|
|
|
 |
|
| Radiation Exposure and Cancer |
|
Radiation is the emission (sending out) of energy from any source. The
light that comes from the sun is a source of radiation, as is the heat
that is constantly coming off our bodies. When talking about radiation,
however, most people think of specific kinds of radiation such as that
produced by radioactive materials or nuclear reactions. Most forms of
radiation have not been linked to cancer. Only high frequency radiation
(ionizing radiation and ultraviolet radiation) has been proven to cause
genetic damage, which can lead to cancer.
The hazards of exposure to some kinds of radiation were recognized
shortly after the discovery of the x-ray in 1895. Skin reactions were
observed in many people working with early x-ray generators, and by
1902 the first radiation-caused cancer was reported in a skin sore.
Within a few years, a large number of such skin cancers had been
observed. The first report of leukemia (a cancer of the bone marrow) in
radiation workers appeared in 1911. Marie Curie, the discoverer of
radium, and her daughter are believed to have died of radiation-caused
leukemia. Since that time, many studies have confirmed the
cancer-causing effects of some types of radiation.
Ionizing
Radiation
Radiation can generally be defined as being ionizing or non-ionizing.
Ionizing radiation consists of high-energy waves that are able to
penetrate cells and can cause ionization in different parts of the
cell. Ionization is the development of a positive charge in a molecule
(group of atoms) that is normally neutral (without a charge). Ionized
molecules are unstable and quickly undergo chemical changes. This can
lead to the formation of free radicals that can damage the molecule or
other molecules around it.
One type of molecule that is sensitive to ionizing radiation is DNA,
the part of the cell that contains the genes (blueprints) for each
person's characteristics. Ionizing radiation can lead to a mutation
(change) in a cell's DNA, which could contribute to cancer, or to the
death of the cell. All cells in the body can be damaged by ionizing
radiation. The amount of damage is related to the dose of radiation
received by the cell. While the process of cellular change from
radiation takes only a fraction of a second, other changes such as the
beginning of cancer may take years to develop.
Types of ionizing radiation include x-rays, gamma rays, cosmic rays,
and particles given off by radioactive materials such as alpha
particles, beta rays, and protons. These forms of radiation have
different energy levels and can penetrate cells to different extents,
but all are capable of causing ionization.
People may be exposed to 3 main types of ionizing radiation:
- Natural background
radiation comes from cosmic
rays from our solar
system and radioactive elements normally present in the soil. This is
the major contributor to worldwide radiation exposure.
- Non-medical synthetic
radiation occurs as a result
of above ground
nuclear weapons testing that took place before 1962 as well as
occupational and commercial sources.
- Medical radiation
comes in the form of diagnostic x-rays and other
tests, as well as from radiation therapy. Radiation therapy is
currently used to treat some types of cancer and involves dosages many
thousand times higher than those used in diagnostic x-rays.
Does
Ionizing Radiation Cause Cancer?
Ionizing radiation has been shown to induce (cause) cancer in many
different species of animals and in almost all parts of the body. It is
one of the few scientifically proven carcinogens (cancer-causing
agents) in human beings, although it appears to be a relatively weak
carcinogen compared to many chemical agents. Many years may elapse
between the radiation exposure and the appearance of the cancer.
The types of cancer that can be caused by radiation can also occur
naturally (without increased exposure to radiation), but some occur
more frequently as a result of radiation. For example, a higher
percentage of small cell lung cancers occur in uranium miners as a
result of exposure to alpha radiation (a form of highly ionizing
radiation).
Organs differ in their sensitivity to the effects of radiation. The
thyroid gland and bone marrow are most sensitive to radiation, while
the kidney, bladder, and ovary seem to be least affected. Some forms of
leukemia, a type of cancer that arises in the bone marrow, appear to be
the most common radiation-induced cancers.
Evidence that ionizing radiation causes cancer comes from studies of
atomic bomb survivors in Japan, persons exposed to large amounts of
x-rays, and from certain occupational exposures, such as workers with
lung exposure to alpha radiation. These studies, however, generally
involved relatively high-dose exposure - greater than 10 centigray. (A
centigray (cGy) is a standard unit of radiation dose.) Therefore, the
risk estimates for lower doses of radiation have to be estimated from
the high-dose data, and may not be accurate.
