طراحی و بهینه‌سازی طیف نوترونی برای درمان تومورهای عمیق مغزی به روش BNCT با کاهش آسیب رسیده به پوست

Boron neutron capture therapy (BNCT) is an effective method for treatment of deep seated brain tumors. This method consists of two stages: injection of boron compound in the patient body, and then irradiation of the region tumors with the neutron beam. It allows for delivery of high linear energy transfer (LET) radiation (particles 4He and 7Li nuclei) to tumors at the cellular level whilst avoiding unnecessary dose deposition to healthy tissue. The proper neutron energies for BNCT is 1eV–10keV, namely epithermal energy range. Neutrons can slow down to the thermal energies via passing through the different tissue before reaching the tumor. Neutrons with higher or lower energies and &gamma-radiation are extremely undesirable and should be avoided as much as possible of the spectrum. Therefore, a good spectrum shaping is an essential requirement for BNCT. The following neutron-producing charged particles reactions are considered mainly for use in accelerator based neutron capture therapy: 7Li(p,n)7Be, 9Be(p,n)9B, 9Be(d,n)10B and 13C(d,n)13N. The 7Li(p,n)7Be reaction is excellent for producing neutron. Neutrons from this reaction have a relatively narrow energy spectrum which requires less moderation than those generated from other reactions. In this paper, we investigate the feasibility of using 7Li(p,n)7Be reaction with irradiation of 2.5MeV-20mA proton beam for neutron production in order to treatment deep seated brain tumors. the serious drawback of this source is the low melting point of Li target (180 °C) and its low thermal conductivity (84.7 W/m °k). To overcome this problem, a cooling system was optimized and a beam shaping assembly (BSA) was proposed for decreasing of the flux of fast neutrons (E>10 keV). The proposed BSA based on 7Li(p,n)7Be reaction contains: BeO as moderator, graphite as reflector, Cd as thermal neutron filter and BeO as collimator. Our results show 1.08×109 n/cm2s epithermal neutron flux at the beam port of the proposed BSA. Although the designed beam meet IAEA criteria, however, considering the differences between skin and healthy tissue in BNCT leads to high neutron dose in skin. To overcome this problem, the BSA is designed so that the dose in skin reduced as much as possible. The simulated Snyder head phantom is used to evaluate dose in tissues due to the irradiation of designed neutron beam. Dosimetric evaluation in the simulated head phantom shows that our designed beam is effective to treat deep-seated brain tumors with the reduction of damage to the skin in a reasonable time. Our optimization is based on Monte Carlo calculation using MCNPX code. Keywords: BNCT, 7Li(p,n)7Be Reaction, deep-seated tumors, Skin, Dose evaluation