Cancer: What it is, Causes, Risk Factors, Types and More!

Written by Jeff Behar

Defining Cancer

Cancer is the general name for a group of more than 100 diseases in which cells in a part of the body begin to grow out of control. Although there are many kinds of cancer, they all start because abnormal cells grow out of control and are able to invade other tissues. Cancer cells can spread to other parts of the body through the blood and lymph systems.

Most cancers are named for the organ or type of cell in which they start - for example, cancer that begins in the colon is called colon cancer; cancer that begins in basal cells of the skin is called basal cell carcinoma.

Cancer types can be grouped into broader categories. The main categories of cancer include:

Carcinoma - a cancer which is derived from the lining cells, or epithelium, of an organ. There are 4 major types of epithelium in the body (Glandular, squamous, transitional, and pseudostratified). Some types are only found in a few select organs such as the lung (pseudostratified) or urinary bladder (transitional). Carcinomas can arise from any of these epithelial types. For example, breast carcinoma is most commonly derived from the lining cells of the milk producing glands. A carcinoma with a glandular growth pattern is an adenocarcinoma. Common adenocarcinomas include prostate, colon, and breast. A carcinoma with a growth pattern resembling the squamous lining cells is termed a squamous cell carcinoma. Common squamous cell carcinomas are found in the esophagus and skin. However, any of these organs may have either type of carcinoma arising from it, although these latter diagnoses are exceedingly rare.
Central nervous system cancers - cancers that begin in the tissues of the brain and spinal cord.
Leukemia - cancer that starts in blood-forming tissue such as the bone marrow and causes large numbers of abnormal blood cells to be produced and enter the blood.
Lymphoma - a cancer derived from the white blood cells that are present in the lymphoid tissues of the body. These sites most commonly include the lymph nodes and spleen. However, lymphomas may arise from any organ and body site.
Sarcoma - cancer that begins in bone, cartilage, fat, muscle, blood vessels, or other connective or supportive tissue.

How common is cancer?

Today, millions of people are living with cancer or have had cancer. The risk of developing most types of cancer can be reduced by changes in a person's lifestyle, for example, by quitting smoking, limiting time in the sun, being physically active, and eating a better diet.

Half of all men and one-third of all women in the US will develop cancer during their lifetimes.

Risk Factors
Doctors often cannot explain why one person develops cancer and another does not. But research shows that certain risk factors increase the chance that a person will develop cancer. These are the most common risk factors for cancer:

Growing Older
Family history of cancer
Poor diet, lack of physical activity, or being overweight
Tobacco
Sunlight
Ionizing radiation
Certain chemicals and other substances
Some viruses and bacteria
Certain hormones
Alcohol

Many of these risk factors can be avoided. Others, such as family history, cannot be avoided. People can help protect themselves by staying away from known risk factors whenever possible.

If you think you may be at risk for cancer, you should discuss this concern with your doctor. You may want to ask about reducing your risk and about a schedule for checkups.

Over time, several factors may act together to cause normal cells to become cancerous. When thinking about your risk of getting cancer, these are some things to keep in mind:

Not everything causes cancer.
Cancer is not caused by an injury, such as a bump or bruise.
Cancer is not contagious. Although being infected with certain viruses or bacteria may increase the risk of some types of cancer, no one can "catch" cancer from another person.
Having one or more risk factors does not mean that you will get cancer. Most people who have risk factors never develop cancer.
Some people are more sensitive than others to the known risk factors.

The sections below have more detailed information about the most common risk factors for cancer.

Growing Older

The most important risk factor for cancer is growing older. Most cancers occur in people over the age of 65. But people of all ages, including children, can get cancer, too.

Family History of Cancer

Most cancers develop because of changes (mutations) in genes. A normal cell may become a cancer cell after a series of gene changes occur. Tobacco use, certain viruses, or other factors in a person's lifestyle or environment can cause such changes in certain types of cells.

Some gene changes that increase the risk of cancer are passed from parent to child. These changes are present at birth in all cells of the body.

It is uncommon for cancer to run in a family. However, certain types of cancer do occur more often in some families than in the rest of the population. For example, melanoma and cancers of the breast, ovary, prostate, and colon sometimes run in families. Several cases of the same cancer type in a family may be linked to inherited gene changes, which may increase the chance of developing cancers. However, environmental factors may also be involved. Most of the time, multiple cases of cancer in a family are just a matter of chance.

Poor Diet, Lack of Physical Activity, or Being Overweight

People who have a poor diet, do not have enough physical activity, or are overweight may be at increased risk of several types of cancer. For example, studies suggest that people whose diet is high in fat have an increased risk of cancers of the colon, uterus, and prostate. Lack of physical activity and being overweight are risk factors for cancers of the breast, colon, esophagus, kidney, and uterus.

