Neoplastic Transformation of Plateau-Phase Mouse 10T 1/2 Cells Following Single and Fractionated Doses of X Rays

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Abstract
The induction of cell transformation and cell killing in plateau-phase 10T1/2 mouse cells by single and fractionated doses of X rays was investigated. The single dose-transformation curve was composed of a 1st phase (doubling dose = 0.2 Gy [gray]), a 2nd phase (doubling dose = 1 Gy), and a 3rd phase (no increase of transformation at doses more than 6 Gy). Two-dose fractionation experiments revealed that transformation damage was reduced when total doses of 0.93, 1.86, and 3.72 Gy were given with two equal fractions separated by intervals of 15 h, as compared with the equivalent single doses. This not only represents repair of subtransformation damage but also is consistent with previous findings of repair of potential transformation damage. The fractionated dose-transformation curve obtained 3 h after the first (conditioning) dose indicated that damaged cells had recovered, at least in part, from transformation damage, as shown by the reappearance of the 1st phase of transformation induction. No alteration of sensitivity to the 2nd dose seemed to develop. However, the transformable fraction of the cell population was apparently increased by the conditioning dose. From the present results with single and fractionated doses, a resolution between repair of sub- and potential transformation damage did not seem possible. Plateau-phase cells were found to be useful for dose fractionation studies.