Thyroid cancer overdiagnosis revisited

Evidence has accumulated showing that an increase in thyroid cancer incidence reflects overdiagnosis of clinically unimportant lesions due to the rise in the use of neck ultrasonography. In the manuscript we examine the hypothesis that the rise in thyroid cancer incidence in Russia is largely caused by overdiagnosis.

Incidence and mortality rates of thyroid cancer for Russia overall and its administrative regions were abstracted from the statistical database of the Ministry of Health of Russia. For incidence trends, we calculated the percentage change, linear regression coefficient and p-value. The calculation of excess cases was based on expected age-specific distributions assuming that the incidence of thyroid cancer increases exponentially with age, as predicted by the multistage model of carcinogenesis.

Over the study period (1989-2015) the age standardized incidence of thyroid cancer has tripled in Russian women and doubled in men. Strong support for the hypothesis that the increase in thyroid cancer incidence may be artificial is evident from age-specific incidence trends: increases in incidence in middle age but not in older ages, thereby altering the age curves from the expected exponential shape to an "inverted U" shape. The number of observed cases of thyroid cancer exceeded the expected number by 138, 325 or 70 % of all cases diagnosed with thyroid cancer. We attribute the excess cases to detection by ultrasonography clinically unimportant lesions. This is supported by a very high incidence -to-mortality ratio, low case fatality, high and growing prevalence of thyroid cancer.

Although there is an evidence that exposure to iodine 131 (¹³¹I) is an important cause of the increase in incidence of thyroid cancer in high-risk populations, we have shown that this increase could largely be attributed to overdiagnosis associated with ultrasonography screening. Overdiagnosis is the only explanation of the increase in thyroid cancer incidence in low-risk regions.