Neutron Contamination Unexpected Dose in Varian LINAC

Introduction: Neutron contamination is very harmful for the patients receiving radiation therapy due to the high relative biological effect of neutrons. Medical linear accelerators head are made up of materials with high atomic numbers, and High-energy photons interaction with heavy materials can produce neutron contamination. In this study, neutron equivalent dose, neutron spectrum, and the effect of field size on neutron contamination dose is calculated by the FLUKA Monte Carlo code system. Materials and Methods: In this study, all parts of the Varian Clinac 2100 accelerator head’s elements, such as target, flattening filter, jaws, MLC, shields, primary and secondary collimators are simulated by the FLUKA code system. LINAC performance energy was considered 18MV and a water phantom with dimension of 140×140×35 cm3 was simulated for neutron dose calculation. To investigate the effect of field size on neutron contamination, larger treatment fields such as 10×10, 20×20, 30×30 and 40×40 was formed by the jaws, and the smaller ones like 1×1, 3×3 and 5×5 was formed by the MLC.   Results: The results show that the Neutron contamination dose can be up to 2.54 mSv per 1Gy photon dose at isocenter. Neutron to photon dose ratio is 0.004 at isocenter and more than 8.6 in out-of-field areas. Furthermore, the neutron dose changes at the surface of the water phantom is only 15% that show the protective elements of the LINAC head are not able to control the neutron contamination flux. The most of neutron dose distribution is at the surface of the water phantom, So it can increases the secondary cancer risk in patients, including skin cancer. The effect of treatment field size on neutron contamination shows that the larger field size formed by the jaws contribute to higher in-field neutron dose, and using of smaller field size formed by the MLC, cause the lower in-field neutron dose. Conclusion: The neutron contamination dose can not be neglected, and the effective parameters in neutron contamination exasperation should be thoroughly investigated. Furthermore, the harmful effects of the neutron contamination should be minimized by the appropriate protective shields