Though Testicular cancer remains relatively uncommon, rates of the disease have risen in many countries since the 1970s, a new study suggests.

Testicular cancer is known to be most common among white men, and worldwide, rates of the disease are still highest in the U.S., Canada, Australia and Europe - particularly in Nordic countries such as Denmark and Norway.

But researchers at National Cancer Institute in Bethesda, Maryland, have found that rates of the disease rose between 1973 and 1997 in many parts of the world - including traditionally low-incidence countries.

It’s “implausible,” the researchers report in the International Journal of Cancer, that better diagnosis of the disease explains the rise, since there is no widespread screening for the cancer and most men are still diagnosed only after symptoms arise.

Instead, Dr. Mark P. Purdue and his colleagues speculate, changes over time in certain risk factors for Testicular cancer may be at work. What those factors are, however, is unclear, according to the researchers.

Testicular cancer, which usually arises in young men, remains a relatively rare disease, even in countries considered high-incidence. In the U.S., the disease accounts for about 1 percent of all cancers diagnosed in men.

Researchers have identified certain risk factors for the disease - such as white race, family history of testicular cancer, or being born with an undescended testicle or certain other congenital abnormalities.

But the role of factors related to lifestyle and environment has been hard to pin down.

According to Purdue’s team, some studies have suggested that men whose mothers gave birth to them at an older age have an elevated risk of testicular cancer, possibly due to elevated levels of maternal estrogen during pregnancy.

Other theorized risk factors include low birthweight, younger age at puberty and certain viral infections in childhood. None of these, however, have been conclusively tied to testicular cancer.

More research into how suspected risk factors have changed over time in different populations “may yield important insight into the causes of the widespread increase in testis cancer risk,” Purdue and his colleagues write.

Their study is based on 25 years’ worth of data from international cancer registries. Overall, rates of Testicular cancer between 1973 and 1997 were highest in Denmark and lowest in Zimbabwe.

But over time, rates of testicular cancer rose in every population studied - by 60 percent, on average - though the increases were strongest mainly in the regions that have traditionally had the highest incidence. In the U.S., the incidence among white men climbed 47 percent, compared with 23 percent among black men.

And in Nordic countries, cancer rates rose anywhere from 59 percent to 86 percent.

One standout was Puerto Rico, a low-incidence region that nonetheless had the biggest jump in the rate of Testicular cancer, at 220 percent. This, however, is a reflection of the “extremely low” incidence seen in Puerto Rico in the 1970s, the researchers note.

On the brighter side, there were some signs that in the 1990s, the rise in testicular cancer was leveling off in the U.S. and certain other regions. Further research of more recent data, according to Purdue’s team, should help show whether rates are indeed stabilizing.

SOURCE: International Journal of Cancer, July 10, 2005.