Total-Head (Brain) X Irradiation of Mice and Primary Factors Involved

1. White Swiss (CFW) mice, male and female, were exposed at 30 days of age to total-head X irradiation given in a single dose in order to determine the LD50/30. The levels of exposure ranged from 500 to 4000 r (22 MeV) betatron, and from 500 to 3500 r (200 kVp) X-ray irradiation. 2. The mouse LD50/30 for total-head betatron X irradiation was found to be 1600–1650 r. The LD50/30 for total-head 200 kVp X irradiation was found to be 1250 r. There appears a difference in the lethal responses of approximately 30-day-old mice to the two types of radiation. Betatron irradiation is only about 75–78 per cent as effective as 200 kVp X irradiation to the whole head. 3. Total-head X irradiation does not commonly induce death by bacteraemia or by septicaemia. A leucocytosis occurs and this is in contrast to results obtained by whole-body X irradiation. When a bacteraemia occurs it is an agonal infection. 4. Neither white nor red cell counts are depressed in total-head X-irradiated mice and where there is an elevation in the blood count there is also dehydration and blood concentration. 5. Total-head X irradiation at the LD50/30 level with 22 MeV and 200 kVp induces (a) haemorrhage of the brain, (b) dehydration and its effects upon the haemopoietic system and gastro-intestinal tract, (c) delayed pneumonia of various types, (d) weight loss, (e) hair loss in the area irradiated, and (f) possible adrenal and other organ effects. 6. Total-head X irradiation does not induce death by a single means; it is a cumulative effect upon the central nervous system and apparently other tissues without primary bacterial effects. The cause of the sudden failure in mice recovering or apparently recovered from total-head X irradiation is unexplained. 7. The loss of weight occurring after total-head X irradiation of mice has been evaluated for the various dosage levels used, and the ratio of head weight to body weight at various ages is presented. 8. The findings, in general, indicate that when Swiss mice are exposed to total-head irradiation the degree of injury appears to be a function of the dose.