Influence of head cover on the neutron dose equivalent in Monte Carlo simulations of high energy Medical linear accelerator

Neutron contamination of radiotherapeutic photon beam occurs when energies higher than 10 MeV are used in radiotherapy. To correctly assess the neutron doses that medical personnel and patients receive, it is highly important to know the spectra of the produced photoneutrons. One of the most common ways to determine such spectrum is to perform Monte Carlo simulations of the accelerator. Major issue in the Monte Carlo modelling is that the manufacturers often does not provide full specifications of the accelerators head, so some parts of the head are omitted from the simulation. Within this paper we present a model that includes head cover compared to the one where it is omitted, as it can often be found in the references. Neutron fluxes, spectra, mean energies and place of origin are compared in isocenter, at the point 1 m above target and the point 1 m aside from the target, in both models. In all the considered planes the flux change was found to be more than 20 %, with a significantly change in neutron energy, what is also important in neutron dosimetry. Ignoring the head cover in the Monte Carlo modelling of the high energy electron linear accelerators in radiotherapy, will introduce a large uncertainty of neutron doses assessing a patient, or a medical professional.