NEW YORK, Mar 15 (Reuters Health)--Exposure to radiation, such as that released by the Chernobyl nuclear plant accident, takes a heavier toll on very young children, according to new study findings.

Among children living in Belarus, thyroid cancer was more common and more severe in children who were younger than 2 years old at the time of the 1986 accident than in those who were older, researchers report. In addition, the rate of childhood thyroid cancer was considerably higher in girls than in boys.

Radiation released by the nuclear accident at Chernobyl led to an increased rate of thyroid cancer in children living in regions surrounding the plant. Previous research has shown that younger children were at greatest risk of developing cancer, but whether or not a child's age affected the severity of the disease has not been studied closely, according to a team of researchers led by Dr. Jamshid Farahati, of the University of Wurzburg in Germany.

In the current study, the research team looked at 483 children with thyroid cancer who were living in Belarus at the time of the accident. All of the children were younger than 8 years old when exposed to radiation from the plant.

As has been reported before, the greatest number of cancer cases occurred in children who were younger than 2 years old at the time of the accident, Farahati and colleagues report in the March 15th issue of Cancer, a journal of the American Cancer Society. The investigators also found that the rate of cancer was 60% higher in girls than in boys.

But the researchers note that cancer tended to be more severe in the youngest children. In youngsters under age 2, tumors had grown outside the thyroid gland 62% of the time, compared with just 40% in children aged 6 to 8 years old. In addition, the youngest children were more likely to have cancer spread to the lymph nodes and other parts of the body, according to the report. But despite the differences in disease severity, the lag time between radiation exposure and cancer diagnosis was similar in all children.

The authors explain that thyroid cells in infants and toddlers divide rapidly as the glands mature. Since cancer depends on mutations that occur during cell division, each time a cell divides is an opportunity for cancer to take hold. In contrast, older children have more mature thyroid glands, so the pace of cell division is slower, which may explain in part why the risk of thyroid cancer is lower in older children. And since thyroid glands in younger children tend to be smaller than in older kids, the same amount of radiation may have more of an effect, leading to more severe cancer, according to the report.

After an average of nearly 4 years of follow-up, none of the children had died from cancer, Farahati and colleagues report. However, they state that these children should continue to be followed to see what effect, if any, age at the time of radiation exposure has on long- term survival. SOURCE: Cancer 2000;68:1470-1476.

Cancer, Volume 88 Issue 6, (15 March 2000), pp.1470-1476: "Inverse association between age at the time of radiation exposure and extent of disease in cases of radiation-induced childhood thyroid carcinoma in Belarus". Article Abstract