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

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Abstract
The relative biological effectiveness (RBE) of negative pions in the Bragg peak region was compared to the plateau region and to gamma rays. LAF1 mice, bearing 5-day-old lymphoma ascites tumors, were irradiated in the peak and plateau regions of a 90-MeV pion beam for 40 hr. in temperature-controlled holders. The animals were then sacrificed; lymphoma cells were withdrawn and titrated into adult female LAF1 mice.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. Peak doses were calculated 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 [plus or minus] 15 rads. The plateau points were not reliable. A replicate experiment was performed with 60Co gamma rays, yielding a survival-curve D0 of 3501 50 rads. If the gamma-ray D0 is taken as a base line, an RBE of 5.4 [plus or minus] 1.8 is obtained for negative pions in the peak region, based on the ratio of peak-region D0 to 60Co D0.

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