What is bladder cancer?

To understand bladder cancer, it helps to know about the normal structure and function of the bladder.

The bladder is a hollow organ in the pelvis with flexible, muscular walls. Its main function is to store urine before it leaves the body. The average adult bladder holds about 2 cups of urine. Urine is made by the kidneys and is then carried to the bladder through tubes called ureters.

When you urinate, the muscles in the bladder contract, and urine is forced out of the bladder through a tube called the urethra. In women, the urethra is very short and opens just in front of the vagina. In men, the urethra is longer. It passes through the prostate gland and the penis, and opens at the tip of the penis.

The wall of the bladder has 4 main layers.

    The innermost lining is made up of cells called urothelial or transitional cells, so this layer is called the urothelium or transitional epithelium.

    Beneath the urothelium is a thin layer of connective tissue, blood vessels, and nerves, which is called the lamina propria.

    Next is a thick layer of muscle called the muscularis propria.

    Outside of this muscle, a layer of fatty connective tissue separates the bladder from other nearby organs.

Knowing about these layers is important in understanding how bladder cancer grows. Most bladder cancers start in the urothelium. As the cancer grows into or through the other layers in the bladder, it becomes more advanced  and harder to treat.

Types of bladder cancer

Bladder cancers are divided into several types based how their cells look under a microscope. Different types can respond differently to treatments.

Transitional cell (urothelial) carcinoma

This is by far the most common type of bladder cancer. More than 9 out of 10 bladder cancers are this type. The cells from transitional cell carcinomas (TCCs) look like the urothelial cells that line the inside of the bladder.

Urothelial cells also line other parts of the urinary tract, such as the lining of the kidneys (called the renal pelvis), the ureters, and the urethra, so TCCs can also occur in these places. In fact, patients with bladder cancer sometimes have other tumors in the lining of the kidneys, ureters, or urethra. If someone has a cancer in one part of their urinary system, the entire urinary tract needs to be checked for tumors.

Bladder cancers are often described based on how far they have invaded into the wall of the bladder:

    Non-invasive bladder cancers are still in the inner layer of cells (the transitional epithelium) but have not grown into the deeper layers.

    Invasive cancers grow into the lamina propria or even deeper into the muscle layer. Invasive cancers are more likely to spread and are harder to treat.

A bladder cancer can also be described as superficial or non-muscle invasive. These terms include both non-invasive tumors as well as any invasive tumors that have not grown into the main muscle layer of the bladder.

Transitional cell carcinomas are also divided into 2 subtypes, papillary and flat, based on how they grow

    Papillary carcinomas grow in slender, finger-like projections from the inner surface of the bladder toward the hollow center. Papillary tumors often grow toward the center of the bladder without growing into the deeper bladder layers. These tumors are called non-invasive papillary cancers. Very low-grade, non-invasive papillary cancer is sometimes called papillary neoplasm of low-malignant potential and tends to have a very good outcome.

    Flat carcinomas do not grow toward the hollow part of the bladder at all. If a flat tumor is only in the inner layer of bladder cells, it is known as a non-invasive flat carcinoma or a flat carcinoma in situ (CIS).

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

Other cancers that start in the bladder

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

Squamous cell carcinoma: In the United States, only about 1% to 2% of bladder cancers are squamous cell carcinomas. Under a microscope, the cells look much like the flat cells that are found on the surface of the skin. Nearly all squamous cell carcinomas are invasive.

Adenocarcinoma: Only about 1% of bladder cancers are adenocarcinomas. The cancer cells have a lot in common with gland-forming cells of colon cancers. Nearly all adenocarcinomas of the bladder are invasive.

Small cell carcinoma: Less than 1% of bladder cancers are small-cell carcinomas, which start in nerve-like cells called neuroendocrine cells. These cancers often grow quickly and typically need to be treated with chemotherapy like that used for small cell carcinoma of the lung.

Sarcoma: Sarcomas start in the muscle cells of the bladder, but they are rare. More information about sarcomas can be found in our documents Sarcoma - Adult Soft Tissue Cancer and Rhabdomyosarcoma.

These less common types of bladder cancer (other than sarcoma) are treated similar to transitional cell cancers, especially for early stage tumors, but different drugs may be needed if chemotherapy is required.