Tumor

Tumor, broadly interpreted, any abnormal local increase in size of a tissue or organ. Medical research at the microscopic level, however, has made clear that swelling can be due either to the infiltration of cells from another part of the body (see Inflammation) or to the proliferation of cells originating within the affected site itself; only the latter circumstance is called a tumor.

Tumors are classified as either benign or malignant, although the benign-malignant distinction is not universally useful. The most important property rendering a tumor malignant is the ability to invade nearby or distant tissues; this spread to distant tissues is called metastasis (Cancer), and it usually occurs by means of the blood or lymph vessels. Some so-called benign tumors can kill without metastasizing. Chief among these are brain tumors called gliomas, which can grow large enough to exert substantial pressure on nearby brain structures and destroy respiratory function. A liver tumor can kill by destroying the vital functions of that organ, even without metastasis. Cells in malignant tumors are also sometimes said to have lost their characteristic function, but blood cells form tumors, called myelomas, in which the cells retain the ability to form antibodies. Tumors of the uterus, called hydatidiform moles, are benign, but they can be forerunners of the cancer called choriocarcinoma. The clearest cases of benign tumors are skin moles and warts.