The rapid increase in thyroid cancer incidence that has occurred since the 1990s – considered an “epidemic of overdiagnosis,” has extended beyond high-income countries to less affluent settings, where unnecessary – and sometimes opportunistic – screening could continue to thrive.
“The impact of overdiagnosis on the increasing incidence of thyroid cancer highlighted in our report is a warning sign for countries with growing economies, where diagnostic technologies are increasingly and routinely offered, usually in exchange for payment, despite evidence that the harms far outweigh benefits,” the authors say.
“Overdiagnosis could turn healthy people into patients, and expose them to unnecessary harms and lifelong treatments,” say Mengmeng Li, PhD, of the International Agency for Research on Cancer, Lyon, France, and colleagues in their article published in Lancet Diabetes & Endocrinology.
With their previous research showing high rates of overdiagnosis in high-income countries, for this new analysis, they sought to evaluate whether similar patterns were occurring in less affluent settings.
They examined data from population-based cancer registries in 26 countries on four continents, looking at all cases of thyroid cancer reported between 1998 and 2012 in men and women aged 15 to 84 years.
A global public health problem
The results showed that while the incidence of thyroid cancer steadily increased from 1998 to 2002 and from 2008 to 2012 in all high-income countries, similar trends were also seen in less affluent nations, particularly in Belarus, China, Colombia, and Lithuania.
The increases were consistently greater among middle-aged women aged 35-64 years in all countries.
To determine what proportion of the higher incidence was overdiagnosis, the authors turned to historic age-specific thyroid cancer incidence data prior to the introduction of ultrasound and then looked at the progressive departure from that pattern, likely the result of the increased detection by ultrasound of thyroid nodules in middle-aged adults.
The results showed the proportion of thyroid cancer cases in women estimated to be attributable to overdiagnosis between 2008 and 2012 was as much as 93% in South Korea, 91% in Belarus, 87% in China, 84% in Italy and Croatia, and 83% in Slovakia and France.
Proportions attributable to overdiagnosis were lower in Denmark (66%), Norway (65%), Ireland (63%), United Kingdom (58%), Japan (55%), and Thailand (44%).
Women were much more likely to be overdiagnosed than men, with an approximate ratio of 3:1 in all countries; however, mortality and prevalence of thyroid cancer in autopsies were similar between genders.
Although researchers only looked at data up until 2012, Dr. Li said that, even in that year, “the amplitude of the phenomenon” was “already large and is increasing rapidly over time.”
Figures for periods subsequent to those assessed in the study “are likely to be higher.”
And the overdiagnosis is particularly remarkable in the context of the true risk of thyroid cancer, senior author Salvatore Vaccarella, PhD, told this news organization.
“What is surprising is the magnitude of this. Without overdiagnosis, thyroid cancer would probably still be a relatively rare cancer,” he said.
“Currently, it is the fifth most commonly diagnosed cancer in women of all ages and is third in women under 50 years of age. And the rates are still rising fast.”
“Overdiagnosis of thyroid cancer is still rapidly expanding in many high-income countries, and for the first time, we document and quantify the phenomenon also for several middle-income socioeconomically transitioning countries,” he observed. “In short, it is a global public health problem.”