Cultured human kidney cells were irradiated while suspended in a gelatin cylinder using a 400-MeV/u neon ion beam produced by the Berkeley BEVALAC. The Bragg peak was transformed to a width of 4 cm with a ridge filter, and cell survival (colony-forming ability) was determined as a function of depth and dose. A cell survival curve in the broadened peak region appeared to be exponential, and RBE values in the peak and plateau regions were approximately 3.0 and 1.8, respectively, at the 10% survival level. Thus, the results indicate that the biological effect is enhanced even when the narrow peak is broadened. The secondary particles (ions lighter than neon and neutrons) produced by neon ions do not cause much cell killing beyond the range of neon ions and, thus, do not seem to be a limiting factor for the use of neon ion beams in radiotherapy.

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