Nuclear MCNP simulation of the photoneutron dose contribution in linac radiotherapy treatments with multileaf collimation systems

External Bremsstrahlung photon beams produced by medical linac accelerators (Linac) are the most commonly used radiation therapy treatments. The case of treatments which include photon energy beam exceeding 10 MeV produce an undesired dose received by patients due to photoneutron production in the accelerator head. Currently, sophisticated techniques such as multileaf collimators are used for a better definition of the target volume irradiation. In these cases it is important to evaluate the photoneutron dose produced by the excitation of the high atomic number materials (mainly tungsten) constituting the collimator leaves in order to optimize the radiation therapy treatment. A Monte Carlo simulation has been developped to calculate the photoneutron dose arising from an Elekta Precise unit head using a 15 MeV photon beam. The simulation has been performed using the MCNP5 code. Results are presented for the facility at Hospital Universitari Clínic de València setting a square 10 cm × 10 cm collimator configuration.