A Quantitative Description of Latent Injury from Ionizing Radiation 337 



as depicted, until it intersects the curve of diminishing lethal threshold to cause 

 the animal to die prematurely. 



One indication of this has been obtained by Baxter (14) in fruit flies. 

 These flies normally live for about fifty days and lose half their life span if 

 exposed to 75,000 r in a single dose. They die on day 26 approximately whether 

 irradiated on day 1, day 25, or any day in between. Presumably recovery 

 is very rapid in this species, and the irreversible component has the same 

 eff'ect when laid down at any time which is early enough in life to allow the 

 whole potential life-shortening to be made manifest. 



OSINGLE DOSE 

 • DIVIDED DOSES 



100 200 300 400 



DOSE IN PERCENT OF LDsq-SO DAYS 



Fig. 4. Life shortening in per cent of normal span as a function of LD50 for 

 rodents exposed to single doses or divided doses of x- or gamma-radiation. 

 In the case of divided doses the radiation was stopped sufficiently long before 

 death, or was at a sufficiently low daily level, that life shortening was caused only 

 by irreversible injury, all, or nearly all, acute injury presumably having been 

 repaired. The scatter of data is quite high for low divided doses, there being almost 

 as many (omitted for simplicity) which show prolongation as shortening of life. 

 The single dose curve rises more rapidly than linearly as LD50 is approached. The 

 sources of the data are given in (16). LD50 is from 500 to 700 r for most of these 

 strains. There is no established reason why data from different species should 

 form a consistent pattern in this mode of plotting. They are less consistent than 



data on single strains. 



Incomplete observations by Hursh and Casarett (13) indicate that a 

 given dose shortens life by about the same fraction of the normal expectancy 

 in groups of rats exposed in early adult life or beyond middle age. 



Direct measurements of residual injury as reduction in LD50 have been 

 made no later than a few months after an initial dose. Such direct determinations 

 when extended will be a more satisfactory test of the validity of the hypotheses 

 depicted in Fig. 3 than the life-span data mentioned above. 



Another factor to be discussed is whether the irreversible injury, or the 

 constant a, is independent of dosage. It appears definitely to be larger with 

 fast neutrons and alpha rays than with x- or gamma-rays (3). As measured 

 by life-span shortening, it is also greater for single substantial doses of x- or 

 gamma-rays than for divided doses even though all are delivered at the same 

 dose rate in roentgens per minute. 



Figure 4 shows the after eff'ects of single and divided doses on life span 

 in a number of strains of rats and mice. These data are plotted on the assump- 

 tion that strains of different life spans and different LDjo's will lose the same 

 fraction of their life spans per unit dose measured in LD5Q. Existing data 



