Cancer is the name given to a collection of related diseases. In all types of cancer, some of the body’s cells begin to divide without stopping and spread to the surrounding tissues.
Cancer can begin almost anywhere on the human body, which is made up of trillions of cells. Normally, human cells grow and divide to form new cells as the body needs them. When cells grow or become damaged, they die, and new cells take their place.
When the cancer develops, however, this orderly process is broken. As cells become more and more abnormal, old or damaged cells survive when they must die, and new cells are formed when they are not needed. These additional cells can be divided without stopping and can form growths called tumors.
Many cancers form solid tumors, which are masses of tissue. Blood cancers, such as leukemias, usually do not form solid tumors.
Cancerous tumors are malignant, which means they can spread or invade nearby tissues. In addition, as these tumors grow, some cancer cells can rupture and travel to distant locations of the body through the blood or lymphatic system and form new tumors away from the original tumor.
Unlike malignant tumors, benign tumors do not spread or invade nearby tissues. However, benign tumors can sometimes be quite large. When they are removed, they usually do not grow back, while malignant tumors sometimes do. Unlike most benign tumors in other parts of the body, benign brain tumors can be life-threatening.
Differences between cancer cells and normal cells
Cancer cells differentiate themselves from normal cells in many ways that allow them to grow out of control and become invasive. One important difference is that cancer cells are less specialized than normal cells. That is, while normal cells mature into very distinct cell types with specific functions, the cancer cells do not. This is one reason that, unlike normal cells, cancer cells continue to divide without stopping.
In addition, cancer cells are able to ignore signals that normally tell cells to stop dividing or they begin a process known as programmed cell death, or apoptosis, that the body uses to get rid of unneeded cells.
Cancer cells may be able to influence normal cells, molecules, and blood vessels that surround and nourish a tumor, an area known as the microenvironment. For example, cancer cells can induce normal nearby cells to form blood vessels that supply the tumors with oxygen and nutrients, which they need to grow. These blood vessels also remove residues from tumors.
Cancer cells are also capable of evading the immune system, a network of specialized organs, tissues and cells that protect the body from infections and other conditions. Although the immune system usually removes damaged or abnormal cells from the body, some cancer cells are able to “hide” from the immune system.
Tumors can also use the immune system to stay alive and grow. For example, with the help of certain immune cells that normally prevent a wild immune response, cancer cells can actually prevent the immune system from destroying cancer cells.
How cancer arises
Cancer is a genetic disease, that is, it is caused by changes in the genes that control the functioning of our cells, especially how they grow and divide.
Genetic changes that cause cancer can be inherited from our parents. They can also arise during a person’s life as a result of errors that occur as the cells divide or due to DNA damage caused by certain environmental exposures. Environmental exposures that cause cancer include substances such as chemicals in tobacco smoke and radiation, such as ultraviolet rays from the sun. (Our Cancer Causes and Risk Factors page has more information.)
Each person’s cancer has a unique combination of genetic changes. As the cancer continues to grow, additional changes will occur. Even within the same tumor, different cells may have different genetic changes.