The relative biological effectiveness of 160 MeV protons--II. Biological data and their interpretation in terms of microdosimetry.

Abstract The radiobiological effectiveness of 160 MeV protons was measured relative to 60 Co γ-rays, using Chinese hamster cells cultured in vitro . Separate experiments were performed with cells irradiated in suspension, or attached to plastic tissue culture flasks. Proton irradiations were performed in the incident plateau of the depth dose profile and with the Bragg peak spread out to cover 10 cm. In all cases the relative biological effectiveness (RBE) for protons relative to gamma rays was 1.2 for doses in excess of about 200 rad. The attached cell experiments indicate an increasing RBE at low doses, which is consistent with the microdosimetric measurements.