Effect of Negative Pions on the Proliferative Capacity of Ascites Tumor Cells (Lymphoma) in Vivo

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 The animals were then sacrificed; lymphoma cells were withdrawn and titrated into adult female 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>.