The incidence of thyroid cancer cases has increased markedly since the late 1990s, although rates vary widely between countries, and the increases have been largely confined to the papillary subtype, suggests a new population-based study spanning 25 countries.

The researchers say this is “likely to be due to the effect of intense scrutiny of the thyroid gland…in middle-aged individuals” with sensitive imaging such as ultrasonography or CT.

There is “a lot of evidence” to suggest that this is leading to overdiagnosis, lead author Adalberto Miranda-Filho, PhD, told Medscape Medical News. The degree of overdiagnosis depends on the “diagnostic pressure” resulting from opportunistic screening.

The potential for cancer overdiagnosis, including thyroid cancer, was recently estimated to be about 20% for five common types, and there have been calls to relabel low-risk lesions as something other than cancer.

Miranda-Filho does not believe that subclinical papillary thyroid cancer should be renamed, because that would “require agreement of pathologists and other researchers,” although he said it is “important to explain that not all ‘cancers’ can evolve into a life-threatening disease.”

Screening for thyroid cancer is not currently recommended because it could increase the risk for patient harms without improving outcomes, explained Miranda-Filho, of the Cancer Surveillance Branch, International Agency for Research on Cancer (IARC), Lyon, France.

The research was published on March 1 in The Lancet Diabetes and Endocrinology.

Papillary Thyroid Cancers Increased in All Countries

Thyroid cancer, which includes a range of histologic subtypes, accounts for 3% of cancer diagnoses globally, the team writes. It is estimated that there were 586,000 new patients in 2020.

Recent evidence suggests the incidence has risen substantially in many high- and medium-income countries over the past 30 years, although there is wide variation between and within populations. Moreover, mortality rates for thyroid cancer have decreased or remained stable.

To investigate this further, the French researchers examined data on thyroid cancer incidence collated by the IARC for the period 1998–2012. They focused on 25 countries from Europe, the Americas, Asia, and Oceania, which have cancer registries that cover more than two million of their population.

The analyses were restricted to individuals aged 20 to 84 years, and the proportion of cases of unspecified thyroid cancer had to be less than 10%.

Data on almost 150,000 thyroid cancer cases were examined, including 59,499 from South Korea, 15,535 from the United States, 15,158 from Canada, 8684 from the United Kingdom, 8106 from Australia, and 3806 from China.

Across all countries, papillary thyroid cancer was the main contributor to thyroid cancer cases. Despite wide variations between countries, it was the only histologic subtype that increased in incidence in all countries.

For the period 2008–2012, the age-standardized incidence rate for papillary thyroid cancer in women ranged from 4.3 to 5.3 per 100,000 person-years in the Netherlands, the United Kingdom, and Denmark to 143.3 cases per 100,000 person-years in South Korea.

Among men, the age-standardized incidence rate ranged from 1.2 to 1.6 per 100,000 person-years in Thailand, Bulgaria, and the Netherlands to 30.7 per 100,000 person-years in South Korea.

Over the whole study period, rates of papillary thyroid cancer increased in both men and women, with large variability between countries. The increases in women were rapid and exceeded 20 cases per 100,000 in several countries, most notably, South Korea.

Interestingly, incidence rates in South Korea, China, Japan, and Turkey were low and stable until the 2000s and then rose markedly, whereas the increase in incidence in the United States, Austria, Croatia, Germany, Slovenia, Spain, Lithuania, and Bulgaria stabilized around 2009.

Not Much Change in Incidence Rates for Other Subtypes of Thyroid Cancer

For other histologic subtypes of thyroid cancer, the trends in incidence rates were relatively stable and low.

However, some countries, such as the United States, China, South Korea, Turkey, and some Northern European countries, saw increases in follicular thyroid cancer, albeit at much lower rates than for papillary thyroid cancer.

Overall, age-standardized rates for follicular thyroid cancer ranged from 0.5 to 2.5 per 100,000 person-years among women and from 0.3 to 1.5 per 100,000 person-years among men. Rates for medullary thyroid cancer were less than one per 100,000 person-years among men and women. For the anaplastic subtype, rates were less than 0.2 per 100,000 person-years.

The team notes that small decreases in anaplastic thyroid cancer rates across the study period were recorded in 21 countries, including Colombia, Lithuania, Bulgaria, Slovenia, Germany, and Norway.

However, Miranda-Filho said that overall, the incidence rate of anaplastic thyroid cancer “seems to be not affected by the intensity of screening,” whereas the “lack of evidence” on changing trends for follicular disease suggests that opportunistic screening allows only non-life-threatening tumors to emerge “from a large reservoir of subclinical asymptomatic neoplasms in the thyroid glands.”

The study was supported by the French Institut National du Cancer, the Italian Association for Cancer Research, and the Italian Ministry of Health to Centro di Riferimento Oncologico di Aviano. The authors have disclosed no relevant financial relationships.

Lancet Diabet Endocrinol. Published online March 1, 2021. Abstract

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