the effect of field size and distance from the field center on neutron contamination in medical linear accelerator

objective: using megavoltage photons generated by medical linear accelerator is a common modality for the treatment of malignant. the crucial problem for using photon beams >8mv is the photoneutron yields that increase the risk of secondary cancer that treated with high-energy photon beams. the contaminated neutrons produced in different components of the accelerator head and rely on many parameters. the purpose of this study was to determine the effect of field size on the neutron dose equivalent in center and far from it at the elekta sl 75/25 18 mv linear accelerator. methods: neutron dosimetry was carried out with cr-39 films with using of chemical etching technique. the measurement was done at isocenter, 25 cm and 50 cm far from it at 100 cm ssd for squared field with 5 up to 30 cm side. results: the results revealed that the neutron dose equivalent increased with increasing field sizes especially for 5*5 cm2 field size. it was decreased with increasing distance from the isocenter.conclusion:the effect of field size on neutron contamination depend on amount of field aperture where in small field size 5×5 cm2 less variation need for significant change but for larger field size 10×10 cm2 this variation must be larger. the contaminated neutron outside photon field is independent of field size.