Neutron beam dose profiles in tissue equivalent materials

The effectiveness of boron neutron capture therapy (BNCT) is determined by the ability to direct both the neutron-absorbing compound ({sup 10}B) and the neutrons to its target preferentially and away from the normal sensitive structures. A converging neutron beam, which has lower current density per unit area in regions located away from the focus, can simultaneously deliver higher number of neutrons to the target positioned at the focus while minimizing damage to tissues not at the focus. A converging beam is created by using a neutron guiding and focusing device made with polycapillary fibers, based on the principle of a Kumaknov lens. A lens is constructed with more than 1000 polycapillary fibers, and each fiber contains more than 1000 hollow channels {approx}10{mu}m in diameter. Neutrons undergo multiple total external reflections within the narrow channels, and the fibers are arranged in an array so that the exiting neutrons are directed toward a common focus. A variation of the lens is a monolithic one, which is made of a single module within which each channel is tapered toward the focus. Both types of lens have been shown to focus the beam of slow neutrons (<25 meV) to a submillimetre focus with amore » gain in current density approaching two orders of magnitude.« less