The intrinsic radiosensitivity of human cell lines (five tumor and one nontransformed fibroblastic) was studied in vitro. The survival curves were fitted by the single-hit multitarget, the two-hit multitarget, the single-hit multitarget with initial slope, and the quadratic models. The accuracy of the experimental results permitted evaluation of the various fittings. Both a statistical test (comparison of variances left unexplained by the four models) and a biological consideration (check for independence of the fitted parameters vis-à-vis the portion of the survival curve in question) were carried out. The quadratic model came out best with each of them. It described the low-dose effects satisfactorily, revealing a single-hit lethal component. This finding and the fact that the six survival curves displayed a continuous curvature ruled out the adoption of the target models as well as the widely used linear regression. As calculated by the quadratic model, the parameters of the six cell lines lead to the following conclusions: (a) The intrinsic radiosensitivity varies greatly among the different cell lines. (b) The interpretation of the fibroblast survival curve is not basically different from that of the tumor cell lines. (c) The radiosensitivity of these human cell lines is comparable to that of other mammalian cell lines.
Survival of EMT6 tumor cells has been measured by the colony method in vivo and in vitro in the presence of oxygen and in anoxic conditions after irradiation with 645 MeV helium ions of the Saturne synchrotron at Saclay. All studies have been realized in the middle of the spread out Bragg peak. The obtained RBE is comparable for cultured cells and for in vivo tumor cells. Its value is 1.4 at 10 -1 survival level but it reaches 1.6-1.8 at 0.5 survival level. In vitro experimental results have been submitted to statistical analysis; after helium ion irradiation, the extrapolation number n is significantly reduced. The magnitude and the kinetics of repair of potentially lethal damage after γ-ray and helium ion irradiation are comparable.