RADIORESISTANCE

Journal Article · Tue Jan 01 00:00:00 EST 1963 · Laval Medical (Canada)

OSTI ID:4697626

Baker, D G

9 8 9 9 : osed to whole-body irradiation from a Co/sup 60/ source at various dose rates up to 100 r per 23 hours day. At the highest dose rate (100 r/day), there was progressive injury with lethal termination after about 2 to 3 weeks. At doses below 80 r, the same initial reaction was noted but became less severe as time passed until, by about 10 days from the start of exposure, recovery appeared to compensate for injury. Both nitrogen and phosphorus balance in these rats were closely related to the body-weight changes and food intake and showed the same recovery pattern. None of these animals died during, or within 30 days from the end of irradiation. Mice were exposed to a fixed dose of x rays (630 rad) but deiivered at dose rates of from 1 to 180 rad/ min. A probit plot of per cent mortality against the dose rate on a log scale shows that the data fit 2 straight lines. The slope of the line between 10 and 100 rad/min is steeper than that between 0 and 10 rad/min. The lower dose rates possibly allow for the recovery of some critical system which at higher dose rates becomes limiting. Experiments to determine whether this dose rate effect can be explained in terms of the classical recovery factor, as proposed by Blair et al., were conducted. Mice received a fixed dose of 200 r whole-body x irradiation at a constant dose rate of 40 r/min. Immediately following this, 3 groups received an additional exposure to 430 r at either 40, 20, or 10 r/min. Two days following the initial dose, 3 other groups were also exposed to 430 r at dose rates of 40, 20, or 10 r/min. Six remaining groups were similarly exposed at 8 and 16 days intervals. This experimental regimen was then repeated except that the initial dose of 200 r was given at either 10, 20, or 40 r/min, and the challenging dose of 430 r given at a constant dose rate of 40 r/min. The 30-day mortality was observed. When the dose rate of the final dose was varied, it was apparent that there was no dose rate effect. When the dose rate of the initial exposure was varied, however, a highly significant dose rate effect was observed. The data indicated that the recovery function contained a factor which was independent of the interval between exposures (up to 16 days) but was determined by the rate of injury, the higher rate producing the more serious injury. Attempts were made to determine what effect exposures given at dose rates known to prevail in the early failout area of an atomic explosion are likely to have on the acute radiation response. Actual observations indicate that under these conditions, the dose rate from the gamma component of failout follows the empirical relation R/sub t/ = R/sub 10/t/sup -1.2/ where R/sub t/ is the rate at time t. T o simulate this regimen of exposure a Co/sup 60/ device was arranged which would vary the dose rate continuously so as to obey the required power function of time. Thus mice were given various doses of radiation as simulated fallout, lasting 96 hr; at a constant dose rate over the same 96 hr; and in a few min at a dose rate of 65 r/min. Irradiation delivered either as in a simulated fallout or at a constant dose rate over 96 hr doubled the LD/sub 50/30/ compared to an acutely delivered dose. Also, under these experimental conditions the fallout exposure was less effective than the slow regimen as far as 30-day mortality was considered. The results raise the possibility of the elaboration of some sort of transient, nonspecific protective substance against the toxic products produced by cells injured or killed as a result of the radiation. In an effort to account for the varying effectiveness of fixed doses of radiation given at different dose rates it is proposed that a particular metabolic process may be overwhelmed, hence, it becomes limiting. This hypothesis was tested by irradiating populations of the ciliated protozoan Tetrahymena periformis at different doses analyzed for various metabolic intermediates. Metabolic studies and dose response relations were examined using the cells surviving acute and chronic exposures. The

• Research Organization: Univ. of Toronto
• NSA Number: NSA-17-027143
• OSTI ID: 4697626
• Journal Information: Laval Medical (Canada), Vol. Vol: 34; Other Information: From Symposium on Radiosensitivity, Laval Univ., Quebec, Sept. 1962. Orig. Receipt Date: 31-DEC-63
• Country of Publication: Country unknown/Code not available
• Language: English

Related Subjects

BIOLOGY AND MEDICINE
AGE
BODY
CANADA
COBALT 60
DOSE FRACTIONATION
EQUATIONS
FALLOUT
FOOD
GAMMA RADIATION
LETHAL DOSE
METABOLISM
MICE
MICROORGANISMS
NITROGEN
NUCLEAR EXPLOSIONS
PHOSPHORUS
PROTOZOA
QUANTITY RATIO
RADIATION DOSES
RADIATION EFFECTS
RADIATION INJURIES
RADIOSENSITIVITY
RATS
RECOVERY
SEX
SURVIVAL TIME
TETRAHYMENA PYRIFORMIS
WEIGHT
X RADIATION