Thyroid cancer risk persists decades after radiation

People who survived the atomic bombings of Hiroshima and Nagasaki as children continued to have a higher-than-normal risk of thyroid cancer more than 50 years after radiation exposure, according to a new study.

The thyroid gland, located at the front of the neck, releases hormones that help regulate the body’s metabolism. The gland works particularly hard during times of fast growth and development in kids and teens.

Previous studies have shown thyroid cells are particularly vulnerable to ionizing radiation - the kind produced by the Chernobyl nuclear meltdown or the atomic bombings of Japan, for example.

Thyroid cancer is one of the most radiosensitive cancers,” said Dr. Kiyohiko Mabuchi of the National Cancer Institute in Bethesda, Maryland, who worked on the new study.

And, he added, “Younger (thyroid) tissue may be more sensitive to radiation - that’s one of the hypotheses.”

By tracking new cancer diagnoses in people who were in Japan during the bombings in 1945, and those who were not, Mabuchi and his colleagues found little evidence that adults exposed to the radiation were more likely to develop thyroid cancer later on.

But for kids it was a different story. The researchers calculated that 36 percent of the 191 thyroid cancers that eventually developed in people who were kids or teens at the time of the attacks were likely due to radiation exposure.

The rest would have been expected in a similar, unexposed group.

In total, there were 371 thyroid cancers diagnosed between 1958 and 2005 in about 105,000 atomic bomb survivors, according to findings published in the International Journal of Cancer.

Radiation researcher John Boice from Vanderbilt University in Nashville, Tennessee, said the new study confirms findings from the United States that people remain at higher risk of thyroid cancer for decades after they’re exposed to radiation.

“The risk appears to last pretty much your entire life,” he told Reuters Health.

That’s because thyroid cells are permanently damaged by radiation, he explained - unless the radiation dose is so high that they’re killed completely.

“Most people, even scientists, have a misconception that the risk is only a few years and then it goes away,” Mabuchi added.

On the other hand, the risk of leukemia - another cancer tied to radiation exposure - is known to peak a few years after the exposure and then return to normal, he said.

The American Cancer Society estimates that about 56,000 Americans will be diagnosed with thyroid cancer in 2012, and close to 2,000 will die of the disease.

The researchers agreed that it’s not clear whether the findings have implications for kids who were living near the Fukushima Daiichi power plant in Japan, which suffered a meltdown last March following an earthquake and tsunami.

Boice said the Japanese government likely responded quickly enough with evacuations to minimize that risk.

“One of the fortunate things around Fukushima was that the population exposure was extremely, extremely small,” said Boice, who wasn’t involved in the new paper.

Even among atomic bomb survivors, he pointed out, the risk of thyroid cancer was very low for people who only got a small dose of radiation. “And, it appears around Fukushima and in Japan that the exposures to kids were below a level where there’s been any detectable increase (in cancer risk),” Boice said.

Researchers are still calculating radiation exposures after Fukushima. A typical head CT scan delivers about 2 millisieverts (mSv) worth of radiation, compared to 350 mSv and higher exposures among people who were evacuated after Chernobyl. The average dose to survivors of the atomic bombings was somewhere in between, the new study suggests.

Still, Mabuchi said people should not ignore the possible long-term effects of radiation exposure, especially for kids.

That can include medical radiation, such as from CT scans, he said. Though the risk from each individual scan is small, it’s not negligible - especially for scans to the head and neck, he added.

SOURCE:  International Journal of Cancer, online July 31, 2012.

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