In medical applications, high-energy linear accelerators are increasingly used. However, during a cancer treatment, unwanted photoneutrons caused by high-energy photon beams (>10 MV) increase the secondary cancer risk. This study aimed to investigate the effects of the square field size and distance to the isocenter on the neutron contamination emitted by an Elekta Versa HD medical linear accelerator using a Thermo Scientific RadEye neutron detector. The measurements were carried out for 18-MV photons with different square field sizes and different distances from the isocenter. It was clearly seen that the neutron dose equivalent decreases with both the increasing distance from the isocenter and square field size. Also, in order to analyze the neutron dose equivalent variation with field sizes and distances, Monte Carlo code Fluka was used. It was shown that MC-Fluka modeling and experimental results were consistent with each other. It was concluded that it is crucial to take into consideration the unwanted neutron dose in the radiation treatment.