The RBE of components of a <tex-math>$\pi ^{-}\text{-meson}$</tex-math> beam, contaminated with electrons and π- decay products, was determined for polyploidy induction in a mammalian cell system in vivo. The RBE in the plateau region of this beam was 1 relative to cobalt-60 gamma rays, whereas in the Bragg peak region it was 2.15. For pion effects only in the peak region, RBE was 2.37; for "star" effects only, it was 3.64. On the basis of theory and preliminary observation, it is believed that RBE values for cell killing exceed the values found for polyploidy induction. We conclude that the diminished oxygen effect and low ratio of suface dose to depth dose displayed by π- beams having a Bragg peak region, coupled with the Bragg peak region's relatively high RBE, indicate that π- beams may be useful in tumor radiotherapy.

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