Dose-effect relationship and the significance of fractionated and protacted radiation for the frequency of fetal malformations following X-irradiation of pregnant C3H-mice

The dose-response curved for the individual malformations all indicate a threshold value at about 0.75-1 Gy followed by an exponential increase with an exponent of 2 or more. Taking all the malformations together, a significant increase up to 0.75 Gy is found. For the whole of the dose range up to 2.5 Gy the data are consistent with a linear quadric dose-response relationship. From the fractionation experiments the Elkind type of recovery is seen in which the increase in the radiation effect after a period of some hours is a consequence of the accumulation of cells in the radio-sensitive G 2 phase. In our experiments on the distribution of cell in the cell cycle an increase in the radiosensitivity occurs simultaneously with a pronounced increase in G 2 + M. The differences between the malformation and resorption frequencies for the morning and afternoon irradiation can of course not be explained by changes in distribution in the cell cycle. The increase in the malformation and resorption rate after protracted irradiation might also be explained from knowledge of cell proliferation as a significant fraction of cells are likely to be irradiated during the G 2 + M phases due to the brief duration of these phases in relation to the exposure time. However, an increase is only seen in some types of malformations, such as those of the skeleton, while for other types, such as those of the eyes, no increase was found but definitely no decrease as in the case of fractionation. (orig.)