What is cancer?

Cancer is the uncontrolled growth of abnormal cells anywhere in a body. The abnormal cells are termed cancer cells, malignant cells, or tumor cells. Many cancers and the abnormal cells that compose the cancer tissue are further identified by the name of the tissue that the abnormal cells originated from (for example, breast cancer, lung cancer, colon cancer). Cancer is not confined to humans; animals and other living organisms can get cancer. Below is a schematic that shows normal cell division and how when a cell is damaged or altered without repair to its system, the cell usually dies. Also shown is what can occur when such damaged or unrepaired cells do not die and become cancer cells and proliferate with uncontrolled growth; a mass of cancer cells develop. Frequently, cancer cells can break away from this original mass of cells, travel through the blood and lymph systems, and lodge in other organs where they can again repeat the uncontrolled growth cycle. This process of cancer cells leaving an area and growing in another body area is termed metastatic spread or metastatic disease. For example, if breast cancer cells spread to a bone (or anywhere else), it means that the individual has metastatic breast cancer.
There are over 200 types of cancers; most can fit into the following categories according to the National Cancer Institute:

• Carcinoma: Cancer that begins in the skin or in tissues that line or cover internal organs
• Sarcoma: Cancer that begins in bone, cartilage, fat, muscle, blood vessels, or other connective or supportive tissue
  
  
• Leukemia: Cancer that starts in blood-forming tissue such as the bone marrow and causes large numbers of abnormal blood cells to be produced and enter the blood
• Lymphoma and myeloma: Cancers that begin in the cells of the immune system
• Central nervous system cancers: Cancers that begin in the tissues of the brain and spinal cord

In the U.S., according to the National Cancer Institute in 2010, the most common cancers (excluding non-melanoma skin cancers) are listed below.

Cancer type	Estimated new cases	Estimated deaths
Bladder	70,530	14,680
Breast (female-male)	207,090-1,970	39,840-390
Colon and rectal (combined)	142,570	51,370
Endometrial	43,470	7,950
Kidney (renal cell)	53,581	11,997
Leukemia	43,050	21,840
Lung (including bronchus)	222,520	157,300
Melanoma	68,130	8,700
Non-Hodgkin lymphoma	65,540	20,210
Pancreatic	43,140	36,800
Prostate	217,730	32,050
Thyroid	44,670	1,690

The three most common cancers in men, women and children in the U.S. are as follows:

• Men: Prostate, lung, and colorectal
• Women: Breast, colorectal, and lung
• Children: Leukemia, brain tumors, and lymphoma

The incidence of cancer and cancer types are influenced by many factors such as age, sex, race, local environmental factors, diet, and genetics. Consequently, the incidence of cancer and cancer types vary depending on these variable factors. For example, the World Health Organization (WHO) provides the following general information about cancer worldwide:

• Cancer is a leading cause of death worldwide. It accounted for 7.4 million deaths (around 13% of all deaths) in 2004 (statistics published in 2009).
• Lung, stomach, liver, colon, and breast cancer cause the most cancer deaths each year.
• Deaths from cancer worldwide are projected to continue rising, with an estimated 12 million deaths in 2030.

Different areas of the world may have cancers that are either more or less predominant then those found in the U.S. One example is that stomach cancer is often found in Japan, while it is rarely found in the U.S.
The objective of this article is to introduce the reader to general aspects of cancers. It is designed to be an overview of cancer and cannot cover every cancer type. This article will also attempt to help guide the reader to more detailed sources about specific cancer types.