The number of thyroid cancer cases per year has doubled since 1970, although the total tally is still relatively small compared to other cancers and survival rates are high. Less than 5% of the 35,000 people diagnosed with a thyroid tumor in the US die from this relatively slow growing cancer.
Still, the gradual increase in thyroid cancer cases prompted various research groups to examine the cause. One study in 2006 from Dartmouth College asked whether thyroid cancer really was on the rise or whether doctors were just getting better at detecting smaller tumors that may have gone unnoticed in previous decades.
The authors Drs. Louise Davies and H. Gilbert Welch concluded in their article published in the Journal of the American Medical Association “increasing incidence reflects increased detection of subclinical disease, not an increase in the true occurrence of thyroid cancer. “
The researchers found that nearly 50% of the thyroid cancer cases between 1973 and 2002 involved tumors that were small—less than one centimeter—and were largely non-aggressive types of thyroid cancer. The authors believe this helps to explain the observation that mortality rate didn’t change over time even though the number of cases per year went up. They reason that the cancers weren’t any more life-threatening, they were just discovered in patients earlier.
Early detection of thyroid cancers has likely increased because diagnostic instruments have improved, and machines like highly sensitive ultrasounds are more commonplace in practitioners’ offices and clinics. In addition, the smaller thyroid cancers frequently are picked up when a patient is imaged by specialized Xrays, CAT scans or MRI for other health concerns.
Most thyroid cancer patients come from referrals explains Dr. Jennifer Sipos, an endocrinologist at the University of Florida, who was quoted in the National Cancer Bulletin. These patients have “nodules that are incidentally found…on an [imaging procedure] taken for another reason."
Not all physicians are convinced that the current increase in thyroid cancer is wholly attributable to better detection and increased surveillance. Ironically, they suggest that some of the blame may be due to radiation exposure from CT scans. The use of these instruments has increased from a few million per year in the 1980’s to more than 62 million per year in 2006.
There haven’t been sufficient studies to support this claim, but CT scans do emit radiation levels that could increase cancer risk, especially in young people, according to Drs. David J. Brenner and Eric Hall from Columbia University Medical Center.
"Given the relatively short latency period for radiation-induced thyroid cancer…it is quite possible that CT is influencing current thyroid cancer rates in the United States in young people," says Dr. Brenner, of the Center for Radiological Research at Columbia. A large study for the National Cancer Institute in collaboration with researchers in the UK is underway to examine this issue.
Davies, L. and H.G. Welch, 2006. “Increasing Incidence of Thyroid Cancer in the United States, 1973 -2002,” JAMA article: http://jama.ama-assn.org/cgi/content/full/295/18/2164
NCI notice Feb , 2008
Papillary Cancer: The Most Common Type of Thyroid Cancer, Endocrine web