A giant U.S. study meant to help decide whether cellphones cause cancer is coming back with confusing results.
A report on the study, conducted in rats and mice, is not finished yet. But advocates pushing for more research got wind of the partial findings and the U.S. National Toxicology Program has released them early.
The findings are giving new life to the longstanding debate over whether cellphone use might cause cancer.
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They suggest that male rats exposed to constant, heavy doses of certain types of cellphone radiation develop brain and heart tumors.
But female rats didn't, and even the rats that developed tumors lived longer than rats not exposed to the radiation.
The National Toxicology Program, part of the National Institutes of Health, is still analyzing the findings.
What they do not show is whether humans are at any risk from using cellphones, or whether using a headset or keeping them away from the head and body might make a difference.
Brain tumors are rare. About 23,770 malignant tumors of the brain or spinal cord will be diagnosed this year, according to the American Cancer Society, and 16,000 people will die from them. And there has not been an increase in rates since the 1990s, when cellphones started to be used.
The partial report covers what the researchers considered the most worrying findings.
"The occurrences of two tumor types in male Harlan Sprague Dawley rats exposed to RFR (radiofrequency radiation), malignant gliomas in the brain and schwannomas of the heart, were considered of particular interest, and are the subject of this report," the team writes in its report. Sprague Dawley rats are a common type of lab rat.
"These findings appear to support the International Agency for Research on Cancer (IARC) conclusions regarding the possible carcinogenic potential of RFR." IARC, the cancer arm of the World Health Organization, said in 2011 it was possible the devices might cause cancer and recommended further study.
Cellphones are used by billions of people around the world — 92 percent of American adults own a cellphone, according to Pew Research Center. Children as young as toddlers carry and use the devices for hours on end, so any hint that they might somehow cause cancer would be of enormous concern.
Previous studies have had very mixed results. One study found that holding a mobile phone next to the head might warm up brain cells, but there was not any clear consequence of that. Studies on rats are considered of only partial reliability, because lab rats have their own unpredictable vulnerabilities to cancer.
Scientists at the toxicology lab scheduled a briefing for later on Friday to explain the partial results.
For the experiment, a contract lab in Chicago generated 900 MHz cellphone signals, both in the GSM and CDMA radio systems — the two major systems used by cellphone providers. They directed the signals into cages with pregnant rats, and then kept the signals focused on the rat pups for 10 minutes on, 10 minutes off, nine hours a day, as they grew for two years.
So called control rats were raised nearby without radiation exposure.
There was little effect on the newborn rats, except the pups appeared to be very slightly lighter when the mothers were exposed to the signals.
The radiation-exposed rats lived slightly longer than the control rats. "A low incidence of malignant gliomas" was seen in male rats exposed to the GSM signals. Gliomas are a type of brain tumor. There was also evidence of pre-cancerous changes in brain cells called hyperplasia.
"Cardiac schwannomas were observed in male rats in all exposed groups of both GSM- and CDMA-modulated RFR, while none were observed in controls," the team added. That's a type of heart tumor.
"No biologically significant effects were observed in the brain or heart of female rats," they added.
The findings are strong enough to suggest that the radiation exposure caused the tumors in the males, the team concluded.
The full report is due out next year.
The unusual release of the partial report included comments for expert reviewers - outside experts who read reports, ask questions and point out possible weaknesses. They included cancer experts at NIH and three outside veterinarians: David Dorman of North Carolina State University, Russell Cattley of Auburn University and Michael Pino, a pathology consultant.
The rats got significantly more radiation than the levels considered safe for humans, noted Diana Copeland Haines, of the Frederick National Laboratory for Cancer Research, part of the National Cancer Institute. But she said she agreed with the conclusion that the rats' tumors were likely caused by the radiation.
Dr. Michael Lauer of the NIH disagrees and says there's just not enough information to say whether the experiment shows the radiation caused the tumors.
"I suspect that this experiment is substantially underpowered and that the few positive results found reflect false positive findings. The higher survival with RFR, along with the prior epidemiological literature, leaves me even more skeptical of the authors' claims," he wrote in his review.
One British expert was also skeptical. "These partial findings don't cause me any real concern about health risks from mobile phone use," said Kevin McConway, emeritus professor of applied statistics at Britain's Open University.