K values and gain factors of fast neutrons and x-rays for a mammalian cell system (CFU) in vivo

The relationship between radiosensitivity and oxygen tension is defined by two parameters; m, the maximum ratio of radiosensitivity obtained by variation of the oxygen tension, and K, the oxygen tension at which the radiosensitivity equals (m + 1)/2. It is shown here that the ratio of K values for two radiations, e.g., X-rays and neutrons, can be determined from the sensitivities of cells in anoxic, well-oxygenated, and one intermediate condition, irrespective of the absolute oxygen tension of the latter. For mouse bone marrow stem cells irradiated in dead mice and in mice breathing air or breathing 5.8% O2 in N2, the ratio Kₓ/K_n was 1.46 ± 0.45. This ratio implies a Gain Factor for neutrons, with respect to cells at low (but not zero) oxygen tension, which is slightly greater than that expected if Kx is less than, or equals, Kn. The findings are discussed with reference to fast-neutron radiotherapy.