The Kinetics of Chromatid Aberrations Induced in Chinese Hamster Cells by Tritium-Labeled Thymidine

That two points of chromosome damage must exist simultaneously for a twobreak chromosome aberration to arise is a well-documented fact. Sax (see Lea, 1, for review) first demonstrated this phenomenon with his now-classic dose-rate and dose fractionation experiments. The points of damage remain capable of interaction with each other for a measurable period of time. If a dose of sparsely ionizing radiation is administered over a period of time shorter than this time, all the breaks induced by that dose of radiation can interact with one another provided they are within a certain distance of one another. If the radiation is administered over a period of time greater than the rejoining time, breaks induced by the first portion of the dose will close before the entire dose is administered and, therefore, will not be able to interact with subsequently induced breaks. The yield of twobreak aberrations obtained with low-dose-rate irradiations, consequently, will be lower than the yield obtained with high-dose-rate irradiations. In the case where the dose-response curve of a particular aberration type conforms to the equation Y = bo + b1D + b2D2, where Y is the yield, bo the spontaneous rate, bi the coefficient of the linear term, b2 the coefficient of the dose square term, and D the dose, the dose-square term (b2D2) contributes proportionately more to the yield of aberrations at the higher doses than at the lower doses. A lowering of the dose rate will result in a decrease in the coefficient (b2) of the dose-square term. This, of course, is contingent upon the time of irradiation being longer than the rejoining time. Thus, at low dose rates, although the dose-square term would still be present, the dose-response curve would be expected to be dominated by the linear term (blD). This follows from the observation that at low doses the dosesquare term does not contribute as much to the yield of aberrations as the linear