Program Information
Monte Carlo Calculation of Photo-Neutron Dose Produced by Circular Cones at 18 MV Photon Beams
N Banaee1 , E Hosseinzadeh2 , H Nedaie3*, (1) Department of Engineering, Science and Research Branch, Islamic Azad University,Tehran, Iran , (2) Department of Medical Radiation, Engineering Faculty, Central Tehran Branch, Tehran, Iran, (3) Odette Cancer Centre, University of Toronto, Toronto, Canada.
Presentations
SU-I-GPD-T-345 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall
Purpose: Small photon fields, due to their precision on delivering maximum dose to the target are one of the most useful methods in radiotherapy techniques. One of the methods for shaping small photon beams is applying circular cones, made of lead. Using this method in high energy photon beams due to neutron contamination is a crucial issue. The aim of this study is to calculate neutron contamination at presence of circular cones irradiating by 18MV photons using Monte Carlo MCNP6 code.
Methods: Initially Varian Clinac 2100 C/D linear accelerator producing 18MV photon beams was simulated and after validating the code by ion chamber measurements, various circular cones with diameters of 5, 10, 15, 20 and 30 mm were also simulated. Then, number of neutrons, neutron equivalent dose and absorbed dose per Gy of photon dose were calculated along the central axis.
Results: Number of neutrons per Gy of photon dose had their maximum value at depth of 2cm and these values for 5, 10, 15, 20 and 30 mm circular cones were 9.02, 7.76, 7.61, 6.02 and 5.08 (n.cm-2.Gy-1), respectively. Neutron equivalent doses per Gy of photon dose had their maximum at the surface of the phantom and these values for mentioned collimators were 1.48, 1.33, 1.31, 1.12 and 1.08 (mSv.Gy-1), respectively. Neutron absorbed doses had their maximum at the surface of the phantom and these values for mentioned collimators sizes were 103.74, 99.71, 95.77, 81.46 and 78.20 (µGy.Gy-1), respectively.
Conclusion: As the field size gets smaller, number of neutrons, neutron equivalent and absorbed dose per Gy of photon will be increased. Also, neutron equivalent dose and absorbed dose at the surface of phantom are maximized and then these values will be decreased by increasing depth.
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