National Cancer Institute Study of Brain Tumors and Use of Cellular Telephones: Questions and Answers
- Posted: December 21, 2000
- Why was the study done? Brain tumors are uncommon but often cause serious disabilities. Some types of brain tumors, particularly the most common type, glioma, have poor survival rates. (Question 1)
- Why were cell phones included in the NCI study? The number of people using cellular telephones has increased dramatically during the past decade and is likely to continue increasing for the foreseeable future. According to the Cellular Telecommunications Industry Association, there are currently about 107 million mobile phone subscribers in the United States, increasing at a rate of about 2 million per month. (Question 2)
- Who were the participants in the study? The NCI study included 782 brain tumor cases and 799 controls from three medical institutions: St. Josephs Hospital and Medical Centerin Phoenix; Brigham and Womens Hospital in Boston; and Western Pennsylvania Hospital in Pittsburgh. The study included brain tumor patients recently diagnosed with glioma (489 cases), meningioma (197 cases) or acoustic neuroma(96 cases). (Question 2)
1. Why was the study done?
Brain tumors are uncommon but often cause serious disabilities. Some types of brain tumors, particularly the most common type, glioma, have poor survival rates.
Very little is known about the causes of malignant or benign brain tumors. To pursue possible clues about causes from previous studies and identify promising new leads, the National Cancer Institute (NCI) began a comprehensive study of possible environmental and genetic causes of malignant and benign brain tumors in 1994. The present report focusing on cellular telephones is one part of the much larger study and was intended to determine whether use of these devices increases the risk of brain tumors.
2. Why were cell phones included in the NCI study?
The number of people using cellular telephones has increased dramatically during the past decade and is likely to continue increasing for the foreseeable future. According to the Cellular Telecommunications Industry Association, there are currently about 107 million mobile phone subscribers in the United States, increasing at a rate of about 2 million per month (http://www.wow-com.com).
Use of hand-held cellular phones involves placing a small transmitter which emits radio frequency radiation (microwave radiation) next to the head. There has been widespread public concern that radiation from cellular phones might cause tumors of the brain and nervous system. Unlike ionizing radiation, which, at high doses, is known to cause brain tumors, it is not known whether the non-ionizing radiation from cell phones poses a cancer risk. Because so many people use cellular phones, it is important to learn whether it does, and to provide reassurance if it does not.
3. Who were the participants in the study?
The NCI study included 782 brain tumor cases and 799 controls from three medical institutions: St. Josephs Hospital and Medical Centerin Phoenix; Brigham and Womens Hospital in Boston; and Western Pennsylvania Hospital in Pittsburgh. The study included brain tumor patients recently diagnosed with glioma (489 cases), meningioma (197 cases) or acoustic neuroma(96 cases).
The controls are people who were admitted to the same hospitals as the brain tumor cases for a variety of non-cancerous conditions. Controls were matched with cases by hospital, sex, race, age, and distanceof residence from hospital. This type of study is known as a case-control study. The study was restricted to adults who were age 18 or older who received care at one of the participating hospitals, resided within50 miles of the hospital, and could understand English or Spanish. Data collection began in 1994 and was completed in 1998.
4. How was cellular phone use measured?
The extent of use of hand-held cellular phones was assessed by interview. A card displaying photographs of examples of different types of portable phones was shown, including hand-held, car, and transportable cellular phones, plus cordless phones. Of these, only hand-held cellular phones have elicited widespread concern as a possible cause of brain tumors(see question 13).
To help ensure that study participants did not confuse cordless phones with hand-held cellular phones, the interviewers described distinguishing features, including that cellular phones have their own phone and billing numbers, whereas cordless phones can be on the same number as other household phones. Names of local cellular service providers were mentioned by the interviewer, as these often were different from the regular (non-cellular)vendor.
