Today the majority of cancer radiation therapy treatments are performed at medical electron linear accelerators (linacs). The accelerated electrons can be used directly for irradiation of the tumor or for the generation of highly penetrating bremsstrahlung photons inside a high Z target. The photon radiation field can be formed by primary collimators (jaws) and so called multi leaf collimators (MLC) generally made of tungsten to achieve a good tumor conformity. The use of higher photon energies has some advantages over lower energies such as the better sparing of tissue near the body surface due to a longer dose build-up. However photons with energies higher than ~7 MeV can additionally to the interaction with bound electrons undergo inelastic reactions with nuclei. These photonuclear reactions lead to the emission of fast neutrons which contaminate the primary photon field. The neutrons might penetrate through the collimators and deliver out-of-field dose to the patient when electrons with energies over 7 MeV are used for bremsstrahlung generation [1]. Furthermore the  materials inside the linac head as well as the air inside the treatment room get activated which might deliver dose to the medical employees even when the linac is not in operation. A detailed knowledge of these effects is essential for adequate radiation protection of the employees and an optimal patient treatment.