Photoneutron production in tissue during high energy bremsstrahlung radiotherapy.

The total yield of photoneutrons from soft-tissue-equivalent material irradiated with bremsstrahlung of end-point energy from 6 to 28 MeV has been measured using the 35 MeV University of Melbourne Betatron and a 4 pi Halpern-type neutron detector. The results are compared with calculations based on the cross sections for neutron production of the constituent elements of tissue (H, C, N and O). It is shown that the less common isotopes of the constituent elements (2H, 13C, 15N, 17O, 18O) must be included to obtain agreement with the measured yields. The dose imparted to tissue by the photoneutrons is calculated for the energy range from 6 to 30 MeV and is compared with other available estimates. Further, the dose equivalent due to all photonuclear reactions in tissue is estimated for energies of 24 and 30 MeV.