Unwanted neutrons in radiation therapy treatments are typically generated by photonuclear reactions. High energy beams emitted by medical linear accelerators (LinAcs) interact with high atomic number materials situated in the accelerator head and release neutrons.As photoneutrons scatter easily, they can leave the planned radiation field and be spread throughout the treatment bunker, contributing patients to receive a low (but non-negligible) dose, which is not estimated by treatment planning systems. Moreover, since neutrons have a high relative biological effectiveness, even low neutron doses may imply significant exposure of patients. It is also important to study activated radioisotopes for radiation protection purposes, as persons not present during irradiation (e.g. medical staff) may be exposed to them even when the accelerator is not operating.The Monte Carlo code MCNP6 has been used in this work for quantifying the neutron contamination inside the treatment room. Walls and LinAc head activation processes by these neutrons have also been estimated by simulation. To that, a detailed model of particles transport inside the bunker has been carried out using a meshed geometry model. The LinAc studied is an Elekta Precise electron accelerator with a treatment photon energy of 15 MeV.

This work presents a Monte Carlo simulation using the last version of MCNP, v. 6.1.1, of a Varian CLinAc emitting a 15MeV photon beam. The main objective of the work is to estimate the photoneutron production and activated products inside the medical linear accelerator head. To that, the Varian LinAc head was modelled in detail using the manufacturer information, and the model was generated with a CAD software and exported as a mesh to be included in the particle transport simulation. The model includes the transport of photoneutrons generated by primary photons and the (n, γ) reactions which can result in activation products. The validation of this study was done using experimental measures. Activation products have been identified by in situ gamma spectroscopy placed at the jaws exit of the LinAc shortly after termination of a high energy photon beam irradiation. Comparison between experimental and simulation results shows good agreement.

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