CANCER

Cancer is the common name for a group of 100 or more chronic, progressive diseases, all characterized by abnormal and continuous multiplication of cells in a particular tissue or organ without reference to the needs of the body. This commonly gives rise to a solid mass or tumor composed of such cells (e.g., in the lung, breast, or brain), but it can also affect almost any tissue or organ, including the blood-forming cells of the bone marrow (leukemias), the immune defense system (lymphomas), and the soft tissues, such as muscles, cartilage, or blood vessels (sarcomas). Invasion of the organ or tissue of origin by a malignant tumor–called a neoplasm, or new growth–can itself be fatal, but cancer mortality arises mainly from the tendency of most cancers to metastasize elsewhere in the body and to disable or destroy vital organs such as the brain, lung, liver, or bone marrow.

Cancer afflicts all animals as well as humans. The ultimate cause of all cancers is failed control of the growth, reproduction, or senescence (aging) of cells. This is due in turn to inherited and/or acquired damage to cellular DNA, giving rise to a malignant clone comprising all the descendant cells of the original cancerous cell. Inherited susceptibility can greatly increase the risk of developing cancer, but it appears to account for only a small proportion of all cancers, and congenital malignancy is extremely rare. Cumulative genetic damage acquired over the course of life accounts for the occurrence of most cancers. Knowledge in this domain is likely to increase rapidly with further progress in cancer genetics following the completion of the Human Genome Project.

Among the known environmental causes of cancer, use of tobacco (smoking, chewing, sucking, or inhaling) is the most important and most widely recognized. Tobacco use probably accounted for up to a third of all cancers in 2000, and a higher proportion of all cancer deaths, since it is a cause of some of the most fatal cancers–lung, larynx, pharynx, esophagus, and pancreas. Other causes include:

• Exposure to certain chemicals and other substances. For example, benzene exposure is a cause of leukemias, and exposure to asbestos produces mesothelioma of the lung lining and abdomen.
• Ionizing radiation. X-rays and ?-radiation cause solid tumors as well as leukemias.
• Solar or artificial ultraviolet radiation. Exposure produces skin cancers including melanoma.
• Obesity. Obesity increases the risk of breast and colon cancers.
• Infection by certain bacteria. For example, Helicobacter pylori is a cause of stomach cancer.
• Infection by certain viruses. Certain human papilloma viruses (HPV) cause cervical cancer; some hepatitis viruses cause liver cancer; human immunodeficiency virus (HIV) causes Kaposi sarcoma and other cancers.
• Infection by certain parasites. For example, some liver flukes can cause biliary tract cancers, and schistosomes are a cause of bladder cancer.

Many cancers can be prevented by avoidance of exposure to the underlying cause or risk factor. Thus, 90 percent of lung cancers that are directly attributable to tobacco use would not occur in the absence of exposure; almost all mesotheliomas would be avoided if asbestos exposure were eliminated. Vaccination against hepatitis B can prevent associated liver cancers, and vaccination against HPV will almost certainly prevent cervical cancers. Mass population screening, in which selected groups of the entire population are systematically invited for regular diagnostic tests, has been shown to reduce mortality from some cancers by detecting tumors at an early stage before they are clinically detectable or give rise to symptoms, and when treatment is more effective for most cancers. The cancers most widely screened for are breast cancer in women aged 50 to 69 (using mammography), and cervical cancer in women of reproductive age and up to age 60 (using a cervical smear or direct visualization). Screening for bowel cancer by detection of occult blood in the stool is also likely to reduce mortality. Mortality from cancers that do occur would be greatly reduced if all patients were diagnosed earlier, investigated thoroughly, and treated appropriately.

Around the turn of the twenty-first century, the International Agency for Research on Cancer (part of the World Health Organization) estimated that about 10 million people worldwide developed cancer every year, and that 6.9 million cancer deaths occurred in 2000, or 12.4 percent of the global death toll. Estimates of global mortality from the most common cancers are shown in Table 1. The economic cost of cancer is huge: over $100 billion dollars a year in medical expenditure and lost productivity in the U.S. alone in 2000. The human cost is incalculable.

The total number of cancers arising in a population depends on the size of the population, its age and sex structure, and the age- and sex-specific risks of developing cancer. All three components differ

TABLE 1

between countries and populations, and over time. Population growth and aging both lead to an increase in the annual number of new cancers. About half of all cancers in 2000 arose in developed countries, with just a quarter of the world's population, but the proportion of cancers arising in developing countries is set to rise substantially as the relatively young populations in those countries increase and become older. About half of all cancers occur over the age of 65 years. Primary prevention to reduce the risk of developing cancer at any given age is, and will remain, the most effective long-term strategy for cancer control.

See also: Diseases, Chronic and Degenerative; Tobacco-Related Mortality.

bibliography

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Michel P. Coleman