The role of the 14N(n,p)14C reaction in neutron irradiation of soft tissues

The cell-killing potential of the N-14(n,p)C-14 reaction was considered with regard to neutron absorption in human nuclear DNA and respiratory phosphates for: (A) 10(12) thermal neutrons in 1 kg of soft tissue, (B) a mono-energetic beam of 2 MeV neutrons incident in 1 kg of soft tissue such that the total collision kerma was 10 J/kg, and (C) an evenly distributed 0-66 MeV neutron beam, also incident in 1 kg such that the total collision kerma was 20 J/kg. For case (A) 0.0017 N-14(n,p)C-14 reactions could be expected per DNA double strand, case (B) 0.053, and case (C) 0.0039. The probabilities that a proton emitted outside the nucleus would cross nuclear DNA were estimated from N-14 tissue content for adult skeletal muscle, liver, and kidney tissues, for (1) nuclear DNA being concentrated in a sphere of 1.8 mu m diameter, and (2) nuclear DNA being evenly distributed in a spherical nucleus 5 mu m in diameter. It was concluded that even in a nitrogen-rich tissue exposed to a collision kerma of 20 J/kg by a 0-66 MeV fast neutron beam, the N-14(n,p)C-14 reaction directly kills at most 10 cells in every 1000, 4 of these by DNA nitrogen absorption and 6 by the N-14(n,p)C-14 protons emitted elsewhere in the cell. However, the dose due to the N-14(n,p)C-14 reaction should be measured where exposure to thermal neutron fluxes is significant. For therapeutic neutron doses the number of respiratory phosphate molecules in which the N-14(n,p)C-14 reaction occurs is insignificant, and doses from C-14-decay after neutron therapy are also negligible. (C) 2010 Published by Elsevier Ltd.