To understand bladder cancer, it helps to know about the normal structure and function of the bladder.
The normal 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.
During urination, 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 a very short tube that opens just above 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 several layers. A layer of cells lines the inside of the ureter, bladder, and urethra, and parts of the kidney. These cells are called urothelial or transitional cells, so this inner layer is called the urothelium or transitional epithelium. Beneath the urothelium is a thin layer of connective tissue called the lamina propria. Next is a layer of muscle tissue called the muscularis propria. Beyond this muscle, another zone of fatty connective tissue separates the bladder from other nearby organs.
These layers are very important in understanding bladder cancer. Most bladder cancers start in the urothelium. As the cancer penetrates or grows through the other layers in the bladder, it becomes harder to treat.
Types of bladder cancer
Bladder tumors are grouped into several types by how their cells look under a microscope. The type of bladder cancer you have can affect your treatment options. This is because different types can respond differently to treatments such as radiation and chemotherapy.
The main types of cancers that affect the bladder are:
- Transitional cell carcinoma (also called urothelial carcinoma)
- Squamous cell carcinoma
- Adenocarcinoma
- Small cell carcinoma
These same types of cancer can also grow in other places in the urinary tract, such as the lining of the kidney (called the renal pelvis), the ureters, and the urethra. In fact, patients with bladder cancer sometimes have a similar tumor in the lining of the kidneys, ureters, or urethra. Therefore, when someone is found to have a cancer in one part of their urinary system, the entire urinary tract needs to be checked for tumors.
Transitional cell (urothelial) carcinoma
This is the most common type of bladder cancer. About 95% of bladder cancers are this type. The cells from transitional cell carcinomas look like the urothelial cells that line the inside of the bladder.
Two very important traits in predicting the outlook for transitional cell carcinomas are the grade of the cancer and whether or not it is invasive.
Grade: These tumors are divided into grades based on how abnormal the cells look under the microscope. If the cells look more like normal cells, the cancer is called a low-grade cancer. When the cells look more abnormal, the cancer is high-grade. Lower-grade cancers tend to grow more slowly and have a better outcome than higher-grade cancers.
Invasiveness: If the cancer stays in the inner layer of cells without growing into the deeper layers, it is called non-invasive. If the cancer grows into the lamina propria or even deeper into the muscle layer, it is called invasive. Invasive cancers are more likely to spread and are harder to treat. You may also see a bladder cancer 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.
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.
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 typically need to be treated with chemotherapy similar to that used for small-cell carcinoma of the lung.
Other cancers
Cancers rarely start in the muscle cells of the bladder. But when they do, these cancers are called sarcomas. More information about sarcomas can be found in our documents: Sarcoma - Adult Soft Tissue Cancer and Rhabdomyosarcoma.
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