Thyroid cancer risk assessment after the Chernobyl accident fallout in France using an uncertainty analysis
aInstitute for Radiation Protection and Nuclear Safety, BP 17, Fontenay-aux-Roses Cedex, France, bRegional Health Observatory Provence Alpes Côte d’Azur, France, cIsère cancer registry – FRANCIM network, France
Aim The incidence of thyroid cancer has increased by a factor of 5.2 in men and 2.7 in women over the last twenty years, in France. This increase has raised public concern about its association with the nuclear accident at Chernobyl, in 1986. At the request of French authorities, a first study sought to quantify the potential risk of thyroid cancer associated with the Chernobyl fallout in France. This request raised three methodological questions: How to consider the observed trend of thyroid cancer incidence in France? Which risk models should be used to relate exposure to risk? What are the uncertainties associated with the thyroid cancer risk assessment? The aim of this study is to propose an integrated method of thyroid cancer risk assessment after environmental exposures to ionizing radiation. Methods Thyroid cancer risks potentially associated with the Chernobyl fallout in France was estimated for the most at risk group: those who were younger than 15 years old in 1986. Calculations were carried out for the period 1991-2007. The number of spontaneous thyroid cancers was estimated from French cancer registries’ data on the basis of 2 scenarios: one with a constant incidence, the other using the trend observed over the past 20 years. We used age-period-cohort models to analyse the trend of spontaneous incidence from 1978 through 1997. We then applied these models for projections up to 2007. The thyroid doses were estimated from all available data about contamination in France due to the Chernobyl fallout. To estimate the thyroid cancer risk, we used results from a pooled analysis of the principal cohorts of irradiated children published as of now, by E. Ron in 1995. We also performed an uncertainty analysis on the thyroid cancer risk assessment based on spontaneous thyroid cancer incidence rate, thyroid doses and risk coefficient distributions. Results Depending of the scenario, the total predicted number of expected spontaneous cases between 1991 and 2007 ranged from 894 [90% Confidence Interval: 869 - 920] to 1 716 [90% CI: 1 691 - 1 741]. The total predicted number of excess thyroid cancers associated with the Chernobyl fallout in France ranged from 5 [90% CI: 1 - 15] to 63 [90% CI: 12 - 180]. Uncertainties depended first on the spontaneous thyroid cancer incidence rate projection, second on the risk coefficient and last on the thyroid dose distribution. Conclusions These risk calculations indicate that the Chernobyl fallout cannot explain all of the increase of thyroid cancers in France. Associated risks were calculated with on the basis of a non-threshold linear dose-effect relation hypothesis. Under this hypothesis, in a low doses exposure situation, uncertainties associated with the thyroid cancer risk assessment are mainly due to the projection of spontaneous thyroid cancer rate.
Paper presented at the International Symposium on Predictive Oncology and Intervention Strategies; Nice, France; February 7 - 10, 2004; in oral session 891 (Risk factors).