Boron neutron capture therapy (BNCT) is an external beam radiotherapy that utilizes the high propensity of the non-radioactive nuclide boron-10 to capture thermal neutrons, resulting in the prompt nuclear reaction 10B(n,α)7Li. The products of this reaction have high linear energy transfer properties (α-particle approximately 150 keV μm−1, 7Li particle approximately 175 keV μm−1). The path lengths of these particles in water or tissue are in the range of 4.5–10 μm; this leads to an energy deposition that is limited to the diameter of a single cell. Theoretically, it is therefore possible to selectively irradiate those tumor cells that have absorbed a sufficient amount of 10B, while sparing normal cells. With the advent of accelerator-based neutron sources, these basic principles are now becoming available for a broad clinical application and have the potential to open a new chapter in radiation oncology by introducing a kind of “cell surgery.” In this chapter, other isotopes with high cross section for interacting with thermal neutrons are also listed and discussed.