Different genetic and cellular responses to negative pimesons (peak and plateau) in dependence on intrinsic properties of reaction systems: a new concept of RBE.

Abstract

The biological effectiveness of negative pions from the 590 MeV proton accelerator of the Swiss Institute for Nuclear Research (SIN) has been studied and the results of experiments with different end points obtained since the starting of operation of the biomedicalπ−-beam are presented. The dose rate of theπ−-beam (180 MeV/c) in the Bragg peak was 3–5 rad/min, contamination with electrons and myons 13.5%. 140 kV X-rays with the same dose rate have been used for comparison. To avoid complications introduced by protracted irradiation, only very sensitive systems have been chosen. The normal reaction systems included embryonic damages in mice and inhibition of mitotic activity in mouse jejunum. As a sensitive tumor system Ehrlich carcinoma cells were irradiated in vitro and reinoculated into mice. The RBE-values depending on dose were between 1.3–1.7. The ratios of the effects of same doses in peak and plateau showed clinically desired values of 1.4–1.5. Genetic damages (as possible events leading to cell death) were extensively studied at different stages of development of Drosophila germ cells. Nine different types of mutations such as chromosome loss, loss of definite chromosome regions, lethals, translocations etc. were determined. The RBE values varied between 0.4 and 3.3 depending on mutation type and cell stage. The existence of some RBE values for peak pions under 1 have suggested a new concept of high LET action namely the two system theory which gives more importance to the intrinsic properties of the irradiated biological systems. They can be divided in two groups corresponding to their reaction to high LET radiation: 1. The euoxic and modifiable system which is characterized by intrinsic radiosensitivity, high oxygen tension (euoxic cells), high OER values and low RBE values even under 1; and 2. The anoxic and rigid system with high RBE values for high LET (≫ 1), which is characterized by natural radioresistance and (or) hypoxia.