Rising Thyroid Cancer Incidence In Youth May Suggest Overdiagnosis

Global incidence of thyroid cancer among children and adolescents has climbed rapidly in recent years despite low mortality, with wide variation among countries, according to a new study whose authors suggest that overscreening and overdiagnosis are playing a role.

Researchers examined data on thyroid cancer incidence in 49 countries and territories and on mortality in 27 countries for youth aged 0-19 years old, using records from the World Health Organization as well as two other cancer incidence registries. The analysis included 8,049 thyroid cancer cases from 2008 to 2012.

Overall, age-standardized incidence from 2008 to 2012 ranged from as low as 0.4 per 1 million person-years in Uganda and Kenya to as high as 13.4 per 1 million person-years in Belarus, the study found. During this period, incidence rates ranged from 0.6 to 20.4 cases per 1 million per-years for girls and from 0.3 to 6.8 cases per 1 million person-years for boys.

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Mortality rates for children and teens during this period were less than 0.1 deaths per 1 million person years, or less than 10 fatalities annually across all of the included countries and territories in the study.

Almost all countries and territories in the study showed a trend toward increasing incidence rates between the periods 1998-2002 and 2008-2012, and the absolute increase was statistically significant in 44% of locations studied for girls and 31% of locations studied for boys.

"Our study suggests that overdiagnosis may be the major driver of the increases observed in incidence in children, and particularly in adolescents, because of the similarity of the geographical and epidemiological patterns with those observed in adults, for which there is a consensus that overdiagnosis plays a massive role," said lead study author Salvatore Vaccarella of the International Agency for Research on Cancer in Lyon, France.

At the same time, mortality rates have been relatively stable at very low levels or declining in both adults and children, Vaccarella said by email.

"Although, it cannot be excluded that exposure to known or unknown risk factors might have contributed to the rise in incidence, we believe that this is very unlikely, at least to the point of being visible when using surveillance data," Vaccarella said. "Overdiagnosis remains the most likely explanation."

From 2008 to 2012, incidence rates for children under 10 years old were lower than 1.5 cases per 1 million person-years with little variation across countries and territories. Incidence rates increased with age, and were higher in adolescent girls than boys.

One limitation of the study is the lack of data on tumor staging and size for any diagnosed cases, researchers note in The Lancet Diabetes & Endocrinology. This data would help better assess the potential for overdiagnosis of thyroid cancer among youth.

Even so, the results suggest that clinicians should limit screening in children and adolescents to those at elevated risk for thyroid cancer due to factors such as radiation exposure, multiple family members with thyroid cancer, or a predisposition to certain genetic syndromes, said Martin Schlumberger, a researcher at Institut Gustave Roussy and University Paris-Saclay, in France, who coauthored a commentary accompanying the study.

"Clinically detected thyroid cancers in young children are often extensive, and indeed should be treated with great care," Schlumberger said by email. "However, the detection and treatment of small cancers less than 10mm, does not guarantee a better outcome given the excellent prognosis of these cancers and the potential harm of treatment."

—Lisa Rapaport

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