WASHINGTON, USA: Studies on whether mobile phones can cause cancer, especially brain tumors, vary widely in quality and there may be some bias in those showing the least risk, researchers reported on Tuesday.

So far it is difficult to demonstrate any link, although the best studies do suggest some association between mobile phone use and cancer, the team led by Dr. Seung-Kwon Myung of South Korea's National Cancer Center found.

Myung and colleagues at Ewha Womans University and Seoul National University Hospital in Seoul and the University of California, Berkeley, examined 23 published studies of more than 37,000 people in what is called a meta-analysis.

They found results often depended on who conducted the study and how well they controlled for bias and other errors.

"We found a large discrepancy in the association between mobile phone use and tumor risk by research group, which is confounded with the methodological quality of the research," they wrote in the Journal of Clinical Oncology.

The use of mobile and cordless phones has exploded in the past 10 years to an estimated 4.6 billion subscribers worldwide, according to the U.N. International Telecommunication Union.

Research has failed to establish any clear link between use of the devices and several kinds of cancer.

The latest study, supported in part by the U.S. Centers for Disease Control and Prevention, examined cases involving brain tumors and others including tumors of the facial nerves, salivary glands and testicles as well as non-Hodgkin's lymphomas.

It found no significant association between the risk of tumors and overall use of mobile phones, including cellular and cordless phones.

Mild risk

Myung's team said eight studies that employed "high quality" methods to blind participants against bias found a mild increased risk of tumors among people who used mobile phones compared with those who never or rarely did.

An increased risk of benign, not malignant, tumors was also found among people who used the phones for a decade or longer.

The "high quality" studies were funded by the Swedish Work Environment Fund, the Orebro Cancer Fund and the Orebro University Hospital Cancer Fund, Myung's team said.

By contrast, studies that used "low quality" methods to weed out bias found mobile users were at lower risk for tumors than people who rarely used the devices.

Myung's team suggested those results could be marred by random errors and bias because of the quality of the methods.

Funding for some of the lower-quality studies included two industry groups, the Mobile Manufacturers Forum and the Global System for Mobile Communication Association, the researchers said.

Overall, the studies examined were not broad enough to shed light on whether mobile phone use could cause tumors. Myung's team said larger studies of a type called cohort studies are needed to answer that question.

Such studies follow a group of people who share a characteristic, in this case cellphone use, and compare them with other groups over time.

The only cohort study published to date showed no association between mobile phone use and tumors. But the study, conducted in Denmark, relied on telephone subscriptions and did not evaluate actual exposure to mobile phones.