ISMJ

Background: In the field of the treatment by Boron Neutron Capture Therapy (BNCT), an optimized neutron system was proposed. This study (simulation) was conducted to optimize the geometry and composition of neutron system and increase the epithermal neutron flux for the treatment of deep tumors is performed.

Materials and Methods: A neutron system for BNCT was proposed. The system included 252Cf neutron source, neutron moderator/reflector arrangement, filter and concrete. To capture fast neutrons, different neutron filters Fe, Pb, Ni and PbF2 with various thicknesses were simulated and studied. Li (with 1 mm thick) was used for filtering of thermal neutrons. Bi with thickness of 1 cm was used to minimize the intensity of gamma rays. Monte Carlo simulation code MCNPX 2.4.0 was used for design of the neutron system and calculation of the neutron components at the output port of the system.

Results: For different thicknesses of the filters, the fast neutron flux, the epithermal and thermal flux were calculated at the output port of the system. The spatial distribution of the fast neutron flux, the epithermal flux and gamma flux in human head phantom with the presence of 40 ppm of 10B were obtained. The present calculations showed that Pb filter (about 1 cm) at the output port is suitable for fast neutron capture. The thickness of Li filter was determined due to its high absorption cross-section in thermal region. Bi was used as a gamma filter by the reason of it is good for shielding gamma rays, while having high transmission epithermal neutrons.

Conclusion: The epithermal neutron flux has enhanced about 38 percent at the output port of the present system, compared with recent system proposed by Ghassoun et al. At 2 cm depth inside the head phantom, the neutron flux reaches a maximum value about . At this depth, the ratio of the thermal neutron flux to the epithermal flux is about three times, that suggests such a neutron system to treat tumors in the proximity of the depth. In the presence of 10B in the brain, at 2 cm depth the neutron absorption takes place more than other areas of the brain, and consequently the thermal flux is depressed uniformly in the head phantom. Due to high LET and RBE of alpha and 7Li particles (obtained by reaction of boron-neutron), the tumor at the mentioned depth is damaged rather than the around.