Cancer is a group of many related diseases. All cancers begin in cells, the body's basic unit of life. Cells make up tissues, and tissues make up the organs of the body.

Normally, cells grow and divide to form new cells as the body needs them. When cells grow old and die, new cells take their place.

Sometimes this orderly process goes wrong. New cells form when the body does not need them, and old cells do not die when they should. These extra cells can form a mass of tissue called a growth or tumor.

Tumors can be benign or malignant:

• Benign tumors are not cancer. Usually, doctors can remove them. Cells from benign tumors do not spread to other parts of the body. In most cases, benign tumors do not come back after they are removed. Most important, benign tumors are rarely a threat to life.

• Malignant tumors are cancer. They are generally more serious. Cancer cells can invade and damage nearby tissues and organs. Also, cancer cells can break away from a malignant tumor and enter the bloodstream or the lymphatic system. That is how cancer cells spread from the original (primary) tumor to form new tumors in other organs. The spread of cancer is called metastasis.

There are three types of bladder cancer that begin in cells in the lining of the bladder. These cancers are named for the type of cells that become malignant:

• Transitional cell carcinoma: Cancer that begins in cells in the innermost tissue layer of the bladder. These cells are able to stretch when the bladder is full and shrink when it is emptied. Most bladder cancers begin in transitional cells.
  
  
• Squamous cell carcinoma: Cancer that begins in squamous cells, which are thin, flat cells that may form in the bladder after a long- term infection or irritation.
  
  
• Adenocarcinoma: Cancer that begins in glandular (secretory) cells that may form in the bladder after a long-term inflammation or irritation.

Cancer that is only in cells in the lining of the bladder is called superficial bladder cancer. The doctor might call it carcinoma in situ. This type of bladder cancer often comes back after treatment. If this happens, the disease most often recurs as another superficial cancer in the bladder.

Cancer that begins as a superficial tumor may grow through the lining and into the muscular wall of the bladder. This is known as invasive cancer. Invasive cancer may extend through the bladder wall. It may grow into a nearby organ such as the uterus or vagina (in women) or the prostate gland (in men). It also may invade the wall of the abdomen.

When bladder cancer spreads outside the bladder, cancer cells are often found in nearby lymph nodes. If the cancer has reached these nodes, cancer cells may have spread to other lymph nodes or other organs, such as the lungs, liver, or bones.

When cancer spreads (metastasizes) from its original place to another part of the body, the new tumor has the same kind of abnormal cells and the same name as the primary tumor. For example, if bladder cancer spreads to the lungs, the cancer cells in the lungs are actually bladder cancer cells. The disease is metastatic bladder cancer, not lung cancer. It is treated as bladder cancer, not as lung cancer. Doctors sometimes call the new tumor "distant" disease.

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