Neutron Beam Therapy

Abstract

The neutron was discovered by James Chadwick in 1932; by 1938 Robert Stone, in collaboration with Ernest Lawrence, was using a cyclotron produced fast neutron beam to treat human tumors. These original clinical trials were unsuccessful because of the poor understanding of the radiobiology of neutrons. Later, radiobiological studies established tumor hypoxia and radiosensitivity variations at different phases of the cell cycle as rationales for fast neutron therapy. Clinical studies during the early 1970s demonstrated that neutron therapy was a potentially beneficial modality in the treatment of malignant disease and definitive controlled clinical trials followed. Increased interest in neutron therapy lead to the definition of minimum specifications for therapeutic fast neutron beam treatment facilities. These specifications included beam dose rate, penetration, collimation, and reliability. The parameters used to characterize fast neutron beams for therapeutic use include neutron yield (dose rate), the neutron energy spectrum, beam intensity profiles, and microdosimetric data. The most prolific neutron producing nuclear reactions occur when high‐energy proton or deuteron beams are incident on a solid beryllium target, or when the deuterium–deuterium or deuterium–tritium fusion reactions are employed. The relative merits of these different methods of neutron production are discussed in detail. There have been a total of 34 centers that have treated patients with fast neutron therapy of which 6 are currently operational. The state‐of‐the‐art facilities all use cyclotrons to accelerate protons or deuterons to energies >48 MeV; they use multileaf or multirod collimators to shape the beams to the tumor contour. The latest facility uses a compact deuteron superconducting cyclotron, which offers several advantages over conventional room temperature devices.