Radiobiology of neutrons

Tumor responses depend on a number of factors, of which the intrinsic radiosensitivity of the cells and their oxygenation condition can be considered as most important. Variations of the sensitivity in the cell cycle and differences between resting and proliferating cells are considered to play a smaller part. Large differences in intrinsic cellular sensitivity and in RBE values of fast neutrons have been observed for impairment of the clonogenic capacity of cells in culture and in experimental tumors. For most of the treatments commonly applied in radiotherapy, doses per fraction are smaller than 5 Gy of photons and 1.5 Gy of neutrons. In these dose ranges, the survival curves of mammalian cells can be described by the formula: S(D)/S(O)=exp - (a1D + a2D2). Values of a1 range between 10-1 and 1 Gy-1 for photons; they are a factor of 3 to 10 larger for fast neutrons, depending on the type of cell and the neutron energy spectrum. However, a direct correlation between cellular sensitivity and the magnitude of RBE values has not been established. The influence of fractionation is described by the value of a1/a2. It is shown that for several important normal tissues these values are smaller than for cells from a number of experimental tumors. Therefore, with X rays, more sparing of normal tissues is obtained than for these tumors, and the application of neutrons is not expected to provide an advantage in this respect. The presence of hypoxic cells in experimental tumors has been well established. It has been observed, however, that for daily fractionated treatment with doses of less than 3 Gy of photons, the influence of hypoxia is much smaller than for single large doses, because of the process of reoxygenation. Nevertheless the low OER of fast neutrons and other high LET radiation might provide an advantage that could improve treatment results. It is suggested that better methods must be developed to predict the responsiveness of tumors in patients to photons, in order to selectively apply high LET radiations only to tumors that are resistant to photons.
Chemicals/CAS: Oxygen, 7782-44-7