Thyroid cancers in France and the Chernobyl accident: risk assessment and recommendations for improving epidemiological knowledge.

From 1975 to 1995, the incidence of thyroid cancer in the French population increased by a factor of 5.2 in men and 2.7 in women, thereby raising public concerns about its association with the nuclear accident at Chernobyl. A study performed at the request of French health authorities sought to quantify the potential risk of thyroid cancer associated with the Chernobyl fallout in France in order to determine if this risk could be observed through an epidemiological approach. The study focused on the most exposed population: those living in eastern France and younger than 15 y at the time of the Chernobyl nuclear power plant accident (26 April 1986). The number of spontaneous thyroid cancers in this population was predicted from French cancer registry data, and the thyroid doses were estimated from all available data about contamination in France. Associated risks were calculated with different risk models, all based on a linear no-threshold dose-effect relationship. Under this hypothesis, from 1.3 to 22 excess thyroid cancer cases were predicted for the 1991-2000 period, compared with the 212 spontaneous cases (0.5 to 10.5%) predicted, and from 11.2 to 55.2 excess cases were predicted for 1991-2015, compared with the 1,342 spontaneous cases (0.8 to 4.1%) predicted. These risk calculations indicate that the Chernobyl fallout cannot explain the entire increase in thyroid cancers in France, and that it is improbable that an epidemiological study could demonstrate such an excess. The surveillance of thyroid cancers in France should be enhanced.