Fig.1 shows the simulated neutron effective dose distribution around the head of the Varian Clinac with closed jaws and MLC’s. The patient is located near the isocentre (0/0). The neutrons penetrate through the hielding and collimators and cause an effective dose of some mSv per hundred monitor units (MU) in the plain where the patient would be located during a treatment. That causes additional out-of-field dose which might not be negligible when therapy techniques that require many hundred MU’s per fraction as e.g. intensity modulated radiotherapy (IMRT) are applied.