Thyroid cancer also is found more often in older people, and more of them live here than in many other areas.
At a news conference yesterday, advocacy groups offered another possible explanation: proximity to nuclear power plants.
"This area has the greatest concentration of nuclear reactors in the United States," said Joseph J. Mangano, executive director of the Radiation and Public Health Project, based in Ocean City, N.J.
He referred to a circle that extended roughly 90 miles in all directions from the northwestern tip of Bucks County. Within it, he said, are seven nuclear plants and an "epidemic of thyroid cancer."
Mangano, who holds a master's degree in public health, published the results of his research in a small medical journal two months ago and booked the Mayor's Reception Room in City Hall for yesterday's news conference, which included several local environmental groups and cancer survivors.
But the city Health Department knew nothing about the event, and some other scientists dismissed him as a longtime crusader against nuclear energy who cherry-picked data to support his beliefs. Plus, they said, this kind of study cannot prove cause and effect, no matter how thorough it is.
"For example, you could look at rates of colon cancer and incidence of belief in Santa Claus," said Patricia Milligan, a health physicist and nuclear pharmacist at the Nuclear Regulatory Commission. There is a clear correlation - both are much higher in the United States than in Japan. But suggesting that Santa brings cancer down the chimney would be "ridiculous," she said.
Still, no one disputes the validity of Mangano's raw data, taken directly from the National Cancer Institute. Pennsylvania had the fourth-highest rate of thyroid cancer in the nation, about 2,000 cases, 42 percent above normal, in 2006, according to the most recent NCI statistics.
New Jersey is ranked seventh, or 35 percent above the national average. (Mangano's findings, published in the International Journal of Health Services and based on the slightly older data available at the time, had Pennsylvania as No. 1 and New Jersey No. 5.)
Every county in the Philadelphia region has higher-than-expected rates of thyroid cancer.
"It is a provocative paper," but a lot more work needs to be done, said Susan Mandel, a professor of medicine and radiology at the University of Pennsylvania School of Medicine who cowrote the American Thyroid Association's current treatment guidelines. "It raises questions that can potentially be answered."
Thyroid-cancer rates have been increasing for years nationwide. Mandel said recent research identified two main factors.
Sophisticated diagnostic techniques - ultrasound and fine-needle aspiration, or using slim needles to get tissues to be analyzed - are likely finding smaller cancers than was possible with older technology. This raises the reported rate, but does not mean there is more cancer.
The second reason is increased exposure to radiation, mainly from fast-growing use of CAT scans and, to a lesser extent, X-rays. That could include dental X-rays, but only if an individual has gotten a huge number of them, Mandel said.
Both have the potential to show up as higher rates locally because the concentration of high-technology medicine means more cancers are found and more radiation is used to diagnose all sorts of diseases. The Penn Thyroid Center, which is codirected by Mandel, sees nearly 1.5 percent of the 38,000 or so patients diagnosed annually nationwide, she said.
About 1,700 Americans a year die of thyroid cancer, and perhaps 400,000 are living with it, she said.
To show an association - not cause and effect, merely a correlation - between nuclear plants and thyroid cancer would require a large study comparing rates in counties across the country with power plants and without, said Timothy Rebbeck, professor of epidemiology at Penn. Mangano said yesterday that he planned to do that.
A good study should then compare many individuals with thyroid cancer to their exposure histories, Rebbeck said, determining, for example, whether they recently arrived in a county or had lived there for decades.
Cause and effect - nuclear or otherwise - is much harder to prove, but is possible, Mandel said. Thyroid cancer can be caused by radiation in two ways, she said: "internal" or ingested radiation, such as that from Chernobyl and other releases, and "external" radiation, passed through the skin from CAT scans.
Cancers from ingested radiation can be identified because they are more likely than the others to carry a specific genetic mutation, she said.
At the City Hall news conference, Mangano and others agreed that their data proved nothing. But some advocates said the questions alone should be enough to rethink nuclear power.
Eileen Collis, 69, who was diagnosed in 2006 after decades living in the shadow of the Limerick nuclear plant in Montgomery County, still suffers from the effects of debilitating chemotherapy that eradicated her thyroid cancer. Although no doctor has linked her cancer to the power plant, she said, she believes "in my heart" that there is a link.
Contact staff writer Don Sapatkin at 215-854-2617 or firstname.lastname@example.org.