What is cancer?
Cancer is the uncontrolled growth of abnormal cells anywhere in a body. These abnormal cells are termed cancer cells, malignant cells, or tumor cells. These cells can infiltrate normal body tissues. 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 occurs when such damaged or unrepaired cells do not die and become cancer cells and show uncontrolled division and 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 metastasis. For example, if breast cancer cells spread to a bone, it means that the individual has metastatic breast cancer to bone. This is not the same as "bone cancer," which would mean the cancer had started in the bone.
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
The following table (National Cancer Institute 2015) gives the estimated numbers of new cases and deaths for each common cancer type:
|Cancer Type||Estimated New Cases||Estimated Deaths|
|Breast (Female -- Male)||231,840 -- 2,350||40,290 -- 440|
|Colon and Rectal (Combined)||132,700||49,700|
|Kidney (Renal Cell and Renal Pelvis)||61,560||14,080|
|Leukemia (All Types)||54,270||24,450|
|Lung (Including Bronchus)||221,200||158,040|
The three most common cancers in men, women, and children in the U.S. are as follows:
- Men: Prostate, lung, and colorectal
- Women: Breast, lung, and colorectal
- Children: Leukemia, brain tumors, and lymphoma
The incidence of cancer and cancer types are influenced by many factors such as age, gender, 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 8.2 million deaths (around 22% of all deaths not related to communicable diseases; most recent data from WHO).
- 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 13.1 million deaths in 2030 (about a 70% increase).
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. This usually represents a combination of environmental and genetic factors.
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.