Overdiagnosis of Thyroid Cancer in the Marne and Ardennes Departments of France From 1975 to 2014

Introduction - Incidence of thyroid cancer has increased considerably in France in recent years, but the mortality rate has declined only slightly. Part of this increased incidence could be attributable to overdiagnosis. We aimed to estimate the contribution of overdiagnosis to the incidence of papillary thyroid cancer.

Material and methods. - We included all subjects living in the Marne or Ardennes Departments of France and who were diagnosed with papillary thyroid cancer between 1975 and 2014. We recorded socio-demographic variables, i.e. age at diagnosis and sex. Cancer characteristics were also recorded, namely year of diagnosis, International Classification of Diseases (ICD) 10th revision code associated with the papillary cancer, and the pTNM 2002 status. Incidence rates were calculated based on data from the specialised Marnes-Ardennes thyroid cancer registry, for cancers diagnosed between 1975 and 2014, by age category and by five-year period. The population was divided into two groups according to pTNM classification at diagnosis (i.e. localised or invasive). Overdiagnosis was defined as the difference in incidence rates between the invasive cancer and localised cancer groups. This rate was then divided by the incidence rate in the localised cancer group for the most recent period (2010-2014) to obtain the proportion of cancers attributable to overdiagnosis.