Boron Neutron Capture Therapy

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MARUHASHI, Akira, Ph.D. Emeritus and Visiting ProfessorRadiation Oncology Research CenterResearch Reactor InstituteKyoto UniversityAs illustrated in Figure 1, a variety of reactions occur depend-ing on the energy of the neutrons in question. Consequently, fields where there are neutrons exhibit an extremely diverse range of radiation. In addition to the epithermal neutrons that play the central role in BNCT, tissues in the bodies of patients placed in the BNCT irradiation field are exposed to radiation that has leaked from treatment equipment and radiation given off by neutrons. Let’s take a closer look at the following reactions while paying special attention to their relationships with BNCT:Elastic scattering refers to a scattering phenomenon involving neutrons and atomic nuclei in which momentum and kinetic energy are conserved. These reactions include potential scattering and resonance scattering, in which compound nuclei are formed. In the case of the latter, the wave character of the neutrons, based on the de Broglie wave-length, is involved in causing a resonant reaction. This reaction is dominant for many elements with neutrons at or below 10 MeV. Although elastic scattering reactions with hydrogen are primarily used in fast neutron therapy, this reaction is undesirable in BNCT since it is mostly unrelated to 10B concentrations and has similar effects on tumor tissue and healthy tissue. Figure 1. (for example) Various Nuclear Reactions between 16O and NeutronsNeutrons and their reactionsNeutrons are electrically neutral subatomic particles (that is, they have no electrical charge). When existing alone in a vacuum, neutrons decay into a proton and an electron at a half-life of 10.8 minutes. All of the atomic nuclei are composed of neutrons and protons, which are usually approximately equal in number, although there can be up to a 30% differ-ence. The mass of a neutron is nearly equal to that of a proton (the differences between those are about 0.108 %). Depend-ing on their particular combination of nuclear particles, atomic nuclei are either stable (giving rise to a stable isotope [SI]) or unstable (giving rise to a radioisotope [RI]).Neutrons can be characterized by the manner in which they interact with other types of matter. The interactions of neutrons at or below 20 to 30 MeV, the range of energy levels that are used in medical treatment, can be classified using the following conceptual categories: Neutrons and their characteristics1(n, n)(n, n)Elastic scattering reactionsNeutrons, their properties, and their interactions with matter(n,n)(n,p)(n,d)(n,2n)：：：：Elastic scatteringProton emissionDeuteron emissionTwo-neutron emission(n,α)(n,n’)(n,nα)：：：Alpha emissionInelastic scatteringNeutron and alpha emission05100101102103104101520(n,n)(n,α)(n,nα)(n,p)(n,n’)(n,d)(n,2n)Neutron energy (MeV)Neutron reaction probability (relative value)1234Elastic scattering reactionsInelastic scattering reactionsNeutron capture reactions Nuclear transmutation reactions(n, n)(n, n’)(n, γ)(n, x)These interactions occur with qualitative (in terms of reaction content) and quantitative (in terms of reaction probability) differences depending on the energy of the neutrons in ques-tion and the type of matter with which they are interacting. 11

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