Residents of eastern Pennsylvania might not know it, but they're living in the middle of a thyroid-cancer hot spot, according to a public-health advocate.

The eastern side of the state lays claim to six of the nation's top 18 counties with the highest thyroid-cancer rates, according to figures from the Centers for Disease Control and Prevention.

Pennsylvania ranked as the No. 1 state in thyroid-cancer cases between 2001 and 2005, 12.8 cases per 100,000 residents. (New Jersey comes in at No. 5 with 11.8 cases per 100,000.)

Joseph Mangano, the executive director of the Radiation and Public Health Project research group, said yesterday that he believes the spike in cancer is due to the high number of nuclear plants in the area.

At a news conference at City Hall where thyroid-cancer survivors and physicians also spoke, Mangano said that within 100 miles of eastern Pennsylvania, 16 nuclear reactors are operating at seven nuclear plants, the highest concentration in the country.

The emissions from the Limerick and Three-Mile Island plants don't come close to those from the 1945 bombing of Hiroshima or the 1986 Chernobyl accident, but "that doesn't necessarily mean [it's] safer," Mangano said.

"Not only have we documented an epidemic of thyroid cancer in the area, but we have raised a red flag for more and more detailed study of the relationship between the reactor emissions and thyroid cancer," Mangano said.

Mangano, who published his findings in the International Journal of Health Services, said that the only known cause of thyroid cancer is exposure to radiation, specifically radioactive iodine, "one of the 100 man-made chemicals" produced by nuclear energy.

One University of Pennsylvania doctor who has researched thyroid cancer called the findings "provocative" and "intriguing," but added that the author needed to delve more into the subject.

"We do know nuclear plants give off radioactive iodine [and] radioactive iodine can be associated with thyroid cancer," said Susan J. Mandel, a professor of medicine and radiology. "Does it mean it causes it? It requires further investigation to see if it's causing it."

Lehigh County had the highest thyroid-cancer rate; others in eastern Pennsylvania were: Northampton (3rd), Luzerne (6th), York (7th), Bucks (14th) and Lancaster (18th). In New Jersey, Camden was ranked No. 16 and Burlington was 17th.