A review of human carcinogens--part D: radiation.

In June 2009, 20 scientists from nine countries met at the International Agency for Research on Cancer (IARC) to reassess the carcinogenicity of the types of radiation previously classified as “carcinogenic to humans” (Group 1) and to identify additional tumour sites and mechanisms of carcinogenesis (table and panel). These assessments will be published as part D of Volume 100 of the IARC Monographs. Alpha particles, consisting of two protons and two neutrons, are a densely ionising type of radiation with low capacity to penetrate living tissue (less than 0·1 mm). Beta particles are electrons or positrons that are less ionising, but more penetrating (up to a few milimetres). The health hazards resulting from radionuclides that emit these particles largely occur after internal deposition. Epidemiological evidence shows a number of radionuclides that emit alpha or beta particles increase cancer risks at several anatomical sites (table). The Working Group reaffirmed the carcinogenicity of internally deposited radionuclides that emit alpha or beta particles (Group 1). After the Chernobyl accident, a sharp increase in the risk of thyroid cancer was found with exposure to radioiodines, particularly iodine-131, during childhood and adolescence. This increased risk might be due to higher milk intake per unit of body weight among children; a higher thyroid dose per unit of iodine-131 intake from milk; a higher susceptibility per unit of thyroid dose; or a combination of these. Radon exposure occurs mainly through contamination of indoor air by radon released from soil and building materials. Combined analyses of case–control studies now estimate that residential exposure to radon gas is the leading cause of lung cancer after tobacco smoke (8–15% attributable risk in Europe and North America). X-rays and gamma-rays are sparsely ionising electromagnetic radiation that penetrate living tissue, typically producing fast electrons that deposit energy, resulting in tissue damage. Extensive study of atomicbomb survivors shows increased cancer risks at multiple anatomical sites. Current evidence adds to the list of tumours caused by x-rays and gamma-rays (table), and also establishes that in-utero exposure increases the risk of cancer at multiple sites. The Working Group reaffirmed the carcinogenicity of x-radiation and gamma-radiation (Group 1). Neutrons are produced by nuclear reactions and are a main component of cosmic radiation. They are highly penetrating and interact with the traversed tissue, producing protons, other charged particles, and gamma-radiation. Epidemiological evidence is inadequate to assess the carcinogenicity of neutrons, because of co-exposures to other types of radiation. However, the evidence of cancer in experimental animals is sufficient, and mechanistic data show that neutrons transfer their energy in clusters of ionising events— resulting in similar, but more severe, local damage than that induced by x-rays or gamma-rays. On the basis of this evidence, the Working Group reaffirmed the carcinogenicity of neutron radiation (Group 1). Each type of ionising radiation (panel) transfers energy in the form of highly structured tracks of Upcoming meetings Sept 29–Oct 6, 2009 Lifestyle Factors