects of exposure of populations at mean annual doses of only a few millirad in excess of background from various environmental sources. They do, however, add weight to the indication of tumour induction from fractionated exposures, for example from repeated fluoroscopies, in which the total doses are high but individual fractions are low, and so to the possibility of obtaining estimates of low dose effect from them. They also suggest that no very large 'protraction factor' may be needed in reducing dose estimates per rad derived from high doses to infer those applicable at low doses. This conclusion already appeared likely for genetic effects and for changes induced in cell cultures (both for low energy transfer (LET) radiation and even more so for high LET), where primary effects of radiation appear to involve changes induced in single cells. For cancer induction, however, where microdosimetric considerations suggest the initial involvement of groups of cells the expectation is less clear. Direct evidence that occasional malignancies may in certain circumstances be induced in man at absorbed doses of only a few rad, however, indicate the need for continued caution in determining protection criteria in this respect also.