A Monte Carlo study of neutron contamination in presence of circular cones during stereotactic radiotherapy with 18 MV photon beams

Abstract

High-energy photons are being used to treat different kinds of cancer, but it may increase the rate of secondary cancers due to the neutron contamination as well as over exposing of patients and medical staffs in radiation therapy Takam, Bezak, Marcu, and Yeoh, 2011, Radiation Research, 176, 508-520. Due to some difficulties in experimental measurements of neutron contamination, Monte Carlo method is an efficient tool to investigate dose parameters and characteristics in new techniques. The 18-MV photon beam of linac and circular cones have been simulated by MCNP5 code. Various parameters of photon and neutron including mean energy, flux, KERMA, the number of particles crossing a surface at a distance of 100 cm (SSD = 100 cm) as well as the change in photon and neutron spectrum as well as in intensity through the transmission in the circular collimators have been investigated. The results of this study show that the use of a circular collimator decreases neutron dose in the central axis, which is an advantage, but neutron contamination inducing small neutron dose is distributed all over the space. On the surface of phantom, photon dose rate is approximately equal to 3.41E7 (mGy/mA.min) for different collimators, but the neutron dose rate is 1.64E2 (mGy/ mA.min), 2.03E2 (mGy/ mA.min) and 2.52E2 (mGy/mA.min) for diameters of 12, 20 and 40 mm, respectively and it decreases by decreasing the diameter of the collimator. The neutron dose rate decreases from 9.68E7 and 9.74E7 (mGy/min.mA) for open field size 33 cm2 and 55 cm2 to 1.64E2 (mGy/min.mA), 2.02E2 (mGy/min.mA) and 2.52E2 (mGy/min.mA) for collimator diameter of 12 mm, 20 mm and 40 mm. It can be concluded that the use of circular collimators has an advantage of reducing neutron dose in the central axis. It should be mentioned that the off-axis neutron dose surrounding the collimator can be eliminated using an external neutron shield without perturbing the treatment field.