Cancer

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Photo by: Arto

Cancer is a disease characterized by the uncontrolled growth and spread of abnormal cells. Around the world, over 10 million cancer cases occur annually. Half of all men and one-third of all women in the United States will develop some form of cancer during their lifetime. It is one of the most feared diseases, primarily because half of those diagnosed with cancer in the United States will die from it. Cancer is a leading cause of death around the world, causing over 6 million deaths a year. The exact causes of most types of cancer are still not known, and there is not yet a cure for cancer. However, it is now known that the risk of developing many types of cancer can be reduced by adopting certain lifestyle changes, such as quitting smoking and eating a better diet .

Prevalence

Cancer is, in general, more common in industrialized nations, but there has been a growth in cancer rates in developing countries, particularly as these nations adopt the diet and lifestyle habits of industrialized countries. Over one million people in the United States get cancer each year. Anyone can get cancer at any age; however, about 80 percent of all cancers occur in people over the age of fifty-five.

Cancer can affect any site in the body. About one hundred human cancers are recognized. The four most common cancers in the United States are: lung, colon/rectum, breast, and prostate . Together, these cancers account for over 50 percent of total cancer cases in the United States each year.

There is a marked variation among countries in incidence of different cancers. Most of the variation in cancer risk among populations, and among individuals, is due to environmental factors, such as cigarette smoking and certain dietary patterns, that can affect one's risk of developing cancer. For example, individuals living in Australia have the highest worldwide lifetime risk of skin cancer, at over 20 percent, due to the high level of exposure to the sun of people in Australia. People in India have twenty-five times the average risk of developing oral cancer sometime during their lives due to the popularity of chewing tobacco in that country. In fact, India has the world's highest incidence of oral cancer, with 75,000 to 80,000 new cases a year. The population of Japan has the highest rates of stomach cancer in the world due to the high consumption of raw fish by the Japanese.

Types of Cancer

Cancers are classified according to the types of cells in which they develop. Most human cancers are carcinomas, which arise from the epithelial cells that form the superficial layer of the skin and some internal organs. Leukemias affect the blood and blood-forming organs such as bone marrow , the lymphatic system , and the spleen. Lymphomas affect the immune system . Sarcoma is a general term for any cancer arising from muscle cells or connective tissues.

Growth and Spread of Cancer

Cancer develops when cells in a particular part of the body begin to grow out of control. Normal body cells grow, divide, and die in an orderly way. Cancer cells, however, continue to grow and divide without dying. Instead, they outlive normal cells and continue to form new abnormal cancer cells. As most cancer cells continue to grow, they lump together and form an extra mass of tissue. This mass is called a malignant tumor.

As a malignant tumor grows, it damages nearby tissue. Some cancers, like leukemia, do not form tumors. Instead, these cancer cells involve the blood and blood-forming organs and circulate through other tissues, where they grow.

Cancer can begin in one part of the body and spread to others. The spread of a tumor to a new site is called metastasis. This process occurs as cancer cells break away from a tumor and travel through the bloodstream or the lymph system to other areas of the body. Once in a new location, cancer cells continue to grow out of control and form a new malignant tumor.

An image showing the division of cancer cells (left and right) and two healthy white blood cells (above and below). In normal cells, cell division is balanced by cell death, but cancerous cells continue to divide and accumulate, damaging nearby tissues.

[Nibsc/Photo Researchers, Inc. Reproduced by permission.]

Causes of Cancer

The exact cause of cancer is not known. Most cancers result from permanent damage to genes or from mutations, which occur either due to internal factors, such as hormones , immune conditions, metabolism , and the digestion of nutrients within cells, or by exposure to environmental or external factors. A chemical or other environmental agent that produces cancer is called a carcinogen.

Overall, environmental factors, defined broadly to include tobacco use, diet, infectious diseases , chemicals, and radiation, are believed to cause between 75 and 80 percent of all cancer cases in the United States. Tobacco use, including cigarettes, cigars, chewing tobacco, and snuff, can cause cancers of the lung, mouth, throat, larynx, bladder, kidney, esophagus, and pancreas. Smoking alone causes one-third of all cancer deaths in the United States. Heavy consumption of alcohol has also been shown to increase the risk of developing cancer of the mouth, pharynx, larynx, esophagus, liver, and breast.

