What Causes Cancer?
Cancer is a complex group of diseases with many possible causes. In this section you can learn more about the known causes of cancer, including genetic factors; lifestyle factors such as tobacco use, diet, and physical activity; certain types of infections; and environmental exposures to different types of chemicals and radiation.
Genetics and Cancer
Some types of cancer run in certain families, but most cancers are not clearly linked to the genes we inherit from our parents. In this section you can learn more about the complex links between genes and cancer, as well as genetic testing and how it is used.
Heredity and Cancer
Cancer is such a common disease that it is no surprise that many families have at least a few members who have had cancer. Sometimes, certain types of cancer seem to run in some families
What kinds of illness and death are caused by smoking cigarettes?
About half of people who keep smoking will die because of the habit. Each year about 480,000 people in the United States die from illnesses related to tobacco use. Smoking cigarettes kills more Americans than alcohol, car accidents, suicide, AIDS, homicide, and illegal drugs combined.
Medical radiation
X-rays, gamma rays, and other forms of ionizing radiation are used to diagnose and treat some medical conditions. This can be in the form of radiation that penetrates from outside the body, or radioactive particles that are swallowed or inserted into the body.
Imaging tests
Certain types of imaging tests, such as x-rays, CT scans, and nuclear medicine tests (such as PET scans and bone scans) expose people to low levels of radiation in order to create internal pictures of the body. (Some imaging tests, such as MRI and ultrasound do not expose patients to ionizing radiation.)
In adults: The amount of radiation varies depending on the test. For example, the exposure from a 2-view chest x-ray is about 0.1 mSv, while exposure from a regular chest CT is about 7 or 8 mSv. The exposure from a PET/CT scan (which combines a PET scan of the body with a CT scan) can be as high as 30 mSv. Fluoroscopy, which uses x-rays to make real-time moving images, is like getting many x-rays in a row. It exposes people to different amounts of radiation depending on how long it is used. The amount of radiation used in many imaging tests has gone down over time as technology has improved.
In children: Radiation exposure also varies based on the test. Unless the settings on the scanner are adjusted for body size, exposure levels can be higher than they would be for an adult. For example, one study found that an abdominal CT may expose an adult’s stomach to about 10 mSv, while a newborn baby’s stomach would be exposed to 20mSv by getting the same test.
For children, exposure to radiation from imaging tests is of particular concern, because:
These factors mean that for a young child, the risk of developing a radiation-related cancer could be several times higher than for an adult exposed to the same imaging test. The risks from these tests are not known for sure, but to be safe, most doctors recommend that children only get these tests when they are absolutely needed. When such tests are done, it is important to use the minimum amount of radiation needed to get the image.
Radiation therapy
X-rays, gamma rays, and other forms of ionizing radiation offer an effective way to treat certain kinds of cancer. During radiation therapy, high doses of ionizing radiation (much higher than those used for imaging tests) are directed at the cancer, resulting in the death of the cancer cells. However, this can lead to DNA mutations in other cells that survive the radiation, which may eventually lead to the development of a second cancer. Radiation therapy is also sometimes used to treat serious medical conditions besides cancer.
Agent Orange and Cancer
About 3 million Americans served in the armed forces in Vietnam and nearby areas during the 1960s and early 1970s, the time of the Vietnam War. During that time, the military used large amounts of mixtures known as defoliants, which are chemicals that cause the leaves to fall off plants. One of these defoliants was Agent Orange, and some troops were exposed to it. Many years later, questions remain about the lasting health effects of those exposures, including increases in cancer risk.
This article offers a brief overview of the link between Agent Orange and cancer. It does not offer a complete review of all evidence – it is meant to be a brief summary. It also includes some information on benefits for which Vietnam veterans exposed to Agent Orange may be eligible.
Some background on Agent Orange
During the Vietnam War, US military forces sprayed millions of gallons of herbicides (plant-killing chemicals) on lands in Vietnam, Laos, and other nearby areas to remove forest cover, destroy crops, and clear vegetation from the perimeters of US bases. This effort, known as Operation Ranch Hand, lasted from 1962 to 1971.
