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). These are the different types of 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.
The following table (National Cancer Institute 2016) gives the estimated numbers of new cases and deaths for each common cancer type:
|Estimated New Cases
|Breast (Female — Male)
|246,660 — 2,600
|40,450 — 440
|Colon and Rectal (Combined)
|Kidney (Renal Cell and Renal Pelvis) Cancer
|Leukemia (All Types)
|Lung (Including Bronchus)
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.
What Causes It?
In the most basic terms, cancer refers to cells that grow out-of-control and invade other tissues. Cells become cancerous due to the accumulation of defects, or mutations, in their DNA. Certain:
- inherited genetic defects (for example, BRCA1 and BRCA2 mutations),
- environmental factors (for example, air pollution), and
- poor lifestyle choices — such as smoking and heavy alcohol use — can also damage DNA and lead to cancer.
Most of the time, cells are able to detect and repair DNA damage. If a cell is severely damaged and cannot repair itself it undergoes so-called programmed cell death or apoptosis. Cancer occurs when damaged cells grow, divide, and spread abnormally instead of self-destructing as they should.