Survival of mouse lymphoma cells irradiated under hypoxic and hyperoxic conditions has been used to calculate the oxygen enhancement ratio (OER) and the relative biological effectiveness (RBE). X-rays at 230 kV and4 He ions of four different linear energy transfers (LET) from the 184-inch synchrocyclotron and the 88-inch sector-focused cyclotron have been used. For x-rays and Bragg curve "plateau"4 He ions of relatively low LET (17 and$80\ {\rm MeV}\text{-}{\rm cm}^{2}/{\rm g}$), the OER changes very little, ranging from 3.2 to 3.5. Higher LET particles, however, in the vicinities of the Bragg peaks with average LET's of 180 and$220\ {\rm MeV}\text{-}{\rm cm}^{2}/{\rm g}$ are more effective under anoxic conditions so that the OER is less, 1.8 and 2.1. The RBE's were also higher, 1.8 and 1.9, with higher LET particles and hypoxic lymphoma cells but with low LET and either hypoxic or hyperoxic cells the RBE's were all quite close to 1.0. An estimate of the decrease of oxygen effect for a particular radiation can be had from the gain factor, the ratio of OER's ($\text{x-ray}/{}^{4}{\rm He}$ ions). The value of 1.5 for Bragg peak particles from the 184-inch synchrocyclotron and also the RBE of 1.8 are of particular interest because of the use of this radiation for therapy.

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