Oncology. General term for a malignant growth of cells (neoplasm, tumor). Cancer comprises a group of more than 100 diseases in which cells in a part of the body begin to grow out of control. Cancer cells can also invade other tissues and can spread through the lymphatic vessels or blood stream to other parts of the body leading to metastases.
The normal process of cell growth, division and programmed cell death (apoptosis) is disturbed in cancer cells. When, for example DNA, is damaged leading to a mutation that affects normal cell growth and division cancer cells divide without control. When this happens, cells do not die when they should and new cells form when the body does not need them. In the three-stage model, carcinogenesis consists of the stages tumor initiation, tumor promotion and progression in which normal cells are transformed into cancer cells. Newer models are often based on a multi-stage process in which up to 10 mutations are involved. Oncogenes, proto-oncogenes and tumor suppressor genes often play a role in the process of carcinogenesis.