Different mixes of herbicides were used, but most were mixtures of 2 chemicals that were phenoxy herbicides:
Each mixture was shipped in a chemical drum marked with an identifying colored stripe. The most widely used mixture contained equal parts 2,4-D and 2,4,5-T. Because this herbicide came in drums with orange stripes, it was called Agent Orange. Today, Agent Orange refers generally to all the phenoxy herbicides sprayed at the time. (Other types of herbicides were also used, including cacodylic acid and picloram.)
The 2,4,5-T in Agent Orange was contaminated with small amounts of dioxins, which were created unintentionally during the manufacturing process. Dioxins are a family of dozens of related chemicals. They can form during the making of paper and in some other industrial processes. The main dioxin in Agent Orange, 2,3,7,8-tetrachlorodibenzo-p-dioxin, or TCDD, is one of the most toxic.
After a study in 1970 found that 2,4,5-T could cause birth defects in lab animals, the use of 2,4,5-T in Vietnam was stopped. A year later, all military herbicide use in Vietnam ended. During the 1970s, some veterans returning from Vietnam began to report skin rashes, cancer, psychological symptoms, birth defects in their children, and other health problems. Some veterans were concerned that Agent Orange exposure might have contributed to these problems. These concerns eventually led to a series of scientific studies, health care programs, and compensation programs directed to the exposed veterans.
A large class-action lawsuit was filed in 1979 against the herbicide manufacturers, and was settled out of court in 1984. It resulted in the Agent Orange Settlement Fund, which distributed nearly $200 million to veterans between 1988 and 1996.
Although there is now quite a bit of evidence about the health effects of Agent Orange, many questions have not yet been answered.
How were people exposed to Agent Orange?
About 3 million people served in the US military in Vietnam during the course of the war, about 1.5 million of whom served during the period of heaviest herbicide spraying from 1967 to 1969.
In studies comparing Vietnam veterans with veterans who had served at the same time elsewhere, blood TCDD (dioxin) levels were found to be higher among those who had served in Vietnam, although these levels went down slowly over time.
Exposure to Agent Orange varied a great deal. Most of the large-scale spraying in Operation Ranch Hand was done with airplanes and helicopters. However, some herbicides were sprayed from boats or trucks, and some were applied by soldiers with backpack sprayers. Those who loaded airplanes and helicopters might have been exposed the most. Members of the Army Chemical Corps, who stored and mixed herbicides and defoliated the perimeters of military bases, probably also had some of the heaviest exposures. Others with potentially heavy exposures included members of Special Forces units who defoliated remote campsites, and members of Navy river units who cleared base perimeters.
Exposures could have occurred when the chemicals were breathed in, ingested in contaminated food or drink, or absorbed through the skin. Exposure may have been possible through the eyes or through breaks in the skin, as well.
One of the challenges in assessing the health effects of Agent Orange exposure has been determining how much any individual veteran was exposed to (or even what they were exposed to), as very little information of this type is available.
Does Agent Orange cause cancer?
Researchers use 2 main types of studies to try to determine if a substance or exposure causes cancer.
One type of study looks at cancer rates in different groups of people. Such a study might compare the cancer rate in a group exposed to a substance versus the rate in a group not exposed to it, or compare it to what the expected cancer rate would be in the general population. But studies of people can sometimes be hard to interpret, because other factors that are hard to account for might be affecting the results.
In studies done in the lab, animals are exposed to a substance (often in very large doses) to see if it causes tumors or other health problems. Researchers may also expose normal cells in a lab dish to the substance to see if it causes the types of changes that are seen in cancer cells. In these types of studies, other factors are easier to control for, but it’s not always clear if the results in lab dishes or animals would be the same in humans, for a number of reasons.
In most cases neither type of study provides definitive evidence on its own, so researchers usually look at both human and lab-based studies when trying to determine if something might cause cancer.
Source: cancer.org
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