- 30 Page 14, lines 1-9 - "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 ion pairs produced by such radiation results in widely spaced single chromosome breaks which repair themselves readily. For these reasons the relative biological effectiveness of alpha particles, compared to X-rays and Y-rays increases continuously with decreasing dose rate. Thus alpha radiation acquires a greatly increased biological significance relative to soft radiation inthe production of tumors and other health consequences of chromosemal structural changes." Comments: The first two sentences of this paragraph are essentially correct. The third sentence is essentially correct, but it should be pointed out and emphasized that the relative biological effectiveness of alpha particle irradiation, compared to x-radiation and y-radiation as standards, increases with decreasing dose rate not because the effectiveness of alpha radiation changes with change in dose rate but because the effectiveness of the standard radiation (x- or Yradiation) decreases with decreasing dose rate. ‘The effectiveness of the alpha radiation is independent of dose rate and proportional to dose. The 4th sentence of this paragraph is also subject to this qualification and to others of my vreceding comments relating to it. Page 14, Lines 10 to page 15, line 2 - There are several other lines of evidence which reinforce the possibility that alpha interactions with cells play a unique role 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 whichinsoluble alpha emitters are known to accumulate. Anomalously high concentrations of alpha activity have been observed at the bronchial cancer sites(51), at cancer sites adjoining lyn h glands in other organs (52,53) in atherosclerosis plaques 1- 3), 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 35,36) | 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 observations reinforce the possibility that one or more of