Surge in Thyroid Cancer Cases and Deaths Not the Result of Overdiagnosis, Study Says

Over the last 4 decades, the incidence of thyroid cancer has risen by an average 3.6% annualy, according to a recent analysis of cancer registry data. Researchers say the jump in thyroid cancer cases and deaths reflects a true increase in the cancer’s occurrence that cannot be explained by overdiagnosis.

The study, published in JAMA, analyzed data from the National Cancer Institute’s Surveillance, Epidemiology, and End Results cancer registry from 1974 to 2013. It aimed to identify trends in the incidence and mortality of thyroid cancer by patient characteristics and tumor type.

The analysis found an average annual increase of 3.6% in the incidence of thyroid cancer during the study period. The largest spike occurred from 1997 to 2009, when rates increased by 6.7% per year; however, between 2009 and 2013, the incidence rates had generally leveled off. The rising incidence rates over time were observed among patients of all sexes, races, and age groups, and for cases of follicular thyroid cancer, medullary thyroid cancer, and papillary thyroid cancer (PTC).

Patients diagnosed with PTC—the most common type of tumor identified throughout the study period—tended to have better outcomes than the entire population of thyroid cancer patients. Of the PTC patients who died of their cancer, 27% were still alive 10 years after diagnosis, compared with 19% of all patients with thyroid cancer who eventually died.

Thyroid cancer mortality rates also trended upward from 1994 to 2013 by an average of 1.1% annually. The most significant yearly increases in mortality rates were observed among patients who were female, black, white, and diagnosed at age 80 or higher. PTC was the only histologic type of thyroid cancer for which mortality rates increased significantly during the study period, as they rose by 1.7% annually, on average.

Findings of increased cancer incidence and mortality are at times explained as resulting from overdiagnosis, or the ability of new screening techniques to detect smaller cancers that would have previously gone unnoticed. However, the researchers pointed to certain patterns as evidence that overdiagnosis was not the sole driver of the rising thyroid cancer rates. The incidence of advanced-stage and larger-sized PTC had risen, as well as the mortality rates of patients with advanced PTC, which indicates that PTC incidence was actually on the rise and not just appearing a result of overdiagnosis.

The study authors hypothesized that environmental chemicals like pesticides could contribute to this increase in thyroid cancer, but cautioned that additional research was necessary to establish a causal relationship. Instead, they pointed to trends in cigarette smoking and obesity, which can work concurrent with influence over 40% of all new thyroid cancer cases occurring each year in the United States through “insulin resistance, thyroid hormone, and estrogen-related pathways.”

They acknowledged that their observational study could not definitively identify the causes of these trends, but called for further trials to advance the discovery of novel treatments, particularly for high-risk patients with advanced tumors, and emphasized the need for continued vigilance and attention to data.

“It will be important to continue monitoring thyroid cancer incidence and mortality rates over time to see if the observed trends persist,” they concluded.