TUESDAY, July 14, 2009 (HealthDay News) -- Intensified screening doesn't entirely explain the jump in thyroid cancers noted in the United States since 1980, and scientists now believe that other as-yet-unknown factors are to blame.
A new study finds that thyroid tumors of all sizes are being picked up, not just the smaller ones that more aggressive screening would be expected to detect.
"You cannot simply explain this by increased screening, there's a real increased incidence," said Dr. Amy Chen, lead author of a study published online July 13 in the journal Cancer.
Although, "some of this increased incidence is due to increased screening finding smaller tumors," she added.
The findings surprised one expert.
"I wrote a chapter about this for a textbook about a year ago and I came away thinking this [rise in cancers] is a reflection of enhanced diagnostics," said Dr. Bruce J. Davidson, professor and chairman of otolaryngology-head and neck surgery at Georgetown University Hospital in Washington, D.C. But, "it is more disturbing that it's not just small tumors; it seems to be all tumors," he said.
An estimated 37,200 new cases of thyroid cancer will be diagnosed this year, according to the U.S. National Cancer Institute. Fortunately, the cancer is considered highly curable, but the researchers said survival rates have not improved with better detection.
Until now, an uptick in cases seen over the past three decades was attributed to increased use of ultrasound and image-guided biopsy to detect tumors. Some researchers had found that thyroid cancer was diagnosed more often in areas with higher incomes and less in uninsured populations, adding further credence to this theory.
Looking at thyroid cancer cases from 1988 to 2005 reported in a large cancer database, Chen and her team found a higher incidence not just in small tumors, but across all sizes.
The most pronounced increase was seen in primary tumors under 1.0 centimeters -- small ones for which many experts consider it safe to take a wait-and-see approach. The rate for these tumors rose almost 10 percent per year in men (1997 to 2005) and 8.6 percent per year in women (1988 to 2005).
But the authors also saw a 3.7 percent annual increase in tumors exceeding 4 centimeters in men and a 5.7 percent yearly rise in these tumors in women.
Cancers that had spread also increased in men by 3.7 percent annually and in women by 2.3 percent.
Thyroid cancer can be caused by exposure to radiation but there has been no evidence of increased exposure to radiation among Americans.
"People have looked at background radiation and nothing really has come of that that's very useful. And certainly not useful to us in why there would be a bump in incidence in the last 15 years," Davidson said.
Chen proposed in the study that environmental, dietary and genetic issues be explored.
There's more on thyroid cancer at the American Cancer Society.