WASHINGTON (Reuters) - A very large, 30-year study of just about everyone in Scandinavia shows no link between cellphone use and brain tumors, researchers reported on Thursday.

Even though mobile telephone use soared in the 1990s and afterward, brain tumors did not become any more common during this time, the researchers reported in the Journal of the National Cancer Institute.

Some activist groups and a few researchers have raised concerns about a link between cellphones and several kinds of cancer, including brain tumors, although years of research have failed to establish a connection.

“We did not detect any clear change in the long-term time trends in the incidence of brain tumors from 1998 to 2003 in any subgroup,” Isabelle Deltour of the Danish Cancer Society and colleagues wrote.

Deltour’s team analyzed annual incidence rates of two types of brain tumor -- glioma and meningioma -- among adults aged 20 to 79 from Denmark, Finland, Norway, and Sweden from 1974 to 2003. These countries all have good cancer registries that keep a tally of known cancer cases.

This represented virtually the entire adult population of 16 million people, they said.

Over the 30 years, nearly 60,000 patients were diagnosed with brain tumors.

“In Denmark, Finland, Norway, and Sweden, the use of mobile phones increased sharply in the mid-1990s; thus, time trends in brain tumor incidence after 1998 may provide information about possible tumor risks associated with mobile phone use,” the researchers wrote.

They did see a small, steady increase in brain tumors, but it started in 1974, long before cellphones existed.

NO SIGNIFICANT PATTERN

“From 1974 to 2003, the incidence rate of glioma increased by 0.5 percent per year among men and by 0.2 percent per year among women,” they wrote.

Incidence of meningioma tumors rose by 0.8 percent a year among men, and rose by 3.8 percent a year among women starting in the mid-1990s. But this was mostly among women over the age of 60, who were already among those most likely to have brain tumors, they noted.

In addition, it became easier to diagnose these tumors because of better types of brain scans.

Overall, there was no significant pattern, they said.

“No change in incidence trends were observed from 1998 to 2003,” they added. That would have been when tumors would start showing up, assuming it took five to 10 years for one to develop, they said.

It is possible, Deltour’s team wrote, that it takes longer than 10 years for tumors caused by mobile phones to turn up, that the tumors are too rare in this group to show a useful trend, or that there are trends but in subgroups too small to be measured in the study.

It is just as possible that cellphones do not cause brain tumors, they added.

Most scientific studies show no association between cellphone use and brain tumors and researchers trying to find a connection have failed to find any biological explanation for how a mobile phone might cause cancer.

“Because of the high prevalence of mobile phone exposure in this population and worldwide, longer follow-up of time trends in brain tumor incidence rates are warranted,” Deltour’s team advised.