Having a healthy diet, being physically active, and maintaining a healthy weight may help reduce cancer risk. Doctors suggest the following:

Eat well: A healthy diet includes plenty of foods that are high in fiber, vitamins, and minerals. This includes whole-grain breads and cereals and 5 to 9 servings of fruits and vegetables every day. Also, a healthy diet means limiting foods high in fat (such as butter, whole milk, fried foods, and red meat).
Be active and maintain a healthy weight: Physical activity can help control your weight and reduce body fat. Most scientists agree that it is a good idea for an adult to have moderate physical activity (such as brisk walking) for at least 30 minutes on 5 or more days each week.

Tobacco

Tobacco use is the most preventable cause of death. Each year, more than 180,000 Americans die from cancer that is related to tobacco use.

Using tobacco products or regularly being around tobacco smoke (environmental or secondhand smoke) increases the risk of cancer.

Smokers are more likely than nonsmokers to develop cancer of the lung, larynx (voice box), mouth, esophagus, bladder, kidney, throat, stomach, pancreas, or cervix. They also are more likely to develop acute myeloid leukemia (cancer that starts in blood cells).

People who use smokeless tobacco (snuff or chewing tobacco) are at increased risk of cancer of the mouth.

The risk of cancer for people who quit is lower than the risk for people who continue to use tobacco. (But the risk of cancer is generally lowest among those who never used tobacco.)

Sunlight

Ultraviolet (UV) radiation comes from the sun, sunlamps, and tanning booths. It causes early aging of the skin and skin damage that can lead to skin cancer.

Doctors encourage people of all ages to limit their time in the sun and to avoid other sources of UV radiation:

Use sunscreen. Sunscreen may help prevent skin cancer, especially sunscreen with a sun protection factor (SPF) of at least 15. But sunscreens cannot replace avoiding the sun and wearing clothing to protect the skin.
Stay away from sunlamps and tanning booths. They are no safer than sunlight.
It is best to avoid the midday sun (from mid-morning to late afternoon) whenever possible. You also should protect yourself from UV radiation reflected by sand, water, snow, and ice. UV radiation can penetrate light clothing, windshields, and windows.
Wear long sleeves, long pants, a hat with a wide brim, and sunglasses with lenses that absorb UV

Ionizing Radiation

Ionizing radiation can cause cell damage that leads to cancer. This kind of radiation comes from rays that enter the Earth's atmosphere from outer space, radioactive fallout, radon gas, x-rays, and other sources.

Certain Chemicals and Other Substances

People who have certain jobs (such as painters, construction workers, and those in the chemical industry) have an increased risk of cancer. Many studies have shown that exposure to asbestos, benzene, benzidine, cadmium, nickel, or vinyl chloride in the workplace can cause cancer.

Some Viruses and Bacteria

Being infected with certain viruses or bacteria may increase the risk of developing cancer:

Human papillomaviruses (HPVs): HPV infection is the main cause of cervical cancer. It also may be a risk factor for other types of cancer.
Hepatitis B and hepatitis C viruses: Liver cancer can develop after many years of infection with hepatitis B or hepatitis C.
Human T-cell leukemia/lymphoma virus (HTLV-1): Infection with HTLV-1 increases a person's risk of lymphoma and leukemia.
Human immunodeficiency virus (HIV): HIV is the virus that causes AIDS. People who have HIV infection are at greater risk of cancer, such as lymphoma and a rare cancer called Kaposi's sarcoma.
Epstein-Barr virus (EBV): Infection with EBV has been linked to an increased risk of lymphoma.
Human herpesvirus 8 (HHV8): This virus is a risk factor for Kaposi's sarcoma.
Helicobacter pylori : This bacterium can cause stomach ulcers. It also can cause stomach cancer and lymphoma in the stomach lining.

Certain Hormones

Doctors may recommend hormones (estrogen alone or estrogen along with progestin) to help control problems (such as hot flashes, vaginal dryness, and thinning bones) that may occur during menopause. However, studies show that menopausal hormone therapy can cause serious side effects. Hormones may increase the risk of breast cancer, heart attack, stroke, or blood clots.

Diethylstilbestrol (DES), a form of estrogen, was given to some pregnant women in the United States between about 1940 and 1971. Women who took DES during pregnancy may have a slightly higher risk of developing breast cancer. Their daughters have an increased risk of developing a rare type of cancer of the cervix. The possible effects on their sons are under study.

A woman considering menopausal hormone therapy should discuss the possible risks and benefits with her doctor.

Alcohol

Having more than two drinks each day for many years may increase the chance of developing cancers of the mouth, throat, esophagus, larynx, liver, and breast. The risk increases with the amount of alcohol that a person drinks. For most of these cancers, the risk is higher for a drinker who uses tobacco.

Doctors advise people who drink to do so in moderation. Drinking in moderation means no more than one drink per day for women and no more than two drinks per day for men.

Common Cancers
Cancers that are diagnosed with the greatest frequency in the United States include:

Bladder Cancer		Melanoma
Breast Cancer		Non-Hodgkin Lymphoma
Colorectal Cancer		Pancreatic Cancer
Endometrial Cancer		Prostate Cancer
Kidney Cancer		Skin Cancer (Nonmelanoma)
Leukemia		Thyroid Cancer
Lung Cancer

Other types of Cancers

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