Abstract

Previous studies on the variation of dose rate suggested that the duration of radiation was an important factor in the depression of labeled thymidine incorporation into DNA (desoxyribonucleic acid) (1, 2). Preliminary studies on the effects of fractionated doses of radiation suggest that the critical time for the depression of the labeled thymidine incorporation into DNA was in the order of a few minutes. This work was undertaken to further assess the magnitude of the radiation effects on the depression of thymidine incorporation into DNA by varying the temporal relationship between single and divided doses of radiation administered over periods of time ranging from 0 to 256 minutes. These studies have been carried out on semisynchronous populations of liver cells, following partial hepatectomy, as well as asynchronous populations of intestinal cells in the same animal. It has been possible, therefore, to study the effects of different radiation doses given at different times on two different organ systems which, in general, differ markedly in their radiosensitivity. Materials and Methods In this series, the radiation was given locally to the liver and adjacent intestines through a 3 cm cone at the rate of 300 R∕min. The remainder of the animal was shielded. The recorded time between fractionated doses was the time between the end of irradiation of one dose and the beginning of the next. Thus, in the fractionated (750 R × 2) 4-minute experiment, the 4 minutes were allowed between the end of the first dose and the beginning of the second dose. It required 2.5 minutes to give the first and second dose so that a total of 6.5 minutes elapsed between the end of the first dose and the end of the second dose. To compare the effects of a single dose of 1,500 R × 1 with the fractionated dose of 750 R × 2, 4 minutes apart, the 1,500 R dose was initiated so that it ended at the time the first fractionated dose ended. The termination of the single 1,500 R × 1 dose was, therefore, 6.5 minutes before the termination of the second dose. Comparison of a single dose of 750 R × 1 with a divided dose of 750 R × 2 was carried out in a comparable way. For example, the single dose of 750 R × 1 was compared with a divided dose of 750 R × 2 given 8 minutes apart. The single dose of 750 R was initiated so as to end at the same time as the first of the fractionated doses. This was 8 minutes before the second dose was instituted and 10.5 minutes before the end of the second dose. Two additional series of animals were used to compare the effects of divided doses of radiation (375 R × 2) on liver given between 1 and 256 minutes apart, with a single dose of radiation (750 R × 1) at zero time.

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