Bladder cancer most often begins in the cells that line the inside of the bladder. These cells are called urothelial (u-roe-THEE-lee-ul) cells. Urothelial cells are also present in other parts of the urinary tract, such as part of the kidneys and the ureters. When cancer occurs in these cells, it's known as urothelial carcinoma.
The extent of bladder cancer is defined by staging. Cancer that hasn't grown beyond the lining of the bladder where it started is stage 0. Stage I bladder cancer means cancer cells have grown beyond the bladder lining but haven't yet grown into the muscular bladder wall.
With stage II bladder cancer, cancer cells grow into the muscular bladder wall, but the cancer is still confined to the bladder. In stage III bladder cancer, cancer cells spread through the bladder wall into surrounding tissue.
Stage IV bladder cancer occurs when cancer cells spread beyond the bladder to other organs or nearby lymph nodes.