Overdiagnosis of thyroid cancer
Authors: D.G. Zaridze, I.S. Stilidi, A.D. Kaprin, D.M. Maksimovich
Overdiagnosis is the main «driver» of the growing epidemic of thyroid cancer in the world. More than half of cases of thyroid cancer in women are the result of overdiagnosis, i.e. diagnosis of a tumor, the probability of progression and metastasis of which is extremely low.
Our analysis showed that the incidence of thyroid cancer in Russia and most of its administrative regions is growing. At the same time, we observe pronounced interregional variability in the incidence rate. A very highage adjusted incidence rate, especially among women, is registered in the Bryansk region and other high-risk regions. The main cause of the increase in the incidence of thyroid cancer in the regions adjacent to the Chernobyl nuclear power plant is exposure to radioactive iodine (131I ) in childhood and adolescence. However, an increase in the incidence of thyroid cancer in this region at least partly is a consequence of ultrasonography (US) screening and the identification of clinically insignificant lesions. Only 40% of cases of thyroid cancer detected as a result, of screening could be the attributed to radiation. Accordingly, 60% are latent tumors that would never have clinically shown themselves if they had not been identified as a result, of screening.Altai region is also considered a high-risk region because it borders with the Semipalatinsk region of Kazakhstan the region where the former Soviet nuclear test site was located. Inthe Altai region, the introduction of US screening led to an increase in the detection of thyroid cancer from 8,7% in 1992 to 20,3% in 1999.
A marked increase in incidence in Russia, the increase in incidence in middle age and as a result the change in expected exponential pattern of age-specific curve into inverted «U», a very high incidence to mortality ratio, a very high cancer survival rate, which statistically significantly correlates with incidence, a statistically significant correlation between the proportion of tumors detected at the first stage and the incidence of thyroid cancer, a statistically significant correlation between prevalence of thyroid cancer and the incidence is a clear evidence of overdiagnosis. We estimate that the number of overdiagnosed thyroid cancer in Russia over 27 years ranges from 99000 to 138000 cases. We do not have sufficient retrospective data to estimate more accurately the size of thyroid cancer overdiagnosis in Russia. Thyroid cancer incidence data for Russia overall is available only from 1989.