Survival of Cultured Human Cells after Irradiation with Fast Neutrons of Different Energies in Hypoxic and Oxygenated Conditions

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Abstract
Damage to the reproductive capacity of cultured cells of human kidney origin, induced by fast neutrons of different energies, has been studied by the cloning technique. Survival curves have been determined for cells in equilibrium with air or nitrogen, irradiated with beams of fast neutrons with modal energies ranging from 1 to 15 MeV. From these experimental data the relative biological effectiveness and oxygen enhancement ratios of the various neutron beams have been derived. The OER-values of fast neutrons were found to vary only little with the modal energy of the beam, ranging from 1·5 ± 0·1 at 1 MeV to 1·6 ± 0·2 at 15 MeV. The low OER-values indicate that fast neutrons can provide an advantage in the radiotherapy of tumours containing hypoxic cells. Due to their high penetrating power, 15 MeV neutrons are the most promising in this respect. An interpretation of the independence of the OER from the modal energy of the neutron beams used can be given by considering the various distributions of dose in LET. Experiments in which secondary charged particle equilibrium was provided or deliberately avoided have demonstrated that with increasing neutron energy, the proton-recoil component of the energy deposition shows an increasing OER. However, with increasing energy of the neutrons, interactions with C, O and N nuclei play a larger part and the energy dissipated through these interactions mainly corresponds to high LET-values and low OER-values. These two phenomena result in the observed independence of the OER from the neutron energy.