~14- By contrast, the cellular effects of X-rays and y-rays are largely repairable at low dose rates. This stems from the fact that the diffuse distribution of ton pairs produced by such radiation results in widely: spaced single chromosome breaks which repair themselves readily. For these réasons the relarive biological effectiveness of alpha particles, compared to X-rays and y-rays rate. increases continuously with decreasing dose Thus alpha radiation acquires a greatly increased biological sig- nificance relative to soft radiation in the production of tumors and other health consequences of chromosomal structural changes. ; There are several other lines of evidence which reinforce the possibility that aipha interactions with cells play a unique rele in human cancer production. The distribution of cancer sites in the bronchi, in the lymphatic system, in arterial tissue, in the liver and bone, all involve sites at which insoluble alpha emitters are known to accumulate. . Anomalausly high concentrations of alpha activity have been observed at the bronchial cancer sites) » at cancer sites adjoining lymph glands in other organs (52,53) t in atherosclerosis plaques (41-43) ,» at liver cancer sites in thorotrast patients ©") , at bone tumor sites in the radium dial workers (55) | etc. The difficulties of producing lung cancer by external radiation has been pointed out by Warren and Gates (25+36) | The absence of cancers in muscular tissue, except at sites of thorotrast injection or . plutonium injection, also is relevant to this issue. All of these obser- vations reinforce the possibility that one or more of the chromosomal structural changes which characterize a malignant cell’ must be brought about by alpha interactions and not by low intensity X-rays or y-rays. In this connection, the determination of the nature of the structural $ .