The Interdependence of Oxygen Enhancement Ratios for 250kVp X Rays and Fast Neutrons

If mn, the oxygen enhancement ratio (O.E.R.) at high LET, is less than mx, the O.E.R. at low LET, because of increased formation of oxygen within the target sites of the irradiated material, it follows that mx and mn must be interdependent, the ratio mx/mn being linearly related to mx. This relationship has been tested by measuring mn and mx for six strains of bacteria with widely differing sensitivities to ultraviolet rays and therefore with values of mx ranging from 1·8 to 3·5. The predicted relationship has been observed, so the hypothesis is supported by this test, as well as by the independent evidence from measurements, made with a single strain, of the rate of increase of sensitivity with increasing oxygen concentration for neutrons and for X rays. On the other hand, the data do not fit predictions which follow from the hypothesis that O.E.R. is less at high than at low LET because of an increase in the proportion of interactions between neighbouring radicals which replace interactions of isolated radicals with oxygen. The relationship between mx/mn and mx must be general, if it is valid, and should therefore apply also to mammalian cells. This consideration could affect the selection of patients for neutron radiotherapy.