Oncology is the medical specialty devoted to the diagnosis and treatment of cancer, and doctors who specialize in oncology are called oncologists. A cancer is an abnormal growth of cells that have lost normal control mechanisms and thus are able to expand continuously, invade adjacent tissues, and often migrate to distant parts of the body. Cancerous (malignant) cells can develop from any tissue within the body. As cancerous cells grow and multiply, they form a mass of cancerous tissue that invades and destroys normal adjacent tissues. Cancerous cells from the primary (initial) site can spread throughout the body (metastasize). Oncologists distinguish between cancers that affect the blood and blood-forming tissues (such as leukemias and lymphomas) and those that are “solid” tumors.
The term tumor refers to any abnormal growth or mass. Tumors can be cancerous or noncancerous. Not all cancers are tumors. For example, cancers that affect the blood, such as leukemia, do not form a single, solid mass.