Virtually all organs and tissues are susceptible to cancer. Cancers are usually named for their site of origin. Cancer cells that spread to other organs are similar to those of the original tumor, therefore these secondary (metastatic) cancers are still named for their primary site even though they may have invaded a different organ. For example, lung cancer that has spread to the brain is called metastatic lung cancer, rather than brain cancer. Carcinoma in situ refers to a cancer that has not spread. (See neoplasm for more on cancer nomenclature.)
Cancer is the second leading cause of death in the United States. Lung cancer is the leading cause of cancer death in adults; leukemia is the most common cancer in children. Other common types of cancer include breast cancer (in women), prostate cancer (in men), and colon cancer (see also Hodgkin's disease). The incidence of particular cancers varies around the world and sometimes according to ethnic group. For instance, African Americans have comparatively higher cancer rates and cancer mortality rates. It is unclear whether this is due to differences in exposure or to biological susceptibility. The number of diagnosed cases of cancer rose steadily in the United States for decades, but in 1998 it was announced that the number of new cases had begun to decline.