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What Is Cancer?

Throughout our lives, healthy cells in our bodies divide and replace themselves in a controlled fashion. Cancer starts when a cell is somehow altered so that it multiplies out of control. A tumor is a mass composed of a cluster of such abnormal cells.

Most cancers form tumors, but not all tumors are cancerous. Benign, or noncancerous, tumors — such as freckles and moles — stop growing, do not spread to other parts of the body, and do not create new tumors. Malignant, or cancerous, tumors crowd out healthy cells, interfere with body functions, and draw nutrients from body tissues. Cancers continue to grow and spread by direct extension or through a process called metastasis, whereby the malignant cells travel through the lymphatic or blood vessels — eventually forming new tumors in other parts of the body.

The term "cancer" encompasses more than 100 diseases affecting nearly every part of the body, and all are potentially life-threatening, except for some types of skin cancers.

The four major types are carcinoma, sarcoma, lymphoma, and leukemia. Carcinomas — the most commonly diagnosed cancers — originate in the skin, lungs, breasts, pancreas, and other organs and glands. Lymphomas are cancers of the lymphatic system. Leukemias are cancers of the blood and do not form solid tumors. Sarcomas arise in bone, muscle, fat, or cartilage and are relatively rare.

Cancer has been recognized for thousands of years as a human ailment, yet only in the past century has medical science understood what cancer really is and how it progresses. Cancer specialists, called oncologists, have made remarkable advances in cancer diagnosis, prevention, and treatment. Today, more than half of all people diagnosed with cancer are cured. However, some forms of the disease remain frustratingly difficult to treat. For those people who cannot be cured, modern treatment can significantly improve quality of life and may extend survival.

What Causes Cancer?

The fundamental cause of all cancer is a change, or mutation, in the nucleus of a cell. For a healthy cell to turn malignant, its genetic code must be reprogrammed for constant, uncontrolled cell division.

Substances that either start or promote the process are called carcinogens, and there are many types. Scientists theorize that about 10 million of the 300 trillion cells in a human body die and are replaced every second. With such a high rate of cell activity, the potential for occasional malignant cell mutation is high. In a healthy person, special cells from the body's immune system can somehow recognize mutant cells and destroy them before they multiply. Nevertheless, some mutant cells may occasionally evade such detection and survive, causing cancer.

Because the causes of cancer are complicated, experts speak in terms of "risk factors." Any habit, trait, or use of a substance that increases the odds of getting cancer is a risk factor, and the risk for nearly all cancers increases with age. Inherited, or familial, predisposition is a risk factor, although its influence varies from case to case. Researchers continue to identify genes that, when flawed, strongly predispose a person to getting a particular type of cancer. Such genetic predisposition is considered an influential risk factor but by no means guarantees that the person will develop the cancer.

Environmental risk factors relate to where and how we live. Most common cancers are linked to one of three environmental risk factors: smoking, sunlight, and diet. Smoking is linked to cancer of the lung, head and neck area, bladder, kidney, stomach, cervix, and pancreas, as well as to some leukemias. Excessive exposure to sunlight is linked to skin cancer. Dietary factors are associated with some cancers of the gastrointestinal tract, and may be linked to others, such as cancer of the breast, prostate, and uterus. Eating habits suspected of promoting cancer include overconsumption of alcohol, fat, and foods that have been smoked, cured, pickled, or charred. Lack of dietary fiber or antioxidant vitamins and minerals is also believed to be a risk factor.

Many substances in the environment have been identified as carcinogens, but in most cases a very high level of exposure is needed to cause cancer. Environmental carcinogens include various chemicals, gases, and other substances found in air, water, foods, pesticides, tobacco smoke, cleaning products, paints, and many industrial settings; excessive ionizing radiation — the type in X-rays, nuclear radiation, and radioactive waste; and certain viruses, such as the AIDS, hepatitis B, papilloma, and Epstein-Barr viruses. Although stress and certain personality types have been suggested as cancer risk factors, there is no hard scientific evidence to confirm such ideas.

All these factors may contribute to causing cancer, but any single factor may not be enough by itself. Cancer results from a "multifactor hit" of inherited predisposition, general health, carcinogenic exposure, and others. For example, some people exposed to particular carcinogens will develop cancer, while others, exposed just as intensely to the same carcinogens, will not. And as far as we know, most people who get a particular form of cancer are not strongly predisposed genetically to the specific disease. Thus, everyone's cancer risk profile is complex and unique.

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