Thyroid Ca/Radiation in Workplace

Cancer Tied to Workplace Radiation Low doses might play a role in thyroid cancer

Medical workers exposed to low doses of radiation on the job might face an increased risk of thyroid cancer. So, too, might dental, various industrial and nuclear power workers, claims a new Canadian study that examined data on a half-million people. Linking radiation exposure to cancer is nothing new. It's well documented that atomic bomb survivors and those who've received high doses of radiation to treat non-cancerous diseases have a higher incidence of cancer later in life.

Now, however, the new study links higher incidence of cancer -- and specifically thyroid cancer -- to low-dose radiation. The researchers used data collected by the Canadian National Dose Registry, which has monitored radiation exposure of workers since 1951, and compared it to the Canadian Cancer Data Base.

They connected 3,737 workers with cancer (2,098 men and 1,639 women) to radiation exposure between 1969 and l988. Most victims were between the ages of 21 and 85. "The thyroid cancer incidence is greater than would be expected, and the incidence in women is very significant," says lead researcher Willem N. Sont, of Canada's Radiation Protection Bureau.

Sont says this study is the first to focus on medical workers who are exposed to low-dose radiation over a period of time. But he quickly adds that, despite all the linkages found, "we don't know the cause, [and] we cannot say for sure it's from radiation." "We can only speculate," Sont says. Findings appear in the current issue of the American Journal of Epidemiology.

Radiation exposure is measured in doses, taking into account both internal and external exposure to ionizing radiation, which includes gamma, beta and X-rays. The amount of radiation absorbed by a gram of tissue is expressed in millisieverts, or mSv. The average dose received by the study participants was 6.64 mSv, with men receiving a much higher average dose than women: 11.50 mSv vs. 1.75 mSv, the study says.

Not everyone agrees with the researchers' conclusions, however. John Boice, scientific director of the International Epidemiology Institute and a professor of medicine at Vanderbilt University, points out that people normally get between 1 mSv and 3 mSv a year from natural sources, like cosmic rays and the air we breathe. "Thyroid cancer is a major effect of radiation, [but] it comes from exposure in childhood, before the age of 20," Boice maintains.

The fact that the Canadian study finds excesses of thyroid cancers in adults may be attributable to "much better medical care and screening and advancements in detection" rather than radiation exposure, he says. Sont says the researchers also found a higher-than-expected link between low-dose radiation exposure and melanoma, a virulent form of skin cancer. But again, he says, "There are other factors which cause melanoma. A very obvious one is exposure to ultraviolet [light]." With that, Boice concurs. "Melanoma is not associated with ionizing radiation," Boice says. "Basal cell and squamous cell cancers have been linked to high dosages, but when melanoma pops up, we think of other potential causes." Sun exposure generally comes to mind, he says.

Incidence of other cancers -- including testicular, pancreatic and colon cancer -- also was found by the Canadian researchers, but they say it's not possible from this study alone to declare cause and effect. The next step, Sont says, is to look at dose information for workers after l988. "With smaller doses, the cancer is harder to find, and that's why this study will make contributions," he says. "But linking cancer to radiation exposure is more difficult, and more work needs to be done."

Thanks to:Fran Berger HealthScout Reporter

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