Questions were asked about whether a person had used a hand-held cellular phone never, one to five times, or more than five times in their lifetimes. Persons who reported using a hand-held cellular phone more than five times were asked about when they started and stopped using the phone. Regular users were asked about their average minutes of use per day and duration of regular use (regular use was defined as two or more calls per week). The questionnaire also asked about which hand was used to hold the phone when talking.
Information about specific make or model of phone was not collected. Knowing the specific type of phone used does not tell you how much radiationa person was exposed to, because the level of exposure to the brain depends on a variety of factors, such as distance from the nearest cellular base station (see question 11).
5. What were the major findings concerning use of hand-held cellular telephones?
There was no indication of higher brain tumor risk among persons who had used hand-held cellular phones compared to those who had not used them. More importantly, there was no evidence of increasing risk with increasing years of use or average minutes of use per day, nor did brain tumors among cellular phone users tend to occur more often than expected on the side of the head on which the person reported using their phone. Specifically, there was no indication of increased risk associated with use of a cell phone for one hour or more per day, for five or more years, or for cumulative use of more than 100 hours. These findings pertain to all three tumor types considered (glioma, meningioma, and acoustic neuroma).
6. What conclusions can be drawn from the results?
The results pertain primarily to patterns of cell phone use in the United States during the early to middle 1990s. During the period of this study, there was no evidence that use of hand-held cellular phones caused tumors of the brain and nervous system. The findings suggest that, if there was any increase in risk, it was small, particularly for malignant tumors (glioma).
Caution should be used in interpreting the findings for several reasons: (1) Widespread use of cellular telephones is a recent phenomenon, and few people used cell phones before the 1990s in the United States. This study included a small number of persons who reported using a cellphone frequently for more than five years. If there is an increased risk of brain tumors due to use of cellular phones that only appears after five or more years, or only among very heavy users, it is probable that this study would have failed to detect it. (2) This study was designed to be large enough to detect an increased risk of all types of glioma combined. The study was not large enough to detect an increased risk for different subtypes of glioma. (3) Information about duration and frequency of cellular phone use was obtained by interview and thus dependent on the ability of study participants to recall this information accurately. (4) Other factors influence the level of exposure of brain and nervous system tissue in the head to radio frequency radiation from cellular phones ,in addition to amount of use. These include distance from the cellular base station, local topography and vegetation, whether the phone is used indoors or outdoors, design of the particular model of phone, and position of the antenna and phone in relation to the head. (5) During the time period covered by our study, the overwhelming majority of cellular phones in use were analogue phones. Today, most cellular phones use are based on digital, rather than analogue, technology.
Given these limitations, it would be premature to conclude that use of hand-held cellular telephones does not cause tumors of the brain and nervous system.
7. What have other studies shown about the relationship between use of cellular telephones and brain tumors?
The literature is very limited. A recently published study by Muscat et al involved 469 brain cancer (glioma) cases and an equal number of controls. The researchers did not find an association between use of hand-held cellular telephones and the risk of gliomas. A small Swedish study by Hardell et al published in 1999 involving 233 brain tumo cases and 466 controls did not find an association between the amount of use of analogue or digital cell phones and risk of brain tumors. The investigators reported an association between side of the head on which the tumor occurred and side of phone use, but this was based on a small number of brain tumor cases.
8. Are there other studies of cell phones and brain tumors in progress?
Yes. The largest of these is a multicenter, international case-control study involving about 3,000 cases and 3,000 controls, coordinated by the International Agency for Research on Cancer (IARC), based in Lyon,France. Results are expected in several years. (http://www.iarc.fr).
A Danish study, which includes a cohort of 550,000 cellular phone subscribers from 1982 to 1995, is expected to be published soon.
