Skin carcinogenesis following uniform and nonuniform beta irradiation

In practical situations where workers or the general public may be exposed to ionizing radiation, the resulting irradiation is rarely uniform. The risk figures and dose limits recommended by the International Commission on Radiological Protection (ICRP) are based largely on clinical and epidemiological studies of reasonably uniform irradiated organs. The paucity of clinical or experimental data for highly nonuniform exposures has prevented the ICRP from providing adequate recommendations applicable to this practical situation. This weakness has led on a number of occasions to the postulate that highly nonuniform exposures of organs, such as the lung or the skin, could be 100,000 times more carcinogenic than ICRP risk figures would predict. This so-called "hot-particle hypothesis" found little support among reputable radiobiologists, but could not be clearly and definitively refuted on the basis of experiment. An experiment design, based on skin tumour induction in mouse skin, is described which was developed to test the "hot-particle hypothesis." In collaboration with the Radiobiology Department of St. Bartholomew's Hospital Medical College, London, the skin of 1200 SAS/4 male mice has been exposed to a range of uniform and nonuniform sources of the beta emitter 170Th (Emax 1 MeV). Nonuniform exposures were produced using arrays of 32 or 8 2-mm-diameter sources distributed over the same 8-cm2 area as a uniform control source. Average skin doses varied from 2-100 Gy. The results for the nonuniform sources show a 30% reduction in tumour incidence by the 32-point array at the lower mean doses compared with the response from uniform sources. The eight-point array showed an order-of-magnitude reduction in tumour incidence compared to uniform irradiation at low doses. These results, in direct contradiction to the "hot-particle hypothesis," indicate that nonuniform exposures produce significantly fewer tumours than uniform exposures.