A proposal for a new neutron moderator material for Accelerator-Based BNCT

A. BURLON; A. KREINER; D. MINSKY; A. VALDA

Santos, Brasil. Presentación mural

Congreso; V Latinoaerican Symposium on Nuclear Physics, XXVI Reunião de Trabalho sobre Física Nuclear no Brasil; 2003

Sociedade Brasileira de Física

The use of nuclear reactions induced by accelerated particle beams, as neutron sources for Boron Neutron CaptureTherapy (BNCT) appears as a competitive option to nuclear reactors. This technique involves the administration andselective uptake of the capture agent 10B (who has a thermal neutron capture cross section of 3840 b) in tumor cells anda suitable thermal neutron flux. The capture reaction 10B(n,alpha)7Li generates the heavy fragments alpha and 7Li that willdamage the tumor cells. BNCT aims at treating pathologies that cannot be treated satisfactorily with conventionaltherapy (especially brain tumors). The neutron spectrum obtained from a nuclear reaction must be optimized usinga moderator-reflector assembly before its application to a patient. It is important to get an epithermal neutron beamthat termalizes inside the body in order to maximize the thermal flux at the tumor position. In the frame of aproduction-target optimization for accelerator-based BNCT, feasibility studies for a new moderator material havebeen performed. This material consists in successive stacks of Al, polytetrafluoroethylene (PTFE), commerciallyknown as Teflon, and LiF. It is easy to build and its cost is relatively low. The interaction neutron cross section inAl and PTFE presents complementary resonances above 30 keV, which reduces the energy of the fast neutrons. Onthe other hand, the 6Li in the LiF captures the thermal neutrons in order to avoid high doses in the skin, where theboron uptake is greater than in the rest of the healthy tissue. An exhaustive Monte Carlo simulation study has beenperformed evaluating the doses delivered to a cylindrical water-filled head phantom by a neutron production Li-metaltarget based on the 7Li(p,n)7Be reaction. A 35.8 cm diameter lead reflector and a 7.5 cm diameter Al/PTFE/LiFmoderator were considered. Several moderator thickness have been studied and the figures of merit show that it ispossible to reach a 98% Tumor Control Probability for a 6.5 cm deep tumor with a 34 cm thick moderator and a 20mA proton bean at a bombarding energy of 2.3 MeV in 50 minutes treatment time.