Dose-Related
Radiation Effects
Ionizing radiation is probably the only carcinogen with evidence that
its effects are related to dose exposure. The probability that cancer
will result from radiation exposure increases as the dose increases.
However, there is no evidence to suggest that the grade (tendency to
grow and spread) of the resulting cancer is affected by the dose. In
other words, higher doses of radiation do not cause more aggressive
cancers.
Low-Dose
Radiation Exposure
A number of studies over the past 20 years have looked at the impact of
environmental radiation exposure in the dose range of 10 cGy or less.
Careful analysis of this research revealed no significant increase in
the incidence of all cancers combined, or of cancers in specific parts
of the body. Research in this area is continuing.
Types of
Cancer Associated With Ionizing Radiation
Leukemia was at one time thought to be the major cancer to arise from
high-dose radiation exposure, based on the experience with people
exposed to the atomic blasts in Japan. It is now known that other
cancers can result from radiation exposure, although they may take
longer to develop (usually at least 10 to 15 years). Leukemias, by
contrast, begin appearing as early as 2 years after acute radiation
exposure.
Studies of the survivors of the atomic blasts have demonstrated that
high-dose radiation (at least 100cGy) increases the risk of developing
several types of cancer.
- For these survivors, the
risk of developing leukemia is five and a half
times greater than in the general public. Children appear to be twice
as sensitive as adults to the leukemia-causing effects of radiation,
and unborn children exposed to radiation in the uterus are even more
sensitive.
- The risk for developing any
type of cancer in those highly exposed to
an atomic blast is about 50% higher than the risk in those not exposed.
- Female breast cancer risk is
more than twice as high as normal, and
women who are exposed when under the age of 20 are found to be at
higher risk than older women.
- The risk of developing lung
cancer is 50% higher, and the risk for
multiple myeloma is more than twice as high as in the general
population.
Some people serving in the armed forces were exposed to radiation in
nuclear weapons testing during the Cold War era. Information on this
topic is available in our document, "Cancer Among Military Personnel
Exposed to Nuclear Weapons."
Cancers
Caused by Radiation Therapy
Ionizing radiation is an effective way to treat certain kinds of
cancer. During radiation therapy, high doses of ionizing radiation are
directed at the cancer, resulting in the death of the cancer cells.
However, this can lead to DNA mutations in cells that survive the
radiation, which can eventually lead to the development of another
cancer (called a second primary cancer).
An increase in second primary tumors in the area being irradiated has
been observed in patients with several types of cancer following
radiation therapy and/or chemotherapy. Some studies have associated
radiation therapy with an increased incidence of thyroid cancer and
early-onset breast cancer. Overall, however, radiation alone does not
appear to be a very potent cancer-causing agent in second tumors. This
is probably due to the fact that it is often used in a localized area,
which means fewer normal cells are exposed to radiation.
However, treatment for Hodgkin disease, a type of lymphoma, often
delivers lower radiation doses to many areas of the body. These treated
areas include large amounts of normal tissue. Patients with Hodgkin
disease who are treated with radiation therapy are at an increased risk
for developing second primary tumors.
When considering radiation exposure from radiation therapy treatment,
the benefits generally outweigh the risks. However, some combinations
of radiation therapy and chemotherapy are more risky than others.
Additional research is needed in this area so that optimal treatment
can be given that minimizes the risk of the development of secondary
cancers.
Genetic
Susceptibility to Radiation-Caused Cancer
Scientists are making great progress in understanding how certain
changes in DNA can cause cells to become cancerous. DNA is the chemical
that carries the instructions for nearly everything our cells do. We
usually resemble our parents because they are the source of our DNA.
However, DNA affects more than our outward appearance.
Some genes (parts of DNA) contain instructions for controlling when our
cells grow, divide, and die. Certain genes that promote cell division
are called oncogenes. Others that slow down cell division, or cause
cells to die at the right time, are called tumor suppressor genes. It
is known that cancers can be caused by DNA mutations (changes) that
"turn on" oncogenes or "turn off" tumor suppressor genes. Certain
inherited DNA changes can lead to a high risk for developing cancer and
are responsible for the cancers that run in some families.
