Fig. 1

Boron neutron capture therapy is based on the nuclear capture and fission reactions that occur when non-radioactive boron-10, a constituent of natural elemental boron, 80% of which is in the isotopic form of ¹¹B and 20% as ¹⁰B, is irradiated with low-energy (0.025 eV) thermal neutrons or, alternatively, higher-energy (10,000 eV) epithermal neutrons. The latter become thermalized as they penetrate tissues. The resulting ¹⁰B(n,α)⁷Li capture reaction yiels high linear energy transfer (LET) α paricles (stripped down helium nuclei [⁴He]) and recoiling lithium-7 (⁷Li) atoms (a). A sufficient amount of ¹⁰B must be delivered selectively to the tumor (~ 20–50 μg/g or ~ 10⁹ atoms/cell) in order for BNCT to be successful (b). A collimated beam of either thermal or epithermal neutrons must be absorbed by the tumor cells to sustain a lethal ¹⁰B(n,α)⁷Li capture reaction. Since the α paricles have very short pathlengths in tissues (5–9 μm), their destructive effects are limite to boron-containing cells. In theory, BNCT provides a way to selectively destroy malignant cells and spare surrounding normal tissue if the required amounts of ¹⁰B and neutrons are delivered to the tumor cells.