Cancer

Cancer is a disease of uncontrolled cell growth caused by exposure to carcinogens (cancer-causing substances), genetic defects, or viruses. Cancer cells can multiply and form a large mass of tissue called a tumor. Some tumors are limited to one location and can be surgically removed. These tumors may cause little harm and are therefore termed benign. Cancer cells of other tumors may spread, or metastasize (muh-TASS-tuh-size), to surrounding tissue or other organs of the body. Such aggressive tumors are termed malignant. Cancer is a word used usually to describe malignant, not benign, tumors. The study of cancer is called oncology.

A transmission electron micorgraph of two spindle cell nuclei from a human sarcoma. Sarcomas are cancers of the connective tissue (such as bone, cartilage, tendons, and ligaments). (Reproduced by permission of

Photo Researchers, Inc.

)

How cancer cells are formed

The transformation of a normal cell into a cancer cell can occur when the genetic material (deoxyribonucleic acid or DNA) of a cell is changed, or mutated. A tumor is the result of multiple gene mutations within a single cell. Years or decades before a tumor forms, a cell can become weakened by various factors, making it more susceptible to later transformation into a cancer cell. Cancer is often a disease of age, with many occurring after age fifty.

Types of cancer

There are more than 200 different types of cancer, and they are named for the organ or tissue in which they begin to grow. Leukemia refers to cancer of white blood cells (also called leukocytes), and lymphoma is cancer of lymphoid tissue (a connective tissue containing white blood cells called lymphocytes). Melanomas are cancers that begin in melanocytes (skin pigment cells). Cancers that originate in epithelial tissue (cellular tissue that lines cavities such as the stomach or lung) are called carcinomas. Those that begin in connective tissue (such as bone and cartilage) or muscle are called sarcomas.

Causes of cancer

One of the most carcinogenic substances known is tobacco smoke. It is the major cause of lung cancer, which is the leading cause of cancer deaths in both men and women in the United States. In the year 2000, almost 160,000 people in the United States died from lung cancer. In comparison, cancer of the colon and rectum caused over 56,000 deaths that same year. Breast cancer claimed over 41,000 lives, while prostate cancer claimed almost 32,000.

Other carcinogens include certain chemicals, the Sun's ultraviolet light, and radiation. Some viruses can cause cancer by altering the DNA of a host cell and converting the cell's normal genes into cancer-causing genes, or oncogenes. Genetic factors—such as chromosomal

Man undergoing radiation treatment for Hodgkin's disease, a cancer of the lymphatic tissue. The areas to be irradiated appear as illuminated circles on the patient's body. (Reproduced by permission of

Photo Researchers, Inc.

)

abnormalities or the inheritance of faulty genes from a parent—can make some people more likely to develop certain cancers. For instance, people with Down syndrome, a chromosomal abnormality, are susceptible to leukemia.

Treatment

Cancer treatment consists of surgery to remove tumors and radiation to slow tumor growth. Chemotherapy, or drug therapy, is often used to treat cancers that have spread to other parts of the body. Another approach to treatment is to boost the immune system with immune-enhancing drugs or antibodies that can recognize and destroy abnormal cells. A new type of anticancer drug that homes in on cancer cells—leaving healthy ones alone—was introduced in the spring of 2001. The drug, Gleevec, works strikingly well against chronic myelogenous leukemia (one of the four main types of blood cancer) and gastrointestinal stromal tumor (GIST; a rare stomach tumor). Scientists believe the future of cancer treatment lies in the development of a wider range of anticancer drugs and in genetic engineering, whereby healthy genes would be manufactured to replace mutated DNA in transformed cells.

Prevention

Many cancers are preventable. It is estimated that avoidance of tobacco, overexposure to the Sun, high-fat diets, excessive alcohol, unsafe sex, and other known carcinogens could prevent more than 80 percent of all cancer cases. In addition, yearly testing can detect certain cancers and make early treatment possible, providing a better chance of survival.

[ See also Carcinogen ; Nucleic acid ]