A $10 million program on cell phone research was recently announced in the United Kingdom. (http://www.doh.gov.uk/mobilephones/mobilephones.htm). Also, the U.S. Food and Drug Administration (FDA)and the Cellular Telecommunications Industry Association (CTIA) have recently signed a Cooperative Research and Development Agreement (CRADA). FDA will provide scientific and technical guidance for studies that evaluate the health effects of cellular phone use. (http://www.fda.gov/Radiation-EmittingProducts/RadiationEmittingProductsandProcedures/HomeBusinessandEntertainment/CellPhones/ucm116282.htm)
9. Are there any other health risks associated with the use of cellphones?
Yes. One study by Hayes et al in 1997 involving 980 people with pacemakers reported that a cellular telephone held over the pacemaker interfered with a substantial proportion of pacemakers, in some cases leading to symptoms. However, holding the telephone against the ear caused little interference; the main risk occurs when the telephone is held against the chest.
To date, the most significant health effect for cell phone use is an increased risk of car accidents. A 1997 article in the New England Journal of Medicine showed that the risk of a collision was about four times greater when the driver was using either a hand-free telephone or holding the phone to the ear with one hand. There is also evidence from a 1999 study in the Journal of the American Medical Association (JAMA)by Dreyer et al that the heaviest users have more than twice the mortality rates from car accidents than the lightest users.
10. Might the cancer risk due to use of cellular phones be greater in children than in adults?
The NCI study included only adults (age 18 years or older at the time of brain tumor diagnosis), and few children used cell phones prior to 1994,so there are no data concerning possible risks among children. Certain agents known to cause cancer of the brain and nervous system in rats (e.g.ionizing radiation or certain chemicals) have the greatest effect when administered very early in life, while the nervous system is developing. However, the situation with cellular phones is somewhat different insofar as exposure to non-ionizing radiation from cell phones has not been demonstrated to cause brain tumors in humans or experimental animals of any age.
11. How do cellular phones work?
Cellular technology provides a two-way radio communications system between a portable handset and the nearest base-station. A given geographical region is divided into zones or cells, each of which is equipped with a base station. The base station receives radio frequency signals from cell phones and sends radio frequency signals to neighboring stationsand other cell phones. The base station is also connected to the conventional
land-line telephone network. When a call is placed from a cellular phone, a signal is sent from the antenna of the phone to the base station antenna. The base station routes the voice signal though a switching center, where the call can be transferred to another cellular telephone, another base station, or to the local land-line telephone system.
The intensity of radio frequency exposure depends on the power level of the signal which depends, in part, on the distance of the telephone from the base station. The farther the telephone is from the base station antenna, the higher the power level needed to maintain the connection. In a rural area, the cell may extend over many miles and in urban areas may cover only a fraction of a mile.
12. Did the study consider radiation exposure from cellular base station antennas?
No. Radiation levels drop off dramatically with increasing distance from base station antennas. One would have to be very close to the antennas to have much exposure. It is difficult to estimate an individuals exposure to radio frequency radiation from base stations because many persons do not know if they are living, working, or drivingclose to base station antennas, and most probably cannot estimate the distance accurately.
13. What level of radio frequency exposure is associated with the different types of wireless phones (hand-held cellular phones, car cellular phones, transportable cellular phones, and cordless phones)?
The closer the transmitter of the wireless telephone is to the body, the greater the exposure. With hand-held cellular phones, the radio transmitter is in the handset which is typically held against the sideof the head while the phone is in use. With car cellular phones the antenna is mounted on the outside of the car some distance from the user. With transportable cellular phones or bag phones the transmitter is with the battery pack in a portable unit separate from the handset. Cordless phones are not cellular phones. They have a base unit wired to the land-line telephone service and typically operate at a lower frequency and much lower power than other wireless phones. Lower power is typical of cordless phones because the radio signal from the handset only needs to reach the base unit in the home or office versus a distant base station.
If hand-held cellular phones do not cause brain tumors, it would be surprising if the other types of phones do. They involve much lower radiation exposures to the brain.
14. What have been the changes in cellular phone technology since the data were collected?
During the 1980s and early to middle 1990s, most cell phones in the United States operated in the microwave frequency range between 800and 900 MHz (megahertz: a million hertz; a hertz has a frequency of one cycle per second). There has been a trend towards use of higher frequencies. It is unknown whether cancer risk depends on the frequency. There also has been a change from phones based on analogue signal transmission to those based on digital signal transmission, and increased use of devices that keep the radio transmitter farther from the head while the phone is in use (hands-free accessories).
