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.

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