Cancer is an abnormal growth of cells. Cancer cells rapidly reproduce despite restriction of space, nutrients shared by other cells or signals sent from the body to stop reproduction. Cancer cells are often shaped differently from healthy cells, they do not function properly and they can spread to many areas of the body. Tumors, or the abnormal growth of tissue, are clusters of cells that are capable of growing and dividing uncontrollably; their growth is not regulated.
Oncology is the study of cancer and tumors. The term “cancer” is used when a tumor is malignant, which is to say it has the potential to cause harm, including death.
Tumors can be benign (noncancerous) or malignant (cancerous). Benign tumors tend to grow slowly and do not spread. Malignant tumors can grow rapidly, invade and destroy nearby normal tissues and spread throughout the body.
Cancer is malignant because it can be locally invasive and metastatic:
- Locally invasive – the tumor can invade the tissues surrounding it by sending out fingers of cancerous cells into the normal tissue
- Metastatic – the tumor can send cells into other tissues in the body, which may be distant from the original tumor
The original tumor is called the primary tumor. Its cells, which travel through the body, can begin the formation of new tumors in other organs. These new tumors are referred to as secondary tumors. The cancerous cells travel through the blood (circulatory system) or lymphatic system to form secondary tumors. The lymphatic system is a series of small vessels that collect waste from cells, carrying it into larger vessels, and finally into lymph nodes. Lymph fluid eventually drains into the bloodstream.
Cancer is named after the part of the body where it originated. When cancer spreads, it keeps this same name. For example, if kidney cancer spreads to the lungs, it is still kidney cancer, not lung cancer. (The lung cancer would be an example of a secondary tumor.) Staging is the process of determining whether cancer has spread and, if so, how far. There is more than one system used for staging cancer.
Cancer is not just one disease but rather a group of diseases, all of which cause cells in the body to change and grow out of control. Cancers are classified either according to the kind of fluid or tissue from which they originate, or according to the location in the body where they first developed. In addition, some cancers are of mixed types. The following five broad categories indicate the tissue and blood classifications of cancer.
Carcinoma – refers to a cancer found in body tissue known as epithelial tissue that covers or lines surfaces of organs, glands or body structures. For example, a cancer of the lining of the stomach is called a carcinoma. Many carcinomas affect organs or glands that are involved with secretion, such as breasts that produce milk. Carcinomas account for 80 percent to 90 percent of all cancer cases.
Sarcoma – refers to a malignant tumor growing from connective tissues, such as cartilage, fat, muscle, tendons and bones. The most common sarcoma, a tumor on the bone, usually occurs in young adults. Examples of sarcoma include osteosarcoma (bone) and chondrosarcoma (cartilage).
Lymphoma – refers to a cancer that originates in the nodes or glands of the lymphatic system — which produces white blood cells and clean body fluids — or in organs such as the brain and breast. Lymphomas are classified into two categories: Hodgkin’s lymphoma and non-Hodgkin’s lymphoma.
Leukemia – also known as blood cancer, is a cancer of the bone marrow that keeps the marrow from producing normal red and white blood cells and platelets. White blood cells are needed to resist infection. Red blood cells are needed to prevent anemia. Platelets keep the body from easily bruising and bleeding. Examples of leukemia include acute myelogenous leukemia, chronic myelogenous leukemia, acute lymphocytic leukemia and chronic lymphocytic leukemia. The terms myelogenous and lymphocytic indicate the type of cells that are involved.
Myeloma – grows in the plasma cells of bone marrow. In some cases, the myeloma cells collect in one bone and form a single tumor, called a plasmacytoma. However, in other cases, the myeloma cells collect in many bones, forming many bone tumors — called multiple myeloma.
For information about cancer, refer to a specific type of cancer and the causes and risk factors associated with each type.