400 INFLUENCE OF EXPOSURE FACTORS 



This may be due to the possibiUty that fractionation is more Ukely to 

 expose every cell at least once in division, at which time it is susceptible 

 to the radiation. 



Influence of Quality (Change in Energy 

 Absorbed per Gram of Tissue) 



If the effective wave length of photons is shorter the penetrability of 

 the incident beam of radiation is increased. This results in an increase 

 in the depth dose and in the volume of tissue irradiated. It is important 

 in comparing, quantitatively, the effect of two radiations which are 

 widely different in quality to so arrange the exposure and the measuring 

 instruments that the dosage can be expressed in rep (roentgen equiva- 

 lent physical) rather than in "roentgens in air." If this measurement 

 cannot be done satisfactorily one may approach in the following manner 

 the problem of whether two radiations (differing in quality) produce 

 similar effects. A ratio of effectiveness of the two may be determined 

 in arbitrary units for an effect A. If the effects are similar, the same 

 ratio should then hold for effects B, C, etc. Likewise ^ LD^q exposure 

 of one type of radiation plus }/2 LD50 of the other kind should be equal 

 to the effect of LD50 exposure of either type of radiation (72). One 

 should recognize the possibility of recovery between exposures when 

 animals are first irradiated with one and later with another type of 

 radiation. 



Influence of Quality (Change in Specific Ionization) 



A change in quality also produces a change in the ionization densit}^, 

 per micron of path, of the ionizing particle, in tissue (Fig. 3). It may 

 be said that for certain effects (killing of larger microorganisms, chromo- 

 some breaks, etc.) which are apparently due to injury to a small volume 

 of protoplasm the efficiency of the radiation increases with increase in 

 the specific ionization it produces. On the other hand, for effects ap- 

 parently related to changes in a still smaller volume of perhaps only 

 one or a few molecules, cells are irradiated more efficiently by radiations 

 producing a more sparse ionization. For these effects (killing of very 

 small microorganisms and production of a gene mutation) apparently 

 only one or very few ions are required, and thus any additional ones 

 are "wasted." 



Since x-rays and gamma rays produce ionization through the forma- 

 tion of photoelectrons, it is understandable that their effects should be 

 somewhat similar in character and degree to those of beta-particle radi- 



