The effect of acute and fractionated doses of X rays on the growth of the mouse tibia

An exposure-response curve is given for the depression of growth of the mouse tibia after irradiation of one hind leg at three weeks of age with 250 kVp X rays. The mice were killed at about 40 weeks after irradiation, when the deficit was maximal, except for the 2,000 R exposure group killed at 12 weeks after irradiation. The curve is linear from 200 R down to 1,000 R. (The 2,000 R point is well above the line although the standard deviation does touch the line. This shows up more using a logarithmic scale for these results.) The results for 50 R and 100 R lie on a shoulder. There is no significant difference in response to fractionated exposures of 2 × 500 R, 5 × 200 R, or 10 × 100 R. The mean result in each group can be explained using the exposure-response curve for single exposures and measured values for the loss of response with age at irradiation. When total exposures of 2,000 R or 1,000 R are given as two equal fractions, the efficiency is least when the fraction interval is six to 12 hours, after which there is a partial recovery of efficiency which is more obvious after the smaller total exposure. An explanation is suggested on the basis of the observed depression and recovery of mitotic rate in the irradiated cartilage plate.