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The Interphone study, run by the World Health Organization's International Agency for Research on Cancer, tracked 13,000 cell phone users for 10 years.
Researchers from 13 countries, including the U.S., Canada, Britain and Germany, collaborated on the effort. Since tumors can take 10 to 20 years to develop following exposure, the study was an attempt to substantiate ongoing concerns about the role of radiation from cell phones in brain cancer.
But the research, to be released Tuesday after years of delay, managed to confirm only what prior studies have concluded: Scientists just aren't sure if cell phones cause brain tumors.
"The tired refrain 'more research is needed' fully applies in this instance: Without more research, the public's question about the acceptability of cancer risk from mobile phones will remain unanswered," reads an accompanying commentary by Italian researcher Rodolfo Saracci and the University of Southern California's Jonathan Samet.
Given the long-term, global nature of the study, and its lofty $24 million price tag, critics are expressing frustration about the significant flaws that mar its findings -- or the lack thereof.
"I'm not telling people to stop using the phone. I'm saying that I can't tell you if cell phones are dangerous, but I can tell you that I'm not sure that they are safe," Dr. Devra Davis, professor of preventive medicine at Mount Sinai Medical Center in New York, told CNN News.
She added that only adults over age 30 were studied, even though teens are the fastest growing segment of cell phone users. And rather than compare cell phone users to nonusers, the study only looked at the frequency of phone use.
As cell phones rapidly transformed how we communicate, the research, which started in 2000, became outdated as it was happening. It didn't evaluate how hands-free devices or carrying a cell phone in one's pocket might change the risk of brain cancer, or consider trends in texting and phone-based e-mailing.
And in some instances, the study came up with conclusions that the research team dismissed as "improbable." Most notably, cell phone use appeared to reduce the risk of brain tumors in some groups, which is likely due to flaws in the study's methodology.
"We can't just conclude that there is no effect," Elisabeth Cardis, of the Center for Research in Environmental Epidemiology in Barcelona, Spain, told Reuters. "There are indications of a possible increase. We're not sure that it is correct. It could be due to bias, but the indications are sufficiently strong ... to be concerned."
But at least one of the study's authors is defending his team's conclusions.
"I think the results are tantalizing in the sense that overall we're not seeing evidence of an increased risk," study researcher Daniel Krewski told Canada's CBC News.
Krewski, who is with the University of Ottawa, called the results "comforting," but also noted that those who use their phones for more than 30 minutes a day might have cause for concern.
The study found a slight increased risk of glioma, a rare but deadly kind of brain tumor, among the heaviest cell phone users.
But perhaps most frightening for today's cell phone users -- including 289 million Americans -- is that the study was based on outdated usage patterns. A "heavy" caller was defined as someone who used a cell phone for 150 minutes a month, which is what many Americans now use in a single week.
About one-third of the research dollars to fund the study came from the cell phone industry. Already, they're welcoming the study's results -- and putting their own spin on them.
"The overall conclusion of no increased risk is in accordance with the large body of existing research and many expert reviews," Dr. Jack Rowley, research director for GSM, an association of mobile phone companies, said in a statement. "By way of background," he added, "tumors of the nervous system are rare and account for less than 2 percent of all malignancies."
Despite the inconclusive results, the WHO study is, by far, the most comprehensive analysis of a link between cell phones and brain tumors to date.
And we might not get more substantive conclusions for decades. European researchers have just launched a 30-year study of 250,000 people in hopes of coming up with some definitive answers.
