M T Bahreyni Toossi - Medical Physics Department, Faculty of Medicine, Mashhad University of Medical Sciences, Mashhad, Iran
B Khajetash - Medical Physics Department, Faculty of Medicine, Mashhad University of Medical Sciences, Mashhad, Iran
M Ghorbani - Medical Physics Department, Faculty of Medicine, Mashhad University of Medical Sciences, Mashhad, Iran

Background: One of the main causes of induction of secondary cancer in radiation therapy is neutron contamination received by patients during treatment.Objective: In the present study the impact of wedge and block on neutron contamination production is investigated. The evaluations are conducted for a ۱۵ MV Siemens Primus linear accelerator. Methods: Simulations were performed using MCNPX Monte Carlo code. ۳۰˚, ۴۵˚ and ۶۰˚ wedges and a cerrobend block with dimensions of ۱.۵ × ۱.۵ × ۷ cm۳ were simulated. The investigation were performed in the ۱۰ × ۱۰ cm۲ field size at source to surface distance of ۱۰۰ cm for depth of ۰.۵, ۲, ۳ and ۴ cm in a water phantom. Neutron dose was calculated using F۴ tally with flux to dose conversion factors and F۶ tally.Results: Results showed that the presence of wedge increases the neutron contamination when the wedge factor was considered. In addition, ۴۵˚ wedge produced the most amount of neutron contamination. If the block is in the center of the field, the cerrobend block caused less neutron contamination than the open field due to absorption of neutrons and photon attenuation. The results showed that neutron contamination is less in steeper depths. The results for two tallies showed practically equivalent results.Conclusion: Wedge causes neutron contamination hence should be considered in therapeutic protocols in which wedge is used. In terms of clinical aspects, the results of this study show that superficial tissues such as skin will tolerate more neutron contamination than the deep tissues.

Neutron Contamination, wedge, cerrobend block, Siemens Primus linac, Monte Carlo Simulation