What is cancer?
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 from healthy cells, they do not function properly, and they can spread to many areas of the body. Tumors, abnormal growth of tissue, are clusters of cells that are capable of growing and dividing uncontrollably; their growth is not regulated.
Oncology is the branch of medicine concerned with the diagnosis and treatment of cancer.
The number of cells in each body tissue is tightly controlled. The number of new cells made equals the number needed to replace dying cells plus the number needed for growth and development. Unlike normal cells, tumors are clusters of cells that are capable of growing and dividing uncontrollably. Their growth is not regulated. 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:"
locally invasive - the tumor can invade the tissues surrounding it by sending out "fingers" of cancerous cells into the normal tissue.
metastatic - the tumor can send cells into other tissues in the body, which may be distant from the original tumor.
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 cancer has spread and if so, how far. There is more than one system used for staging.
Cancer is not just one disease but rather a group of diseases. All forms of cancer cause cells in the body to change and grow out of control.