according to a recently published global study.
The proportion of thyroid cancer cases attributable to overdiagnosis globally was higher in women (78%) than in men (68%), with this rate varying substantially across countries, wrote Mengmeng Li, PhD, of the Sun Yat-sen University Cancer Center, Guangzhou, China, and coauthors in an October paper in The Lancet Diabetes & Endocrinology.
Overdiagnosis refers to the diagnosis of lesions that would not cause symptoms and that would not progress, if left alone.
Increased testing for thyroid cancer, fueled in large part by the expansion of imaging technologies and progressively more intense and disorganized scrutiny of the thyroid, led many people to be treated for often indolent lesions, exposing them to potential side effects as well as financial and emotional distress.
Li and coauthors estimate that more than 1.7 million people might have been overdiagnosed between 2013 and 2017 in 63 countries.
“Overdiagnosis clearly emerged in some high-resource countries with private-based health systems in which access to healthcare overrules regulatory controls (eg, in the USA) and in some high-quality public health systems with easy and broad access to thyroid gland diagnostic examinations (eg, in Canada),” Li and coauthors wrote. “Conversely, thyroid cancer is less commonly diagnosed in those countries in which access to diagnosis is guided by strong regulatory rules (eg, in Nordic countries).”
Their study drew from almost 40 years of research, including the latest available data from the World Health Organization’s International Agency for Research on Cancer’s (IARC’s) Global Cancer Observatory. Li and coauthors examined patterns in the time trends of thyroid cancer, mortality data, and trends in diagnosis of thyroid cancer before testing became common in many nations.
This approach is needed in estimating overdiagnosis, where it’s not possible to see what’s happening on a case-by-case level, Salvatore Vaccarella, PhD, a scientist at IARC’s Cancer Surveillance Branch, said in an interview.
Researchers can’t tell whether an individual’s detected early-stage cancers would have remained indolent for years or eventually would have put their life at risk, he said. Instead, the patterns emerge through larger studies of the reported cases of cancer like thyroid tumors and then looking at separate datasets on mortality.
“We can only see that as a big phenomenon when we look at population-based data,” Vaccarella said.
Persisting Problem
Recognition of the harms of overdiagnosis has resulted in some reduction of the incidence of thyroid cancer in the United States, Li and coauthors wrote. After adjusting for age, incidence has fallen from 19 per 100,000 women in 2013 to 16 per 100,000 women in 2017. The proportion of thyroid cancer attributed to overdiagnosis has dropped from 76% to 68% in the country.
The paper adds to the evidence suggesting that the rise in screening has not changed mortality rates for thyroid cancer. For example, Li and coauthors reported seeing “a small decrease in thyroid cancer mortality rates over time in some European countries, but this decline (less than 1 per 100,000 women) is marginal compared with the increases in incidence (reaching around 100 per 100,000 women).”
“Moreover, previous data show that the downward mortality trends had begun before the wide use of ultrasonography for early detection and that period and birth cohort effects have been declining, probably due to treatment advances and reduced prevalence of risk factors, such as the reduction in iodine deficiency,” they wrote.
In an interview, Amanda Davis, MD, of AnMed, a nonprofit health system based in Anderson, South Carolina, said the new paper from Li and Vaccarella provides further evidence for a cautious approach to thyroid nodules given concerns about overdiagnosis.
If early detection of cancer via discovery of thyroid nodules actually helped patients, mortality rates would have dropped with expansion of screening and the resulting diagnoses, said Davis, who is an associate program director at AnMed’s family medicine residency program and affiliate professor at the Medical University of South Carolina, Charleston.
In many cases, people learn they have thyroid lesions after being tested for other conditions such as ultrasound done on carotid arteries to check for stroke risk. The most common form of thyroid cancer is the papillary form. Papillary thyroid cancer tends to be slow growing, carries a low risk for distant metastasis, and in many cases poses little risk. Some small (< 1 cm) papillary thyroid cancers can be monitored with active surveillance as opposed to thyroid lobectomy.
“So just finding more nodules incidentally or through screening ultrasound and even finding more papillary cancers via these methods does not make people healthier or decrease mortality,” Davis said.
“So just finding more things and even finding more papillary cancers does not increase our ability to treat people and keep them alive longer,” Davis said.
The 5-year survival rate for thyroid cancer overall is 98.1% and varies from 99.9% for localized disease to 55.3% for distant disease, the US Preventive Services Task Force (USPSTF) said in a 2017 publication in JAMA. The task force that year gave a “D” rating on screening of asymptomatic people for thyroid cancer. That means there’s moderate certainty that screening for thyroid cancer in asymptomatic persons results in harms that outweigh the benefits. The decision to give this “D” rating meant this screening is not recommended. That’s still the panel’s view.
“You can think of it as a “D” for ‘don’t screen for thyroid cancer,’ ” in people who present no symptoms of this illness, John Wong, MD, the vice chair of the USPSTF, said in an interview.
In primary care, the challenge is assessing thyroid nodules detected when people undergo testing for another reason, such as an ultrasound of the carotid artery to check for stroke risk.
Thyroid nodules can be detected by ultrasonography in up to 68% of the general population, reported a study in American Family Physician. Nodules with suspicious features or ≥ 1 cm require fine needle aspiration. The Bethesda System for Reporting Thyroid Cytopathology can be used to classify samples, with molecular testing applied to guide treatment when fine needle aspiration yields an indeterminate result.