1048 RADIATION BIOLOGY 



made in animals exposed chronically to 7 rays, fast neutrons, and X rays. 

 The work of Lorenz, Eschenbrenner, et al. (1946), Lorenz, Heston, et al. 

 (1946), Lorenz, Uphoff, and Sutton (1949), and Lorenz (1951) on 7-ray 

 exposure has been extensive and exceptionally well controlled. Evans 

 (1948) has studied the effects of fast neutrons and, to a lesser extent, the 

 effect of X rays on the hematopoietic system. 



CHRONIC EXPOSURE TO X RAYS AND FAST NEUTRONS 



For purposes of comparison of the biological effectiveness of X rays and 

 fast neutrons, Evans (1948) exposed groups of Swiss mice to daily doses 

 of 80 r (X rays) and 10 "N" (neutron units arbitrary). These doses 

 were chosen as the basis for comparing the median lethal doses of the two 

 radiations that gave a ratio (in effectiveness) of 8.1 r = IN. The radia- 

 tions were given daily for 25 days, and the accumulated dosages were 

 2000 r and 250 N. The hematologic responses were similar in both 

 groups. The fall in the leukocyte values of both groups was comparable 

 through the 30 days of observation at which time the values were below 

 1000/cu mm. 



The erythrocyte and hemoglobin values of the fast-neutron and X-ray 

 exposure groups remained within control limits for about one week and 

 thereafter fell steadily, reaching levels of approximately 2 million and 

 3-4 g, respectively, on the twenty-eighth day. On these findings and 

 other data including survival and histopathologic examination of post- 

 mortem material, Evans concluded that the effects of these two radiations 

 at this dosage level were qualitatively and quantitatively similar. 



CHRONIC EXPOSURE TO REPEATED SMALL DOSES OF FAST NEUTRONS 



In another experiment, Evans (1948) exposed groups of Swiss and 

 CFi male and female mice to dosages of 0.014, 0.07, 0.14, and 1.4 N/day. 

 These dosages, according to his calculations, are equivalent biologically 

 to a value of 1 N = 35 r. Exposure of the Swiss mice was continued for 

 ninety weeks and of the CFi mice for eighty-three weeks. 



Although the mean leukocyte values of all exposure groups were con- 

 sistently lower than those of the control groups, only in the 1.4 N/day 

 exposure group were significant effects observed in the peripheral blood. 

 In this latter group, reduction in the leukocyte values was apparent 

 within four weeks after beginning of exposure, and a reduction in the 

 mean count of about 50 per cent was apparent at twenty-four weeks when 

 the exposure was discontinued. This leukocyte reduction was largely 

 due to the lymphocyte reduction; no effect on erythrocyte, hemoglobin, 

 reticulocyte, or platelet values was noted during the period of exposure 

 of any of the four groups. 



Suter (1947) and Ingram and Mason (1950b) studied the hematologic 

 effect of chronic X irradiation in dogs, rabbits, rats, and monkeys. With 



