When cells divide at an accelerated rate, they often begin to form a mass of tissue called a tumor. The tumor is fed by nutrients that diffuse through neighboring blood vessels and can also grow by forming a substance called tumor angiogenesis (vessel forming) factor. This factor stimulates the growth of an independent blood supply to the tumor.
Tumors can cause destruction in three common ways:
- Tumors put pressure on nearby tissues and/or organs.
- Tumors invade tissues and organs directly (direct extension), often damaging or disabling them in the process.
- Tumors make invaded tissues and/or organs susceptible to infection.
Tumors can also release substances that destroy tissues in close proximity to them.
One of the frightening things about cancer is the possibility of metastasis. This is the process where millions of malignant cells are released from the tumor (the primary) into the bloodstream. Fortunately, most of these cells are killed by trauma produced while traveling within the blood vessel walls, or by circulating cells from the immune system, like the Natural Killer (NK) cells and other T lymphocytes. Other immune cells that battle malignant cells are macrophages, antigen-presenting cells, and substances produced by immune cells called lymphokines. One common lymphokine is called interleukin-2 (IL-2) or interferon. (See How the Immune System Works for details on these different components of the immune system.)
In some cases, the circulating malignant cells survive and adhere to the inner muscular lining of the blood vessel walls. Here the process of tumor formation can begin in a different area of the body (the secondary), causing further destruction.
It is important to note that not all tumors are cancerous. Tumors can be either malignant or benign. A malignant tumor is cancer, and a benign tumor is not. One main difference between a benign tumor and a malignant tumor is that a benign tumor will not spread (metastasize) to distant parts of the body, and usually it will not grow back once removed. A benign tumor is either surgically removed, or it may be left in place and simply observed to see what it does. The decision to remove or observe depends on the tumor's size, type and location.
Next, we'll look at some of the causes of cancer.