Towards Accelerator-based Boron Neutron Capture Therapy of Infectious Diseases

Suitable neutron sources for BNCT have been limited for many years to nuclear reactors. A reactor can produce a sufficient neutron flux for therapy. However, the energy spectrum is usually moderated to the thermal range, which is efficient only for superficial tumors. A neutron source based on a low-energy, high-current light-ion accelerator has the potential for meeting the requirements for a clinical BNCT facility. The flexibility to choose the target material and the ion energy and current allows the design of the most suitable neutron field (energy spectrum and flux) for therapy.Further advantages of accelerator-based neutron sources for BNCT are: (1) Accelerators of small size which can, in principle, be located inside hospitals; (2) Accelerators can be easily turned off when the neutron field is no longer required (hence licensing and regulations are simplified); (3) The capital investment of an accelerator-based BNCT system is much lower than the installation of a nuclear reactor; (4) Accelerators have good public acceptability, and are already being used in hospitals. The main design effort of accelerator targets for BNCT has been on lithium, with the reaction Li(p,n) Beat proton energy of 1.9-2.5 MeV. A major advantage of this reaction is the low energy neutron spectrum that is naturally produced (mean neutron energy in the range of 34- 326 keV). Neutrons of these energies require less moderation in order to get to the BNCT ideal neutron spectrum (1 eV to 10 keV) than those generated in other target materials suggested for this purpose, such as beryllium and carbon. Smaller moderator lengths translate to fewer neutron losses and lower the neutron per unit accelerator current at the patient position. The lithium target efficiency will allow the use of smaller and cost-effective accelerators with a current of- 5 mA . Despite the excellent neutronic qualities of the reaction 7Li(p,n)7Be, a lithium target has been considered as very complicated to build because of the mechanical, chemical and thermal properties of lithium, the major problem being to sustain or dissipate the power generated by the high-intensity proton beam