Neptunium, Radioactive

Carcinogenicity Evidence

Evaluation. There is inadequate evidence in humans for the carcinogenicity of neutrons. There is sufficient evidence in experimental animals for the carcinogenicity of neutrons. Overall evaluation. Neutrons are carcinogenic to humans (Group 1). In making the overall evaluation, the Working Group took into consideration the following: When interacting with biological material, fission neutrons generate protons, and the higher-energy neutrons used in therapy generate protons and alpha particles. Alpha Particle-emitting radionuclides (e.g. radon) are known to be human carcinogens. The linear energy transfer of protons overlaps with that of the lower-energy electrons produced by gamma-radiation. Neutron interactions also generate gamma-radiation, which is a human carcinogen. Gross chromosomal aberrations (including rings, dicentrics and acentric fragments) and numerical chromosomal aberrations are induced in the lymphocytes of people exposed to neutrons. The spectrum of DNA damage induced by neutrons is similar to that induced by X-radiation but contains relatively more of the serious (i.e. less readily repairable) types. Every relevant biological effect of gamma- or X-radiation that has been examined has been found to be induced by neutrons. Neutrons are several times more effective than X- and gamma-radiation in inducing neoplastic cell transformation, mutation in vitro, germ-cell mutation in vivo, chromosomal aberrations in vivo and in vitro and cancer in experimental animals.
Internalized radionuclides that emit alpha-particles are carcinogenic to humans (Group 1). In making this overall evaluation, the Working Group took into consideration the following: (1) Alpha-Particles emitted by radionuclides, irrespective of their source, produce the same pattern of secondary ionizations and the same pattern of localized damage to biological molecules, including DNA. These effects, observed in vitro, include DNA double-strand breaks, chromosomal aberrations, gene mutations and cell transformation. (2) All radionuclides that emit alpha-particles and that have been adequately studied, including radon-222 and its decay products, have been shown to cause cancer in humans and in experimental animals. (3) Alpha-Particles emitted by radionuclides, irrespective of their source, have been shown to cause chromosomal aberrations in circulating lymphocytes and gene mutations in humans in vivo. (4) The evidence from studies in humans and experimental animals suggests that similar doses to the same tissues, for example lung cells or bone surfaces, from alpha particles emitted during the decay of different radionuclides produce the same types of non-neoplastic effects and cancers.
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