The increase in thyroid cancer cases in the United States is the result of better, high-tech diagnostic tests that are picking up minuscule tumors-most of which pose no long-term threat, according to a recent study.

The New Numbers

The number of thyroid cancers in the US more than doubled from 3.6 per 100,000 in 1973 to 8.7 per 100,000 in 2002, according to the report. But that increase was not accompanied by a rise in deaths, which have remained stable at just 0.5 per 100,000, the researchers say.

The New Technology

The study should help settle the controversy about rising thyroid cancer rates. Some investigators have attributed the increase to environmental radiation from sources such as nuclear power plants. However, the report suggests that the real reason for the higher rates may be "increased detection of subclinical disease."

"New tests are available to detect abnormalities that we never saw in the past," says the co-author of the national report, Dr. Louise Davies, an assistant professor of surgery at Dartmouth Medical School.

Ultrasound scans and needle biopsies can detect nodules as small as 2 millimeters (mm), one quarter of the size of a pencil eraser. Before the new techniques were available, doctors were detecting much larger nodules-1.5 centimeters (cm) or larger, explains Davies. Nearly 5O% of the tumors that were detected during this study were 1 cm or less, she notes.

No Threat?

Most cancers of that size grow so slowly that they pose no threat to life, Davies says. "People should know that nodules are common and that there is a lot more thyroid cancer out there than really matters," she says.

As in prostate cancer, autopsies have shown that a fair number of people, perhaps 5%, had thyroid cancers that were not detected while they were alive.

"Some of these [thyroid and prostate tumors] are so small and grow so slowly that they would not be evident during the lifetime of the patient," says Dr. R. Michael Tuttle, an endocrinologist at Memorial Sloan-Kettering Cancer Center in New York City.

The Dilemma

Experts agree that the detection of such small cancers presents a dilemma for physicians. "We need to start considering how aggressively we should treat these tumors," Davies says. "I think it is something people are struggling with."

"We're always looking for the needle in the haystack," Tuttle says. "In the worst-case scenario, we end up overtreating a good number of patients." But overtreatment might be necessary because of the potential of missing the few small cancers that will grow aggressively and become life-threatening, he says. Currently, there is no way to identify which thyroid growths can be safely left alone and which require surgery Tuttle says.

Although the report highlights a dilemma for doctors, it does offer comfort to the public, Tuttle says. What's important for patients to know is that we don't have an epidemic of thyroid cancer, he says.

Located beneath the voice box, the thyroid is a butterfly-shaped gland that produces thyroid hormone, which the body uses to regulate growth and metabolism.

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