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Overview of Cancer

By

Robert Peter Gale

, MD, PhD, Imperial College London

Last full review/revision Sep 2020| Content last modified Sep 2020
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A cancer is an abnormal growth of cells (usually derived from a single abnormal cell). The cells have lost normal control mechanisms and thus are able to multiply continuously, invade nearby tissues, migrate to distant parts of the body, and promote the growth of new blood vessels from which the cells derive nutrients. Cancerous (malignant) cells can develop from any tissue within the body.

As cancerous cells grow and multiply, they form a mass of cancerous tissue—called a tumor—that invades and destroys normal adjacent tissues. The term tumor refers to an abnormal growth or mass. Tumors can be cancerous or noncancerous. Cancerous cells from the primary (initial) site can spread throughout the body (metastasize).

Types of Cancer

Cancerous tissues (malignancies) can be divided into those of the blood and blood-forming tissues (leukemias and lymphomas) and “solid” tumors (a solid mass of cells), often termed cancer. Cancerous solid tumors can be carcinomas or sarcomas. Specific cancers can be further categorized by the organ in which they first develop and the type of cell in which they arise—for example, squamous cell carcinoma of the skin.

Leukemias and lymphomas are cancers of the blood and blood-forming tissues and cells of the immune system. Leukemias arise from blood-forming cells and crowd out the production of normal blood cells in the bone marrow. Cancer cells from lymphomas expand lymph nodes, producing large masses in the armpit, groin, abdomen, or chest.

Carcinomas are cancers of cells that line the skin, lungs, digestive tract, and internal organs. Examples of carcinomas are cancer of the skin, lungs, colon, stomach, breasts, prostate, and thyroid gland. Typically, carcinomas occur more often in older than in younger people.

Sarcomas are cancers of mesodermal cells. Mesodermal cells normally form muscles, blood vessels, bone, and connective tissue. Examples of sarcomas are leiomyosarcoma (cancer of smooth muscle that is found in the wall of digestive organs) and osteosarcoma (bone cancer). Typically, sarcomas occur more often in younger than in older people.

Table
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Most Common Cancers in Men and Women*

Group

Cancer

Men

Women

* The most common cancers are at the top of the list, and go down in decreasing frequency. The order is based on estimates for 2020 from the American Cancer Society. Skin cancer is probably the most common cancer in both men and women, but only one type of skin cancer—melanoma—is required to be reported. How common other types are is less clear. Thus, skin cancer figures are incomplete and are therefore generally excluded from statistics.

Cancer Terminology

The following terms are often used in discussing cancer:

  • Aggressiveness: The degree to which (or speed at which) a tumor grows and spreads

  • Anaplasia: A lack of differentiation of the cancer cells. That is, the cells do not look like normal cells of the same tissue type. Anaplastic cancers are usually very aggressive

  • Benign: Noncancerous. Benign tumors do not invade nearby tissues or spread to distant sites through the bloodstream or lymphatic system (metastasize). However, a benign tumor may still grow in place and cause problems by pressing on nearby tissues.

  • Carcinogen: An agent that causes cancer

  • Carcinoma-in-situ: Cancerous cells that are still contained within the tissue where they have started to grow and that have not yet invaded surrounding normal tissue or spread to other parts of the body.

  • Cure: Complete elimination of the cancer with the result that the specific cancer will not grow back

  • Differentiation: The extent to which the cancer cells have matured, ceased to multiply, and taken on normal cellular functions so that they no longer look like rapidly multiplying and primitive cells

  • Grade: The degree of abnormality of the appearance of cancer cells on microscopic examination—more abnormal appearing cells are more aggressive

  • Invasion: The capacity of a cancer to grow into and destroy surrounding tissue

  • Malignant: Cancerous cells that can invade adjacent tissue and also spread to other parts of the body

  • Malignant transformation: The complex process by which cancerous cells develop from healthy cells

  • Metastasis: Cancerous cells that have spread to a completely new location

  • Neoplasm: General term for a tumor, whether cancerous or noncancerous

  • Recurrence (relapse): Cancerous cells return after treatment, either in the primary location or as metastases (spread)

  • Remission: Absence of all evidence of a cancer after treatment although there may still be cancer in the body

  • Stage: The extent to which cancer has spread

  • Survival rate: The percentage of people who survive for a given period of time after treatment (for example, the 5-year survival rate is the percentage of people who survive 5 years)

  • Tumor: An abnormal growth or mass

Did You Know...

  • The -oma ending on a word means a swelling, growth, or tumor. The first part of the word refers to what the swelling or growth is made of. For example, a meningioma is a tumor that develops in the covering of the brain or spinal cord (the meninges). Many cancer names end in "-oma" but not all "-omas" are tumors. A hematoma is swelling caused by a collection of blood (heme)..

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