高能(15 MV)醫用直線加速器誘發產生之中子能譜與劑量研究

Abstract

In radiation therapy, the high energy photon beams generated by the medical linear accelerators (Linac) were usually used to kill the tumors. Thorough the giant dipole resonances, (,n) photoneutron reactions occurred by the high energy photons interacted with materials of high atomic number.. The energy distribution of induced photoneutrons is from thermal to fast energy. The neutrons induced by high energy photons increase the doses of patients. Dual-TLD method coupling with Cadmium-difference method is usually used to distinguish the doses of thermal neutrons form photons in a mixed radiation field. TLD-600 and TLD-700 were easily used to put inside a Rando phantom to investigate the dose contributed by the thermal neutrons induced by a high energy photon beam. Due to the hole space inside the Rando phantom is small to fill a set of dual-TLD with Cd cover, in this case, the estimated neutrons would include neutrons with energies of thermal and above thermal, so the estimated thermal neutron doses would include a contribution for some fast neutrons. In this study, a system of Bonner sphere spectrometer with gold foils was used to measure the neutron spectra (at different depth of phantom) induced by a Linac (15 MV photon beam). By means of the measured neutron spectra and the Monte Carlo method, the dose ratios contributed by thermal and fast neutrons were assessed and relative correction factors were calculated. In the results, the dose errors of the thermal neutrons due to not subtracting the doses contributed by the higher energy neutrons were 99 %, 45 %, 21 %, 12 % and 12 % for 0, 4, 10, 15 and 20 cm depth in phantom, respectively. On the surface of the phantom, the fast neutrons dominated in the neutron spectrum, therefore, dose of thermal neutrons estimated by dual-TLD method (without using cadmium-difference method) had a maximum dose error. In deeper depths, due to large amount of the fast neutrons were slowing down, estimated dose errors of thermal neutrons decreased.