4 ROBERTS RUGH 



ferentiation. The radiosensitivity of a single cell may vary 1,000 times from 

 its undiflferentiated to its differentiated state. The time has long since passed 

 when data were adequately presented in terms of milligram hours of radium 

 without consideration of the actual absorbed dose (rads), as well as the 

 organ, tissue, or cell exposed. 



There is a confusion of teiTns, such as threshold, safe, permissible, or toler- 

 able doses of ionizing radiations. If by threshold one means an exposure 

 below which nothing happens, it is very doubtful that such a level exists. A 

 single ionization occurring at a critical point on a chromosome may not 

 affect its bearer, but may have permanent and drastic effects on its progeny, 

 which explains the extreme caution of the geneticists. At the level of the 

 atom, any effect brought about by ionizations is probably all-or-none. It is 

 another matter whether such an effect is detectable and, if detectable, 

 whether it is tolerable to the biologic system. Certainly some changes in the 

 central nervous system of the embryo or of the adult are both detectable 

 and tolerable, if by tolerable we mean that the individual is able to survive. 



At the cellular level the changes brought about by the absorption of 

 ionizing radiations are irreversible, irrevocable, and irreparable, but they 

 may still be tolerated by the cell. If the cell survives and is a germ cell, it 

 may contribute a new mutant to its succeeding generations of cells, both 

 detectable and tolerable, but not likely of any benefit. If the cell survives as 

 a somatic cell, it may tolerate the damage and reproduce by mitosis for 

 many years, ultimately to blossom out in two or three decades as a center of 

 a malignancy. The ability to tolerate absorbed radiations is good for sur- 

 vival, but possibly not good for progenies. 



The term permissible dose is used largely in Civil Defense directives and 

 radiologic centers, and its dose level is generally somewhat lower than the 

 tolerable. There is still another aspect of the tolerable dose, and this relates 

 to the ability of the adult to repair or regenerate replacements for damaged 

 tissue, usually scar tissue, or of the embi-yo to redirect its schedule and pattern 

 of differentiation. The words have a wider meaning, therefore, and include 

 reparative processes above the cellular level. A certain amount of radiation 

 may be tolerated by an organism if the remaining tissue is sufficient for 

 survival and can replace the void with protoplasmic mass. This ability is 

 more evident in the embi-yo where undifferentiated cells are being directed 

 and redirected after irradiation toward differentiation along the various 

 routes which result in recognized tissues. Once this is completed, as in the 

 adult, replacement of kind is not usual. 



A corollary of this discussion is the matter of cumulative effects. Certainly 

 one can see how a primitive germ cell, successively exposed to ionizing radia- 

 tions, might well accumulate mutants at various points along its chromo- 

 somes. In the same way, a somatic cell may be bombarded successively by 



