We have attempted to determine the relative biological effectiveness (RBE) of negative pions in the Bragg peak region as compared to the plateau region and to gamma rays. We irradiated <tex-math>${\rm LAF}_{1}$</tex-math> mice, bearing 5-day-old lymphoma ascites tumors, in the peak and plateau regions of a 90-MeV pion beam for 40 hours in temperature-controlled holders. The animals were then sacrificed; lymphoma cells were withdrawn and titrated into adult female <tex-math>${\rm LAF}_{1}$</tex-math> mice. The proliferative capacity of the irradiated tumor cells was evaluated after 8 weeks by observing the percentage of animals developing ascites tumors. Surviving fractions were then calculated from LD50' s of control and irradiated animals. Radiation doses in the plateau region were measured with LiF dosimeters calibrated against cobalt-60 gamma rays. We calculated peak doses from those at the plateau, using a measured average peak-to-plateau ionization ratio of 1.5. Doses in the plateau region ranged from 145 to 250 rads; doses in the peak region ranged from 220 to 380 rads. The survival curve for cells irradiated in the peak region gave a D0 of 65 ± 15 rads. The plateau points were not reliable. A replicate experiment was performed with60 Co gamma rays, yielding a survival-curve D0 of 350 ± 50 rads. If the gamma-ray D0 is taken as a base line, an RBE of 5.4 ± 1.8 is obtained for negative pions in the peak region, based on the ratio of peak-region D0 to <tex-math>${}^{60}{\rm Co}\ D_{0}$</tex-math>.

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