1054 RADIATION BIOLOGY 



2.2 r, however, developed a normochromic anemia of only moderate 

 significance beginning approximately seventy-nine weeks after exposure. 

 Within a period of forty-nine weeks both the male and female guinea pigs 

 of the 4.4-r group developed a moderate normochromic anemia that 

 gradually increased in severity. A reduction of about 25 per cent in 

 hemoglobin and erythrocyte values occurred. Females exposed to 2.2 

 or 4.4 r developed a lesser anemia than the males at these exposure levels, 

 but a severe terminal anemia occurred in most of the animals, both males 

 and females, in these two exposure levels. Within nineteen weeks, 

 female and male guinea pigs exposed to 8.8 r developed an anemia that 

 increased in severity through sixty and sixty-nine weeks, respectively, 

 and all animals died with a terminal anemia. 



The platelets per cubic millimeter in both sexes exposed to 0.11 r daily 

 (8 hours) remained comparable to those of the control animals throughout 

 the experimental period. Female guinea pigs at the 1.1-r level had a 

 normal mean platelet value when first sampled at forty-nine weeks after 

 exposure began: the value was reduced slowly, reaching a mean slightly 

 below control levels that was maintained throughout the remainder of the 

 experiment. The platelet values of the male guinea pigs at this same 

 level, however, fell slowly between the forty-ninth and seventy-ninth 

 weeks, recovered by the hundredth week, and remained within normal 

 limits thereafter. 



In male and female guinea pigs exposed to 2.2, 4.4, and 8.8 r daily 

 (8 hours), the reduction in platelet values was progressive, the degree 

 of reduction roughly corresponding to the daily dose. The terminal 

 platelet counts for individual animals were as low as 20,000/cu mm and, 

 in many cases, lower. All animals exposed at the rate of 8.8 r daily 

 (8 hours) died with anemia or thrombocytopenia, or both. The majority 

 of the animals exposed to 4.4 and 2.2 r daily (8 hours) likewise died of 

 the same disorders. 



Chronic exposure to the three dose rates of 8.8, 4.4, and 2.2 r daily (8 

 hours) is sufficient to produce a terminal anemia or thrombocytopenia, or 

 both, in the guinea pig. Although Lorenz, Heston, et at. (1946) state 

 that it is difficult to decide from the data whether or not dose rate is more 

 important than total accumulated dose in the induction of the severe 

 terminal pancytopenia, they are inclined to believe that the dose rate is 

 the more important factor. This opinion is based on the fact that no 

 terminal anemia has been observed in the 1.1-r group even though the 

 maximum accumulated dose was approximately 2200 r. This dose was 

 sufficient to induce terminal anemia in the 8.8-, 4.4-, and 2.2-r groups. 

 It seems that in the 1.1-r group the injury is compensated for by repair 

 processes, but it is possible that in species with a similar sensitivity of 

 the hematopoietic system, but with a considerably longer life span, 

 injury would eventually overcome the repair process. 



