Cancers that affect human beings are classified in two ways: (1) by primary body site-that is, the part of the body where cancer first develops; and (2) by the type of tissue in which the cancer originates.

Classification by primary body site. Cancer first appears most often in the skin, the female breasts, and the organs of the digestive, respiratory, reproductive, blood-forming, lymphatic, and urinary systems. The number of cases of cancer in these sites varies from country to country. Cancer of the stomach, for example, is much more common in Japan than in the United States or India. Rates of breast cancer are high in the United States and the United Kingdom, but much lower in India and many Asian countries. The following discussion deals with the kinds of cancer that occur most often in many countries.

Skin cancer is the most common type of cancer in the world. Most skin cancers grow slowly and do not spread to other parts of the body. As a result, these cancers are among the easiest to cure. Most people treated for skin cancer recover completely. However, one form of skin cancer called malignant melanoma is particularly dangerous. This type of cancer begins in the skin's pigment cells. If not detected and treated early, it can spread rapidly to other parts of the body.

Cancers of the digestive organs most commonly affect the parts of the large intestine called the colon and the rectum. About half of all people treated for cancer of the colon or rectum survive five years or longer after treatment with no return of the disease. Other digestive organs commonly affected by cancer include the esophagus, liver, pancreas, and stomach.

Cancers of the respiratory system involve the larynx and lungs. In most industrial countries, lung cancer kills more people each year than any other kind of cancer. The death rate is high because many lung cancers have spread to other body sites before they are detected.

Breast cancer occurs in both sexes, but it attacks women about 100 times more often than it strikes men. Most of these cancers occur in women over 40 years of age. Female breast cancer patients whose disease is found and treated before it has spread beyond the breast have good long-term survival rates.

Breast Cancer

Cancers of the reproductive system are relatively common among both men and women. The male organ most often affected is a small gland called the prostate. Most cases occur in men over 50 years old. Proper treatment can cure the disease in its early stages and control more advanced prostate cancer for many years.

The most common cancers of the female reproductive system affect the uterus, the organ in which babies develop. Some cancers arise in the main part of the uterus. Cancer may also affect the lower, necklike part of the organ called the cervix. Cancer of the cervix strikes more younger women than do other cancers of the uterus.

Cancers of the blood-forming and lymphatic systems. Cancer of the bone marrow and other blood-forming organs is called leukemia. In leukemia, immature white blood cells multiply wildly and interfere with production of other vital blood elements.

Cancer may also arise in tissues of the lymphatic system. This system is a network of vessels that returns fluids to the bloodstream and helps fight disease. Lymphatic cancer is called lymphoma. A well-known form of lymphoma is Hodgkin's disease, named after Thomas Hodgkin, the English physician who first described the disorder. Hodgkins disease affects both young adults and older individuals. The most common lymphatic cancer is non-Hodgkin's lymphoma. Cases of non-Hodgkin's lymphoma have risen steadily since the mid-1900's. Many people with AIDS develop this type of cancer.

Cancers of the urinary system. The bladder is the urinary organ most commonly affected by cancer. The majority of bladder cancer patients are men, and most survive five years. Many such cancers are detected early because they arise in the inner layer of the bladder and cause bleeding in the urine.

Classification by tissue. Scientists group cancers based on the body tissue in which tumors begin. Carcinomas develop in epithelial tissue, which forms the outer layer of skin and lines internal body surfaces and organs. Many organs, including the breast, colon, and lung, also contain glands. Cancers that arise in gland tissue, called adenocarcinomas, are among the most common cancers. Cancers that develop in connective tissue are called sarcomas. Connective tissue forms the body's supporting structures, such as bones and cartilage. Sarcomas also form in the breast, digestive system, respiratory system, and reproductive system, but far less often than carcinomas.