The steady climb in the incidence of thyroid cancer since the early 1980s has been attributed to an increase in screening in recent years and the diagnosis of more small tumors. However, work by investigators at the American Cancer Society in Atlanta suggests that other factors - such as environmental exposures, dietary changes, or genetic causes - may play a larger role.

Dr. Amy Y. Chen and her team used the National Cancer Institute's SEER cancer registry to identify more than 30,000 cases of thyroid cancer diagnosed between 1988 and 2005.

The authors note that, while the rate of thyroid cancer was three times higher among women than among men after age was taken into account, rates increased similarly for both genders.

Specifically, among women, the age-adjusted number of cases per 100,000 rose from 6.4 in 1998 to 14.9 in 2005. Among men, the rate of cancer increased from 2.5 per 100,000 in 1988 to 5.1 per 100,000 in 2005.

When the researchers looked at tumor size, they found that the number of tumors of all sizes was increasing. If more screening was responsible for a higher rate of diagnosis, they explain, they would have expected to only see an increase in the number of small tumors.

Despite the better detection of cancers at earlier stages, they note, survival rates did not improve among men or women, the authors write in their report in the journal Cancer. They conclude by calling for more research into the causes of rising rates.

SOURCE: Cancer, August 15 2009.