15. What is the difference between analogue and digital phones?
Analogue phones transmit a constant radio frequency signal and encode conversations through modulation of that signal. Digital phone stransmit information through a series of pulses of radio frequency radiation rather than through modulation of a constant radio discrete frequency signal.
At this point, there is little or no evidence that use of either type of cellular phone increases the risk of cancer. Digital phones typically operate at lower average power levels. Some people have speculated that risk might be related to the nature of modulation of the radio frequency signal rather than the power or rate of energy deposition in tissue. However, there are no epidemiologic data to support this hypothesis.
16. What is the difference between radio frequency radiation and microwave radiation?
Radio frequency radiation covers a range of frequencies (from about 10,000 Hz to 1,000,000,000,000 Hz or 1 million MHz) of which microwave radiation is one type. Cellular telephones operate in the microwave frequency range (about 800 MHz - 2,100 MHz). AM/FM radios, and VHF/UHF TVs operate at lower radio frequencies than cellular phones, while microwave ovens, radar, and satellite-stations operate at higher radio frequencies than those used for cellular phones.
17. What are some of the differences between the types of brain tumors?
Primary brain tumors are tumors that arise in the brain, unlike tumors that begin elsewhere in the body and then spread to the brain. They are classified by the type of cell in which they develop. The most common brain tumors are gliomas. Gliomas develop in the glialcells which make up the soft, spongy tissue that supports the nerve cells in the brain. There are several types of gliomas. One type, astrocytoma, arises from small, star-shaped cells called astrocytes, and can grow anywhere in the brain or spinal cord. In adults, astrocytomas most often arise in the cerebrum, the largest part of the brain that fills most of the upper skull. Glioblastoma is an especially malignant form of astrocytoma. Gliomas are more common among men. When people say brain cancer, they usually are referring to glioma.
Meningiomas are brain tumors that develop in the meninges, the protective membrane covering the brain directly underneath the skull. These tumors are usually benign and grow slowly. They occur most often in women between 30 and 50 years old.
Schwannomas are benign tumors that develop in Schwann cells which produce the myelin that protects the acoustic nerve. (Myelin is a fatty substance that covers and protects nerves.) Acoustic neuromasare a type of schwannoma that occur in the nerve between the brain and the ear. They occur primarily in adults and affect women more often than men.
Among adults, the most frequent types of brain tumors are nervous system gliomas (especially astrocytic gliomas), meningiomas, acousticneuromas, and pituitary gland tumors. Less common types include lymphomas, vascular system tumors, and pineal gland tumors.
18. How common are brain tumors and has the incidence of brain tumors changed over time?
About 17,000 cases of brain cancers are diagnosed in the United States each year. This comprises only 1 percent to 2 percent of the total number of cases of cancer diagnosed each year.
Between 1990-1997, the over-all incidence rates for brain cancer decreased by 1.4 percent (from 6.5 cases to 5.9 cases for every 100,000persons in the United States). The mortality rates from1990 to 1997 have also decreased slightly -- by 0.7 percent ( from 4.3 deaths to 4.1 for every 100,000 persons in the United States).
Looking at long-term trends for specific age groups, it appears that among persons 70 or older, there was a higher incidence of brain cancer in 1991 to 1995 compared to 1975 to 1979. This is in contrast to persons under 70, for whom the rates were similar in both periods. The most dramatic increase was in the 85 or older group, for which the rates in the most recent five-year period were nearly three times higher than in those in 1975 to 1979 (15.7 cases vs. 5.4 cases per 100,000 persons in the United States). The higher rates probably are due, at least in part, to improvements in the ability to diagnose and treat brain tumors in elderly patients. The increased use of CT (computed tomography),MRI (magnetic resonance imaging) and stereotactic biopsy procedures (more precise methods for locating and diagnosing tumors) among physicians for this population during the later time period correlates with the increased incidence trends, and represents a more recent widespread tendency of physicians to aggressively pursue diagnoses in older patients.
