Why Are Thyroid Cancer Cases on the Rise?

While thyroid cancer is surging, the reasons why are very much a matter of debate. Some researchers claim the numbers primarily reflect improved diagnostic and detection techniques, others say the problem goes deeper.

thyroid cancer
(L-R) Drs. Jochen Lorch, Francis Moore and Erik Alexander, leaders of Dana-Farber’s Thyroid Center.

Official statistics depict a disease on the rise. Data collected by the National Cancer Institute (NCI) show the incidence of thyroid cancer – the number of people diagnosed within a given-size population – has more than doubled since the early 1970s. For women, it is the fastest-growing cancer as measured by the number of new cases each year.

Many investigators point to improved methods of detection as the major force behind the increase. Over the past 35 years, the use of ultrasound and fine-needle biopsies has enabled doctors to diagnose thousands of cases that might have been missed in the past. A study published in 2006 concluded that a 140 percent increase in thyroid cancer diagnoses between 1973 and 2002 was largely a result of “increased diagnostic scrutiny.”

If the incidence of thyroid cancer were actually rising, researchers say, detection of thyroid tumors of all sizes would have increased. What researchers found, instead, was that 87 percent of the increase was in small papillary thyroid tumors that were less than two centimeters in diameter. Many of these growths, researchers say, would never have posed a danger to patients’ health, even if they hadn’t been detected.

Still, some researchers argue that view is oversimplistic. Because doctors often can’t predict which growths are likely to be harmful and which aren’t, early detection of any thyroid abnormality is important for proper treatment, they say.

A study published last year attributes much of the increase in thyroid cancer incidence to overdiagnosis. Using NCI data, the investigators found that the incidence rose from 4.9 people per 100,000 in 1975 to 14.3 per 100,000 in 2009 – a nearly 300% jump. When they looked at the percentage of patients who died of the disease over that period, however, they found it remained stable. If doctors were detecting more dangerous cases of thyroid cancer – rather than merely diagnosing more small, innocuous growths – the mortality rate would be expected to climb along with the incidence. Because that didn’t happen, the investigators concluded that the figures indicate an “epidemic of diagnosis” rather than disease.

As with the 2006 study, however, some researchers have taken issue with these findings. They argue that the mortality rate appears flat mainly because it was charted over a relatively long time – 35 years. Had it covered just the last 10 years, critics say, it would show that the death rate from thyroid disease is increasing faster than any other cancer except liver cancer.

“The tricky part is, how do we identify patients with disease that is not going to cause problems over a long period of time or perhaps ever, and how do we find patients who need to undergo aggressive treatment right away because of the nature of their disease?” says Jochen Lorch, MD, director of the Thyroid Cancer Center at Dana-Farber. “As we learn more about what drives thyroid cancer on a molecular level, we will have answers to that question in the future, but this will take a bit more work to figure out.”