A tumor is defined as any abnormal mass of cells resulting from excessive cellular multiplication. In its original context, the word "tumor" was used to describe any swelling of the body, regardless of the cause. At present, the word is used predominantly to refer to a neoplastic mass (also termed a neoplasm). See also: Cell (biology)
A neoplastic mass is a pathological lesion characterized by an uncontrolled proliferation of cells. The cells involved in the neoplastic growth have an intrinsic heritable abnormality, preventing proper cell regulation. The cell proliferation serves no useful function and can be very detrimental to the organism in which the mass grows. Although the stimulus that elicits this neoplastic growth is not always known, genetic elements (including mutations that affect the gene controlling the tumor suppressor protein p53), tumor-causing viruses (for example, RNA retroviruses and DNA viruses), and other mutagens or carcinogens are important factors. See also: Mutagens and carcinogens; Mutation; Tumor suppressor genes; Tumor viruses
Tumors have two basic anatomical components: the parenchyma, which consists of the neoplastic proliferating cells and determines the biological behavior of the neoplasm, and the stroma, which is the supporting framework of connective tissue (including the vascular supply). Tumors are usually classified into benign or malignant (cancerous) categories, although it is not always easy to differentiate the two types. For example, the early phases of many malignancies resemble benign growths. Contrary to what the name might suggest, benign tumors are not necessarily harmless and can indeed sometimes be lethal, depending on their location in the body. See also: Cancer (medicine); Oncology
Benign neoplasms grow slowly, display features (for example, size and structure) similar to normal tissues, function relatively normally, and remain localized. Any harm that they inflict is generally the result of the growing mass causing physical disruptions of nearby vital structures. A benign tumor in the brain, for instance, may create dangerous pressure inside the skull and thereby damage the brain. Fortunately, except when proximity to other vital structures complicates surgery, benign tumors can frequently be removed successfully and permanently.
In contrast, malignant neoplasms tend to grow rapidly and spread throughout the body (that is, they are invasive). Most cells of malignant tumors show some degree of anaplasia (loss of differentiation). Anaplastic cells tend to be larger than normal, are abnormal in shape, and often stain darkly. In addition to infiltrating and destroying surrounding tissue, these cancerous cells have the ability to metastasize; that is, cells from the primary tumor disseminate to other regions of the body, where they produce secondary tumors called metastases. In most cases, the formation of a neoplasm is irreversible because it results from a permanent genetic defect that is passed on to daughter cells. Because of their metastatic properties, malignant neoplasms often recur even if seemingly eliminated by surgery or other treatments. See also: Cancer cell metabolism; Cancer stem cells; Cell differentiation; Circulating cancer cells
Cancer staging is a diagnostic process that evaluates the amount of cancer in the body. Based on the severity of a person’s cancer (including the cancer’s magnitude, original location, and invasiveness), staging can help determine the type of therapy best used to treat it. Surgery to remove abnormal growths is a common first-line recourse, but examples of other important treatments for cancer include immunotherapy (the use of antibodies and other immunological agents, including enhancements of a patient’s own immune system, to reject a tumor), radiation therapy (the use of ionizing radiation to treat the malignancy), and chemotherapy (the use of chemicals or drugs to kill or slow the growth of cancerous cells). See also: Chemotherapy and other antineoplastic drugs; Immunotherapy; Radiation therapy; Surgery