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Bladder cancer starts when cells in the urinary bladder start to grow out of control. As more cancer cells develop, they can form a tumor and, with time, might spread to other parts of the body.

The bladder

The bladder is a hollow organ in the lower pelvis. Its main job is to store urine. Urine is liquid waste made by the kidneys and then carried to the bladder through tubes called ureters. The wall of the bladder is made up of several layers (see image). When you urinate, the muscles in the wall of the bladder contract, and urine is forced out of the bladder through a tube called the urethra.

Types of bladder cancer

Different types of cancer can start in the bladder.

Urothelial carcinoma (transitional cell carcinoma)

Urothelial carcinoma, also known as transitional cell carcinoma (TCC), starts in the urothelial cells that line the inside of the bladder. This is by far the most common type of bladder cancer. If you’re told you have bladder cancer, it's very likely to be a urothelial carcinoma.

Urothelial cells also line the inside of other parts of the urinary tract, such as the part of the kidney that connects to the ureter (called the renal pelvis), the ureters, and the urethra. People with bladder cancer sometimes have tumors in these places, too, so all of the urinary tract needs to be checked for tumors.

There are different subtypes of urothelial carcinoma, which are based on how the cancer cells look under a microscope. Most often the subtype doesn’t affect how the cancer is treated, although some subtypes might be more likely to have gene changes that could affect treatment options.

Urothelial carcinoma with divergent differentiation

Sometimes urothelial cancers contain very small areas that look like some of the other cancer types below (known as divergent differentiation). For example, the cancer may contain areas of squamous or glandular differentiation. Most often, this doesn’t affect treatment options.

Other cancers that start in the bladder

Other types of cancer can start in the bladder, but these are all much less common than urothelial (transitional cell) cancer.

Squamous cell carcinoma

In the US, only about 3% to 5% of bladder cancers are squamous cell carcinomas (SCCs). Seen with a microscope, the cells look much like the flat cells that are found on the surface of the skin.

Adenocarcinoma

Only about 1% to 2% of bladder cancers are adenocarcinomas. These cancers start in gland-forming cells.

Small cell carcinoma

Less than 1% of bladder cancers are small cell carcinomas. These cancers start in nerve-like cells called neuroendocrine cells. These cancers often grow quickly and usually need to be treated with chemotherapy like that used for small cell lung cancer.

Sarcoma

Sarcomas are cancers that start in connective tissues in the body. In the bladder, sarcomas can start in the muscle cells of the bladder, although these cancers are very rare. More information on sarcomas can be found in Soft Tissue Sarcoma and Rhabdomyosarcoma.

Start and spread of bladder cancer

The wall of the bladder has several layers. Each layer is made up of different kinds of cells (see the image above).

Most bladder cancers start in the innermost lining of the bladder, which is called the urothelium or transitional epithelium. As the cancer grows, it can invade into or through the deeper layers of the bladder wall. As the cancer becomes more advanced, it can be harder to treat.

Over time, the cancer might grow outside the bladder and into nearby structures. It might spread to nearby lymph nodes, or to other parts of the body. When bladder cancer spreads, it tends to go to the lymph nodes, the bones, the lungs, or the liver.

Muscle invasive vs. non-muscle invasive bladder cancer

Bladder cancers are often grouped for treatment purposes based on if they have invaded into the main muscle layer of the bladder wall (see the image above):

• Non-muscle invasive bladder cancer (NMIBC) has not grown into the muscle layer. This is also sometimes described as superficial bladder cancer. Included in this group are both non-invasive (stage 0) bladder tumors (see below), as well as some early invasive (stage I) cancers.
• Muscle invasive bladder cancer (MIBC) has grown into the muscle layer of the bladder wall, and possibly deeper. These cancers are more likely to spread, and they tend to be harder to treat.

Flat vs. papillary non-invasive bladder tumors

In non-invasive bladder cancer, the cancer cells are still only in the inner layer (the transitional epithelium) of the bladder wall and have not grown into the deeper layers. These tumors are divided into 2 subtypes, flat and papillary, based on how they grow (see the image above).

Non-invasive flat carcinomas do not grow toward the hollow part of the bladder. These tumors are also known as carcinoma in situ (CIS).

Non-invasive papillary carcinomas grow in thin, finger-like projections from the inner wall of the bladder toward the hollow center. Different terms might be used to describe these tumors, based on how they look under a microscope:

• Papillary urothelial neoplasm of low-malignant potential (PUNLMP): These are very low-grade (slow growing) tumors. They are very unlikely to become invasive, and they tend to have very good outcomes. However, they can sometimes come back after treatment.
• Non-invasive low-grade papillary urothelial carcinoma (LGPUC): The cells in these tumors look a little more abnormal. While these tumors rarely become invasive, they are more likely to come back after treatment.
• Non-invasive high-grade papillary urothelial carcinoma (HGPUC): In these tumors, the cells look very abnormal. These tumors have a high risk of becoming invasive, and sometimes they can be a sign that there’s an invasive cancer nearby.

If either a flat or papillary tumor grows into deeper layers of the bladder, it's called an invasive urothelial (or transitional cell) carcinoma.