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 than healthy cells, they do not function properly and they can spread to many areas of the body.
Tumors can be benign (non-cancerous), or malignant (cancerous). Benign tumors tend to grow slowly and do not spread. Malignant tumors grow rapidly, can invade and destroy nearby normal tissues and can spread throughout the body. The term "cancer" is used when a tumor is malignant.
Cancer is malignant because it can be "locally invasive" and "metastatic":
The original tumor is called the "primary tumor." These cells, which travel through the body, can begin the formation of new tumors in other organs. These 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. When cancer spreads, it is still named after the part of the body where it started. 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 finding out whether a cancer has spread and if so, how far.
The discovery of certain types of genes, which contribute to cancer, has been important in the development of cancer research. Over 90 percent of cancers studied have some genetic alteration. We are born with some of these alterations, while others are sporadic, which means they occur by chance or occur from environmental exposures (usually over many years). There are three main types of genes that can affect cell growth, and are altered (mutated) in certain types of cancers, including oncogenes, tumor suppressor genes and mismatch-repair genes.
These genes regulate the normal growth of cells. Scientists commonly describe oncogenes as similar to a cancer "switch" that most people have in their bodies. What "flips the switch" to make these oncogenes suddenly become unable to control the normal growth of cells and allowing abnormal cancer cells to begin to grow is unknown.
These genes can recognize abnormal growth and reproduction of cancer cells and can interrupt their reproduction until the defect is corrected. If the tumor suppressor genes are mutated, however, and they do not function properly, tumor growth may occur.
These genes help recognize errors when DNA is copied to make a new cell. If the DNA does not "match" perfectly, these genes repair the mismatch and correct the error. If these genes are not working properly, however, errors in DNA can be transmitted to new cells, causing them to be damaged. Usually, the number of cells in any of our body tissues is tightly controlled so that new cells are made for normal growth and development, as well as to replace dying cells. Ultimately, cancer is a loss of this balance due to genetic alterations that "tip the balance" in favor of excessive cell growth.