Igaku butsuri : Nihon Igaku Butsuri Gakkai kikanshi = Japanese journal of medical physics : an official journal of Japan Society of Medical Physics最新文献

Boron neutron capture therapy (BNCT) is a radiation therapy that uses charged particles produced by a nuclear reaction between thermal neutrons and ¹⁰B. A high-intensity neutron source is required to perform BNCT, and it is important to understand the behavior of neutrons. Since BNCT using accelerators has been approved as a medical device, the number of treatment facilities is expected to increase in the future. This article describes the basic knowledge required to understand BNCT in clinical practice, including neutron generation and material interactions, as well as radiation protection considerations specific to BNCT.

Particle therapy uses high-energy charged particles, which cause nuclear reactions with a beam limiting device and a patient, resulting in the generation of high-energy secondary neutrons. These secondary neutrons cause low-dose exposure to organs far from the treatment target, and have a high biological effect due to their energy characteristics, which may cause of secondary cancers after radiotherapy. This article describes the neutron generation mechanism (cross section, and energy spectrum), interaction with the secondary neutron source (beam limiting device, and the patient), measurement of neutrons, and considerations for radiation protection of patients from secondary neutrons, generated by proton and carbon beam radiation therapy.

The Tohoku-Pacific Ocean Earthquake that occurred on March 11, 2011 and the resulting tsunami caused the loss of many people and extensive damage in a wide area. Among the anthropogenic radionuclides dispersed from the Fukushima Daiichi Nuclear Power Plant, ¹³⁴Cs and ¹³⁷Cs have very long half-lives of approximately 2 years and 30 years, respectively, and there are concerns about their uptake into soil and living things. This paper describes a study conducted by the authors' group on radiocesium activity concentrations in the environment.

Exposure of human body to neutrons occurs in radiotherapy using high-energy radiations. This review summarizes knowledges related to biological effects of neutrons, including those obtained in recent projects in Japan and Europe. A study of Japanese atomic bomb survivors with recently revised dosimetry indicated very high relative biological effectiveness (RBE) of 25-80 (as point estimates) regarding cancer risk. Animal studies indicate RBE of 2-100 or even higher regarding cancer induction, which seem to have a peak around ～1 MeV. Evidence suggests that these values depend on the age and sex. Reported RBE regarding the effects on the lens of the eye is in a similar range and sometimes very high. Regarding other tissue reactions, reported RBE values range from 2-10. Experiments at the cellular level have reported RBE of 1-5 regarding cell killing, 2-20 regarding induction of mutations (with a peak at ～1 MeV), and ～1 regarding induction of DNA double strand breaks. A simulation study predicted that the RBE of induction of complex DNA breaks peaks at ～1 MeV with a value of ～17. The complex breaks produced are likely to be far less in amount than simple DNA breaks, leading to a subtle increase in the yield of total DNA breaks; however, these complex damages may be very efficient in inducing mutations and cancer. Thus, the combination of the yield of complex DNA damage and its efficacy in inducing cancer is considered to underlie the high RBE of neutrons regarding cancer risk.