Measurement of RBE, OER, and Recovery of Potentially Lethal Damage of a 645 MeV Helium Ion Beam Using EMT6 Cells

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Abstract
Survival of EMT6 [mouse mammary] tumor cells was measured by the colony method in vivo and in vitro in the presence of O2 and in anoxic conditions after irradiation with 645 MeV He ions. The middle of the spread out Bragg peak was used. The RBE [relative biological effectiveness] was comparable for cultured cells and for in vivo tumor cells. Its value was 1.4 at 10-1 survival level but it reached 1.6-1.8 at 0.5 survival level. In vitro experimental results were submitted to statistical analysis; after He ion irradiation, the extrapolation number n was significantly reduced. The magnitude and the kinetics of repair of potentially lethal damage after .gamma.-ray and He ion irradiation were comparable.