Overweight and obesity are associated with increased risk of cancers of the breast, colon, endometrium, esophagus, kidney, and gallbladder. The following chemicals have been found to cause cancer: coal tars and their derivatives, such as benzene; some hydrocarbons; aniline, a substance used to make dyes; and asbestos. Radiation from a variety of sources, including the ultraviolet light from the sun, is known to lead to skin cancer.

Several infectious agents have also been implicated in cancer. Evidence suggests that chronic viral infections are associated with up to one-fifth of all cancers. These include hepatitis B virus (HBV), which can lead to cancer of the liver; the Epstein-Barr virus, a type of herpes virus that causes infectious mononucleosis and has been associated with Hodgkin's disease, non-Hodgkin's lymphomas, and nasopharyngeal cancer; the human immunodeficiency virus (HIV), which is associated with an increased risk of developing several cancers, especially Kaposi's sarcoma and non-Hodgkin's

Tobacco use is a major cause of lung, lip, mouth, larynx, and throat cancer, and is a contributing cause of many other cancers. In India, where this photo was taken, the prevalence of tobacco use among students approaches 60 percent in some states.

[© AFP/Corbis. Reproduced by permission.]

About 5 to 10 percent of cancers are hereditary, in that a faulty gene or damaged DNA that has been inherited predisposes a person to be at a very high risk of developing a particular cancer. Two genes, BRCA1 and BRCA2, have been found to cause some breast cancers. Other genes have been discovered that are associated with some cancers that run in families, such as cancers of the colon, rectum, kidney, ovary, esophagus, lymph nodes , skin melanoma, and pancreas.

Carcinogenesis Process

All cancers involve the malfunction of genes that control cell growth and division. The process by which cancers develop is called carcinogenesis. This process usually starts when chemicals or radiation damage DNA, the genetic structure inside cells. Viruses induce carcinogenesis by introducing new DNA sequences. Most of the time, when DNA becomes damaged the body is able to repair it. In cancer cells, however, the damaged DNA is not repaired. While normal cells with damaged DNA die, cancer cells with damaged DNA continue to multiply.

There is a long time lag between exposure to a carcinogen and the occurrence of cancer. While cellular mutations cause cancer to develop, it is not exactly clear how this happens. Carcinogenesis is a multistep process, in which as many as ten distinct mutations may have to accumulate in a cell before it becomes cancerous. The fact that so many mutations are needed for a cancer to develop indicates that cell growth is normally controlled through many sets of checks and balances.

The cell cycle is regulated by a large number of cellular genes that are expressed, or exhibited, at different stages of the cycle. The genes code for, or determine, growth factors , growth-factor receptors, and proteins that control gene functions and cell survival. Damaged DNA can lead to cancer because the cell cycle is distorted by the alteration and activation of oncogenes, genes that stimulate cell growth, or by the inactivation of tumor suppressor genes, which ordinarily suppress cell growth. Activated oncogenes drive abnormal, unregulated cell proliferation and lead to tumor formation. Mutations of the tumor suppressor gene p53 are found in about 50 percent of human cancers.

In experimental animals, three stages of chemical carcinogenesis have been identified. These are: (1) initiation, where DNA is irreversibly altered; (2) promotion, which is the multiplication of altered cells; and (3) progression, which involves chromosomal changes, high growth rate, invasiveness, and potential to metastasize.

Prevention

All cancers caused by cigarette smoking and heavy use of alcohol could be prevented completely. Approximately 30 percent of all cancers worldwide are due to tobacco use. Many of the skin cancers could be prevented by protection from sunlight. Certain cancers that are related to infectious exposures, such as HBV, HPV, HIV, and Helicobacter could be prevented through behavioral changes, vaccines , or antibiotics . Research shows that about 30 to 40 percent of all cancers worldwide are due to dietary factors and lack of physical activity, including obesity, and could therefore have been prevented. By making changes in regard to diet, exercise, healthy weight maintenance, and tobacco use, the incidence of cancer around the world could be reduced by 60 to 70 percent.

