Monte Carlo calculation of photo-neutron dose produced by circular cones at 18MV photon beams

Publication date: January–February 2018
Source:Reports of Practical Oncology & Radiotherapy, Volume 23, Issue 1
Author(s): Elham Hosseinzadeh, Nooshin Banaee, Hassan Ali Nedaie
AimThe aim of this study is to calculate neutron contamination at the presence of circular cones irradiating by 18MV photons using Monte Carlo code.BackgroundSmall photon fields are one of the most useful methods in radiotherapy. One of the techniques for shaping small photon beams is applying circular cones made of lead. Using this method in high energy photon due to neutron contamination is a crucial issue.Materials and methodsInitially, Varian linac producing 18MV photons was simulated and after validating the code, various circular cones were also simulated. Then, the number of neutrons, neutron equivalent dose and absorbed dose per Gy of photon dose were calculated along the central axis.ResultsNumber of neutrons per Gy of photon dose had their maximum value at depth of 2cm and these values for 5, 10, 15, 20 and 30mm circular cones were 9.02, 7.76, 7.61, 6.02 and 5.08 (ncm⁻²Gy⁻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 (mSvGy⁻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), respectively.ConclusionsAs the field size gets smaller, number of neutrons, equivalent and absorbed dose per Gy of photon increase. Also, neutron equivalent dose and absorbed dose are maximum at the surface of phantom and then these values will be decreased.



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