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The photo-neutron leakage in the electron linear accelerators used for radiotherapy is primary due to photon interactions with the accelerator head. Neutrons are produced through Giant Dipole Resonance (G.D.R.) when the gamma ray energy is higher than the threshold energy for a (7,n) reaction. For the heavy elements constituting the accelerator head, the threshold lies around 8 MeV and the cross sections have a maximum around 15 MeV. Consequently, gamma beams of energy above 8 MeV can produce undesirable neutrons in the high Z components of the accelerator head. The current trend of using treatment facilities with raised energy and more complex collimation systems, representing an advantage for a more localized therapeutic dose distribution and a better tumor control, leads to an increased photo-neutron production. The undesirable neutron dose to the patient affects the whole body, as neutrons are emitted quite isotropically. In addition, the mean energy of the photo-neutron spectrum produced in the most commonly used LIN AC (with photon maximum energy between 15 and 30 MeV) lies between 1-3 MeV: in this energy range neutrons are highly effective biologically, as pointed out in the publications n. 60 and n. 74 of the International Commission on Radiological Protection (ICRP) [1,2]. ICRP, on the basis of a review of the biological information concerning the damage to human tissue due to ionizing radiation, introduced the "Radiation

Monte Carlo Simulation of the photo-neutron production in the high-Z components of radiotherapy linear accelerators