Pre-therapeutic experiments with the fast neutron beam from the Medical Research Council cyclotron. V. Comparison between the oxygen enhancement ratios for neutrons and x-rays, as observed with Escherichia coli B.

Abstract

While studies on mammalian cellular radiobiology are directly relevant to radiation effects in organised tissues and in the whole mammal, the same is not necessarily true of data obtained with microorganisms as the test system. In one respect at least, however, information gained from the radiobiology of bacteria may be useful as part of this symposium. The relationship between radiosensitivity and oxygen concentration was accurately worked out for the first time with micro-organisms (Alper and Howard-Flanders, 1956), and this relationship was subsequently found to apply in detail also to plant cells (Kihlman, 1958) and to free-living mammalian cells (Deschner and Gray, 1959). It seemed reasonable, therefore, to hope that experiments with micro-organisms might add to the possibility of generalising about the change in oxygen enhancement ratio which occurs when the LET of the radiation is changed, this being at the core of the possible advantage envisaged from using fast neutrons in radiotherapy. Figure 1 shows how changes in oxygen concentration affect the sensitivity of bacteria both to X rays and to fast neutrons from the Van de Graaff generator. It illustrates the principle enunciated by Gray, Conger, Ebert, Hornsey and Scott (1953), that an increase in oxygen tension in the tissues as a whole will increase the sensitivity of anoxic tumour cells in relation to normal, well-oxygenated cells, since these are already at maximum radio-sensitivity.