March 21, 2012 — The controversy surrounding the health risks related to cell phones continues to evolve.
A new study, published online March 8 in BMJ, suggests that there is little evidence linking cell phone use to a higher risk for glioma. The researchers note that the higher risks seen previously are not consistent with the incidence trends they found in the American population.
“Our study looked at actual rates of glioma in the United States, in comparison to projected or expected rates of glioma based on observations from the INTERPHONE study [Int J Epidemiol. 2010;39:675-694] and a study in Sweden explained lead author Mark Little, DPhil, senior scientist at the National Cancer Institute (NCI) in Rockville, Maryland. “We found that actual glioma rates declined over the study period.”
The researchers used population-based data from 1992 to 2008, reported in 12 Surveillance, Epidemiology, and End Results (SEER) registries, to determine glioma incidence.
When they modeled the incidence rates projected in the INTERPHONE study, which showed slight increases in glioma risk among a small number of heavy users, they found that they were not statistically distinguishable from the actual rates in the United States.
However, “when we modeled incidence rates projected from the Swedish study, we found that the actual rates would need to be at least 40% higher than what was observed,” he told Medscape Medical News. “Thus, actual glioma rates in the United States are consistent with the INTERPHONE study, but not with the rates based on the Swedish study.”
Usage Increased, Rates Generally Did Not
From 1992 to 2008, the age-specific incidence rates of glioma remained generally constant (a 0.02% decrease per year), while cell phone usage increased substantially in the American population (from about 0% to almost 100%).
“In general, the study suggests that there is not a large excess risk for glioma associated with cell phone use,” said Dr. Little. “However, it cannot rule out a modest excess risk for a small number of heavy users.”
“In that sense, the study is broadly reassuring,” he added. “However, we recommend continued surveillance of glioma rates for a number of reasons, including changing usage patterns and technology, and because tumor latency may be longer than has been observed to date.”
The link between cell phone use and brain and central nervous system tumors has been widely investigated, but results have been inconsistent. Some studies have not supported the association, whereas others have observed modest to large increases in relative risk, usually of glioma. There have also been some reports of an increased risk for acoustic neuroma.
Last year, the International Agency for Research on Cancer heated up the controversy over cell phone safety when it classified radiofrequency electromagnetic fields as possibly carcinogenic to humans, as reported by Medscape Medical News.
Some of the strongest evidence supporting an association between brain tumors and cell phone use comes from a series of studies led by Lennart Hardell, MD, PhD, from the Department of Oncology, Orebro Medical Center, Sweden (Int J Oncol. 2006;28:509-518; Int Arch Occup Environ Health. 2006;79:630-639; Arch Environ Health. 2004;59:132-137; Pathophysiology. 2009;16:113-122). These studies showed that risk increased with the number of cumulative hours of use, higher radiated power, and length of cell phone use. They also reported that younger users were at higher risk.
A Rising Trend?
Approached for independent comment, Dariusz Leszczynski, PhD, DSc, pointed out that the NCI study does not show that cell phone radiation causes brain cancer, but it does show that brain cancer in the United States is slowly rising.
“This study does not give a precise reason for this trend,” said Dr. Leszczynski, a research professor at the Radiation and Nuclear Safety Authority in Helsinki, Finland.
“Because trends are based on real data, not just assumptions or predictions [as were used in INTERPHONE and the Swedish studies], we can believe that there is a slow increase in brain cancer in the United States,” he explained. “What is important is that for the first time, scientists admit that [this increase]…might be caused by cell phone radiation-induced brain cancer — assuming that this radiation causes brain cancer.”
Dr. Leszczynski noted that the conclusions from the NCI study open the door for more speculation, but emphasized that epidemiology or trend data will not give us the much-needed answer. “We need to show in molecular-level studies that cell phone radiation activates processes in the human body that lead to the development of brain cancer,” he said. “We don’t have such studies yet.”
Currently, only 3 molecular-level studies of cell phone radiation have been conducted in humans — one from Dr. Leszczynski’s lab. “We do not know if this radiation has an effect on human physiology and, if it has, what kind of impact it has,” Dr. Leszczynski explained.
In the NCI study, Dr. Little and colleagues estimated projected rates of glioma by combining the relative risks that were reported in the INTERPHONE and the Swedish study, and adjusted them for age, registry, and sex. They analyzed data for 27,457 cases of glioma, and concentrated on 24,813 cases in adults.
Although glioma rates were generally stable during the study period, low-grade gliomas and those with a poorly specified anatomical location decreased 3.02% and 2.35% per year, respectively. There was also a modest increase in rates of gliomas of the temporal lobe and other specified sites during the study period (0.73% and 0.79% per year, respectively).
The researchers point out that there is no evidence of the 20% decrease in glioma risk (that is, a relative risk of 0.8) seen for regular cell phones users in the INTERPHONE study.
Based on relative risks from the Swedish study, predicted rates of glioma were much higher than, and therefore statistically inconsistent with, observed rates, the authors conclude. “However, based on relative risks from the INTERPHONE study, projected rates could be consistent with the observed data.”
The study was funded by the Intramural Research Program of the National Institutes of Health, and the Division of Cancer Epidemiology and Genetics at the National Cancer Institute. The authors have disclosed no relevant financial relationships.