The Relationship between Diet, Physical Activity, and Cancer

While the exact mechanisms by which diet is related to cancer have not been completely understood, research has shown that food plays a role in cancer prevention. For example, populations whose diet includes at least five servings of fruits and vegetables a day have lower rates of some of the most common cancers. Fruits and vegetables contain many antioxidants and phytochemicals , such as vitamins A, C, and E, and beta-carotene, which have been shown to prevent cancer. It is not completely clear, however, whether it is individual phytochemicals, or a combination of them, or the fiber in fruits and vegetables that result in reduced risk of cancer.

Studies have shown the risk of prostate cancer drops for men who eat tomato products, possibly because of the phytochemical lycopene. In addition, it has been shown that colon cancer declines among those who drink green tea, which contains antioxidants and phytochemicals, and who regularly eat soy products and foods rich in selenium, an antioxidant.

Those who eat a diet low in fat , especially animal fat, also have lower cancer rates, but again it is not clear whether it is the calories , the amount and distribution of body fat, or the likelihood that a low-fat diet is high in fiber, fruits, and vegetables that is protective against cancer. High-fiber diets are thought to reduce the risk of colon cancer because the fiber helps move food through the lower digestive tract, possibly reducing the contact of any carcinogens with the bowel lining.

Scientific evidence indicates that physical activity may reduce the risk of certain cancers. This effect may be due to the fact that physical activity is associated with the maintenance of a healthy body weight. Other mechanisms by which physical activity may help to prevent certain cancers may involve both direct and indirect effects. For colon cancer, physical activity accelerates the movement of food through the intestine, thereby reducing the length of time that the bowel lining is exposed to potential carcinogens. For breast cancer, vigorous physical activity may decrease the exposure of breast tissue to circulating estrogen , a hormone that has been implicated in breast cancer. Physical activity may also affect cancers of the colon, breast, and other sites by improving energy metabolism and reducing circulating concentrations of insulin and related growth factors.

Because of these factors, recommendations of the American Cancer Society to reduce the risk of cancer include: consumption of a mostly plant-based diet, including five or more servings of fruits and vegetables each day; consumption of whole grains in preference to processed or refined grains and sugar; limited consumption of high-fat foods, particularly from animal sources; physical activity; and limited consumption of alcohol.

Nutrition for People with Cancer

People with cancer often have increased nutritional needs. As such, it is important for them to consume a variety of foods that provide the nutrients needed to maintain health while fighting cancer. These nutrients include: protein, carbohydrates , fat, water, vitamins, and minerals . Nutrition suggestions for people with cancer often emphasize eating high-calorie, high-protein foods. Protein helps to ensure growth, repair body tissue, and maintain a healthy immune system. Therefore, people with cancer often need more protein than usual.

Great progress has been made in the fight against cancer, and cancer detection and treatments have improved significantly. However, there is a disparity in cancer death rates between developed and developing countries. Between 80 and 90 percent of cancer patients in developing countries have late-stage and often incurable cancer at the time of diagnosis.

A growing body of evidence shows that simple changes in diet and lifestyle can help prevent many cancers. Further research into the exact mechanisms by which certain diets may help prevent cancer is ongoing.

SEE ALSO Antioxidants ; Functional Foods ; Phytochemicals .

Gita C. Gidwani

Bibliography

American Institute for Cancer Research (1997). Food, Nutrition, and the Prevention of Cancer: A Global Perspective. Washington, DC: Author.

Cooper, Geoffrey M. (1992). Elements of Human Cancer. Boston: Jones and Bartlett.

Tortora, Gerald J., and Grabowski, Sandra Reynolds (2003). Principles of Anatomy and Physiology , 10th edition. New York: Wiley.

Internet Resources

American Cancer Society. "Cancer Facts and Figures, 2002." Available from <http://www.cancer.org/downloads>

National Cancer Institute (2000). "Cancer Facts: Questions and Answers About Cancer." Available from <http://www.nci.nih.gov>