Now that specific genes have been discovered that are associated with
cancer, there is renewed interest in whether some people are more
likely to develop cancers when exposed to radiation. There are some
rare cancers where the interaction between radiation and genetic
factors can be seen. Children treated with radiation therapy for
hereditary retinoblastoma, a malignant eye tumor, are at an increased
risk for developing a certain type of bone cancer called an
osteosarcoma. Similarly, people who have nevoid basal cell carcinoma
syndrome, a type of skin cancer, are at high risk for development of
basal cell cancers in irradiated areas.
Non-Ionizing
Radiation
Non-ionizing radiation is low-frequency radiation that does not have
enough energy to cause ionization in tissues, but may cause adverse
health consequences in other ways. Common types of non-ionizing
radiation include ultraviolet rays, visible light, electromagnetic
fields, infrared radiation, microwaves, and radiofrequency radiation
(radio waves). Electrical appliances, heaters, and cellular phones emit
(send out) non-ionizing radiation waves. Of all the types of
non-ionizing radiation, only ultraviolet rays have been established as
a cancer-causing agent.
Ultraviolet
(UV) Radiation
The sun is the major source of ultraviolet (UV) radiation. Most skin
cancers are a direct result of sunlight exposure. Both basal cell and
squamous cell cancers (the most common types of skin cancer) are found
on sun-exposed parts of the body, and their occurrence is related to
lifetime sun exposure. Melanoma, a potentially fatal type of skin
cancer, is less dependent on sun exposure and can develop on any area
of the body.
If you live in the mid-United States, being in direct sunlight for 30
minutes creates a buildup of a lethal dose of UVR for human cells not
protected by sunscreen. The only other exposure to a carcinogen
approaching this level of exposure is cigarette smoke in very heavy
smokers.
Ultraviolet radiation is divided into three wavelength ranges:
- UVA rays are involved in the
aging of cells and produce some damage to
DNA.
- UVB rays are in the
wavelength range mainly responsible for direct
damage to the DNA, and are thought to cause most skin cancers.
- UVC rays are not present in
sunlight, but are present in mercury lamps.
While UVA and UVB rays make up only 1/10,000,000th of the sun's
wavelengths, they are primarily responsible for the damaging effects of
the sun on the skin.
More information on ultraviolet radiation is available in our
document "Skin Cancer: Prevention and Early Detection."
Electromagnetic
Fields
Electromagnetic radiation is produced by moving electric charges and
may be of natural origin (the sun) or human origin (electronic devices
or power lines). Electromagnetic fields have been the subject of much
controversy. Recent extensive studies of electric utility workers
showed a minimal increase in the risk of brain tumors and leukemia.
However, either of these increases may have been due to chance. Results
from studies on magnetic fields and childhood leukemia have been
suggestive but inconsistent.
While smaller studies have observed a link between cancer and
activities such as the use of electric blankets and television
watching, the most recent and largest study did not find a connection
between electromagnetic fields and cancer.
Additionally, in 1999 the National Institute of Environmental Health
Sciences (NIEHS) released results of an extensive 6-year study stating
that the evidence for a risk of cancer and other human disease from the
electric and magnetic fields (EMF) around power lines is "weak" but
could not totally be discounted, and efforts to reduce exposures when
possible should continue.
Video
display terminals: Video
display terminals (VDTs), or computer
screens, give off several kinds of radiation, most of which is in the
extremely low frequency (ELF) range. Questions have been raised about
possible health problems associated with the use of VDTs, including
cancer and birth defects.
The amount of energy given off by VDTs is far below government exposure
standards, and at this time the available evidence does not support
links to either of these health problems. Research in this area
continues.
Other health-related issues linked to long-term use of VDTs may be of
greater concern, including problems with vision, backaches and other
muscle problems, and stress.
The conflicting data concerning electromagnetic fields will undoubtedly
continue to generate controversy. Clearly, the question of whether or
not electromagnetic fields can cause cancer needs to be answered.
Other
Sources of Non-Ionizing Radiation
Radiofrequency
radiation (radio waves):
Radiofrequency radiation is
emitted from radio broadcast transmitters, citizen band radios, and
heaters. Generally speaking, the potential risk from this type of
radiation exposure is thought to be minimal.
