Bladder cancer is also divided into muscle-invasive and nonmuscle-invasive disease, based on invasion of the muscularis propria (also referred to as the detrusor muscle), which is the thick muscle deep in the bladder wall.
- Muscle-invasive disease is much more likely to spread to other parts of the body and is generally treated by either removing the bladder or treating the bladder with radiation and chemotherapy. As noted above, high-grade cancers are much more likely to be muscle-invasive than low-grade cancers. Thus, muscle-invasive cancers are generally treated more aggressively than nonmuscle-invasive cancers.
- Nonmuscle-invasive disease can often be treated by removing the tumor(s) via a transurethral approach, and sometimes chemotherapy or other treatments are introduced into the bladder with a catheter to help fight the cancer.
Under conditions of chronic inflammation, such as infection of the bladder with the Schistosoma haematobium parasite, squamous metaplasia may occur in the bladder; the incidence of squamous cell carcinomas of the bladder is higher under conditions of chronic inflammation than is otherwise seen. In addition to transitional cell carcinomas and squamous cell carcinomas, adenocarcinomas, small cell carcinomas, and sarcomas can form in the bladder. In the United States, transitional cell carcinomas represent the vast majority (over 90%) of bladder cancers. However, a significant number of transitional cell carcinomas have areas of squamous or other differentiation.
Carcinogenesis and Risk Factors
There is strong evidence linking exposure to carcinogens to bladder cancer. The most common risk factor for bladder cancer in the United States is cigarette smoking. It is estimated that up to half of all bladder cancers are caused by cigarette smoking and that smoking increases a person's risk of bladder cancer two to four times above baseline risk.[2,3] Smokers with less functional polymorphisms of N-acetyltransferase-2 (known as slow acetylators) have a higher risk of bladder cancer than other smokers, presumably because of their reduced ability to detoxify carcinogens.
Certain occupational exposures have also been linked to bladder cancer, and higher rates of bladder cancer have been reported in textile dye and rubber tire industries; among painters; leather workers; shoemakers; and aluminum-, iron-, and steelworkers. Specific chemicals linked to bladder carcinogenesis include beta-naphthylamine, 4-aminobiphenyl, and benzidine. Although these chemicals are now generally banned in Western countries, many other chemicals still in use are also suspected of causing bladder cancer.