19. What other possible causes of brain tumors are being evaluated in the NCI study?
Other factors under consideration include workplace exposures to chemical agents and electromagnetic fields, dietary factors, family history of tumors, genetic factors, home use of selected appliances, reproductive history and hormonal exposures, viruses, medical and dental exposure to ionizing radiation, and other aspects of medical history. Results from these studies will be reported in future publications. For periodic updates, refer to the NCI Web site: http://cancer.gov.
20. Who are the principal investigators for the NCI study?
The principal investigators at NCI are Peter Inskip, Sc.D. and Martha Linet, M.D. of NCIs Division of Cancer Epidemiology and Genetics in Rockville, Md.
21. Where can I find out more about the NCI brain tumor study?
Inskip P.D., Tarone R.E., Hatch E.E., Wilcosky T.C., Shapiro W.R.,Selker R.G., Fine H.A., Black P.M., Loeffler J.S., Linet M.S. Cellular-telephone use and brain tumors. N Engl J Med 2001;344:79-86. ( http://content.nejm.org/)
Inskip P.D., Hatch E.E., Stewart P.A., Heineman E.F., ZieglerR.G., Dosemeci M., et al. Study design for a case-control investigation of cellular telephones and other risk factors for brain tumors in adults. Radiat Prot Dosim 1999;86:45-52.
22. Where can I find out more about cell phones or brain tumors in general?
Hardell L., Näsman Å., Påhlson A., HallquistA., Hansson Mild K. Use of cellular telephones and the risk for braintumours: a case-control study. Int J Oncol 1999;15:113-6.
Cardis E., Kilkenny M. International case-control study of adult brain, head and neck tumours: results of the feasibility study. Radiat Prot Dosim 1999;83:179-83.
Dreyer N.A., Loughlin J.E., Rothman K.J.. Cause-specific mortality in cellular telephone users (Research letter). JAMA 1999;17:1814-6.
Independent Expert Group on Mobile Phones. Mobile phones and health. Chilton, Didcot (United Kingdom): National Radiological Protection Board, 2000.
Web site: http://www.iegmp.org.uk.
Inskip P.D., Linet M.S., Heineman E.F. Etiology of brain tumors in adults. Epidemiol Rev 1995;17:382-414.
Moulder J.E., Erdreich L.S., Malyapa R.S., Merritt J., PickardW.F., Vijayalaxmi. Cell phones and cancer: what is the evidence for a connection? Radiat Res 1999;151:513-31.
Muscat et al . Handheld cellular telephone use not associated with risk of brain cancer. JAMA 2000; 284:3001-3007
Preston-Martin S., Mack W.J. Neoplasms of the nervous system.In: Cancer Epidemiology and Prevention. Edited by Schottenfield D.and Fraumeni JF. Oxford University Press: New York, 1996, pp.1231-81.
Rothman K.J. Epidemiological evidence on health risks of cellular telephones. Lancet 2000;356:1837-1840.
Rothman K.J., Chou C.K., Morgan R., Balzano Q., Guy A.W., FunchD.P., et al. Assessment of cellular telephone and other radio frequency exposure for epidemiologic research. Epidemiology 1996;7:291-8.
National Radiological Protection Board (NRPB) Web site: http://www.nrpb.org/.
World Health Organization International Commission on Non-Ionizing Radiation Protection (select Qs and As): http://www.who.int/emf.
Federal Communication Commission (FCC) RF Safety Program (select Information on Human Exposure to RF Fields from Cellular and PCS Radio Transmitters):
Cellular Telecommunications Industry Association (CTIA): http://www.wow-com.com.
Food and Drug Administration: http://www.fda.gov/cellphones/.
Central Brain Tumor Registry: http://www.cbtrus.org/page2t.htm