Microwaves:
Microwaves have energy levels similar to radio waves and
infrared waves but are of a different frequency. Microwave radiation is
used not only in microwave ovens, but also in navigational technology
such as radar.
Many materials absorb microwave energy, causing them to become hot.
This is how food is cooked in microwave ovens. Microwaves do not make
food radioactive. When microwave ovens are used according to
instructions, there is no evidence that they pose a risk of radiation
exposure to people.
Exposure to high levels of microwaves can have effects on health. Such
exposure can lead to a painful burn or to the development of cataracts
in the lenses of the eyes. Because the testes are very sensitive to
changes in temperature, exposure to high levels of microwaves can alter
or kill sperm. These injuries are caused only by exposure to large
amounts of microwave radiation, however, and the small amount that can
leak from a microwave oven does not cause these problems.
Some pacemakers can be affected if the person with the pacemaker gets
too close to a microwave oven while it is on. This can happen with
other kinds of electronic equipment as well.
Radar
and radar guns: Most forms of
radar use waves in the microwave
range. Questions have been raised about exposure to radar and the risk
of developing cancer, such as in police officers who use radar guns in
traffic enforcement. To date there is very little evidence to support
such a connection, but studies to look at this possibility are ongoing,
and governmental recommendations have been made to reduce any possible
risk.
Cellular
phones: Cellular phones give
off small amounts of low
frequency electromagnetic radiation. People have raised questions about
possible links between the use of cellular phones and cancer, brain
cancer in particular. For more detailed information about cellular
phones, refer to our document, "Cellular Phones."
Additional
Resources
National
Organizations and Web Sites*
In addition to the American Cancer Society, other sources of patient
information and support include:
Centers for Disease Control and Prevention (CDC)
1600 Clifton Road, NE
Atlanta, GA 30333
Telephone: 1-800-CDC-INFO (1-800-232-4636)
Internet Address: www.cdc.gov
*Inclusion
on this list does not imply endorsement by the American
Cancer Society.
The American Cancer Society is happy to address almost any
cancer-related topic. If you have any more questions, please call us at
1-800-ACS-2345 at any time, 24 hours a day.
References
Boelsen R, Jamar S. Advances in Radiation Oncology. Oncology Nursing
Updates: Patient Treatment and Support
2000; 7: 1-11.
Cleaver JE, Mitchell DL. Ultraviolet Radiation and Carcinogenesis. In:
Kufe DF, Pollock RE, Weischelbaum RR, et al., eds. Cancer Medicine
e.6. Hamilton, London: B.C.
Decker, Inc. 2003: 303-312.
Environment, Safety and Health Manual. Available at:
http://www.llnl.gov/es_and_h/esh-manual.html. Accessed February 6, 2006.
Health Effects from Exposure to Power-Line Frequency Electric and
Magnetic Fields. National Institute of Environmental Health Sciences;
1999. Available at:
www.niehs.nih.gov/emfrapid/html/EMF_DIR_RPT/Report_18f.htm. Accessed
February 6, 2006
.
Little JB. Ionizing Radiation. In: Kufe DF, Pollock RE,
Weischelbaum RR, et al., eds. Cancer Medicine e.6.
Hamilton,
London: B.C. Decker, Inc. 2003: 289-302.
Microwave Oven Radiation. 2000. Food and Drug Administration Center for
Devices and Radiological Health. Available at:
www.fda.gov/cdrh/consumer/microwave.html. Accessed February 6, 2006.
Mundt AJ, Roeske JC, Chung TD, Weischelbaum RR. Principles of radiation
oncology. In: Kufe DF, Pollock RE, Weischelbaum RR, et al., eds. Cancer
Medicine e.6. Hamilton,
London: B.C. Decker, Inc. 2003:
585-604.
Occupational Exposure of Police Officers to Microwave Radiation From
Traffic Radar Devices. National Institute for Occupational Safety and
Health; June 1995. Available at:
www.osha-slc.gov/SLTC/radiofrequencyradiation/fnradpub.html.
www.osha-slc.gov/SLTC/radiofrequencyradiation/fnradpub.html
Last
Revised: 02/06/2006
|
|
|
|
|
