A tumor (swelling) is caused by uncontrolled growth of cells starting in a particular organ in the body. Almost all tumors arise from a single cell that has lost growth control and makes a colony or clone of cells that are initially identical.
There are two main categories of tumors: Benign and malignant. Benign tumors are less aggressive, usually don’t invade into tissues that are not their normal residence and don’t metastasize or spread to distant tissues or organs through the vascular systems (lymphatic and blood vessels). Malignant tumors are defined by their ability to invade and metastasize causing significant illness and death. They usually need treatment to remove or destroy them.
Cancer is a general, sometimes imprecise term for a malignant tumor. There are hundreds of different kinds of cancers, and each is a distinctive disease. Malignant tumors are characterized and classified by the location in the body where they start:
- Carcinomas are tumors of epithelium (more below)
- Sarcomas are tumors of supporting tissues (bone, connective tissue, fat, blood vessels, etc.)
- Leukemias are tumors of blood cells
- Lymphomas are tumors of lymph node or blood cells
- Central nervous system tumors originate in the brain.
Carcinomas are the most common tumors in adults (e.g., breast, colon, prostate, lung cancers), and start in the organs that are mostly made up of epithelial cells. These are cells of specialized function that define the organ (for example carcinomas of the skin are malignant tumors arising in the epithelium of the skin, or the epidermis – the covering that is the main functional part of the skin).
Cancers usually arise spontaneously or sporadically (“out of the blue”), but rarely an increased cancer risk can be inherited. The cause of most cancers is unknown, but some are caused by viruses (cervical cancer, liver carcinoma, and a few others). It is thought that exposure to certain chemicals, or excessive radiation can cause cancer, but in only a few instances are the exact causes understood (e.g. smoking is clearly a major cause of lung cancer and increases the risk for many other cancers).
Editor's Note: If you receive a diagnosis of cancer continue to talk with your doctor until what you are hearing makes sense to you. Don't be afraid to ask as many questions as you need to, regardless of how long it takes. Understanding your personal diagnosis is a powerful tool in your survivorship journey.