What Is Prostate Cancer?

Cancer starts when cells in the body begin to grow out of control. Cells in nearly any part of the body can become cancer cells, and can then spread to other areas of the body. To learn more about cancer and how it starts and spreads, see What Is Cancer?

Prostate cancer begins when cells in the prostate gland start to grow out of control. The prostate is a gland found only in males. It makes some of the fluid that is part of semen.

The prostate is below the bladder (the hollow organ where urine is stored) and in front of the rectum (the last part of the intestines). Just behind the prostate are glands called seminal vesicles that make most of the fluid for semen. The urethra, which is the tube that carries urine and semen out of the body through the penis, goes through the center of the prostate.

color illustration showing the prostate and surrounding area (including the location of the urethra, penis, scrotum, rectum, bladder and seminal vesicle)

The size of the prostate can change as a man ages. In younger men, it is about the size of a walnut, but it can be much larger in older men.

Types of prostate cancer

Almost all prostate cancers are adenocarcinomas. These cancers develop from the gland cells (the cells that make the prostate fluid that is added to the semen).

Other types of cancer that can start in the prostate include:

  • Small cell carcinomas
  • Neuroendocrine tumors (other than small cell carcinomas)
  • Transitional cell carcinomas
  • Sarcomas

These other types of prostate cancer are rare. If you are told you have prostate cancer, it is almost certain to be an adenocarcinoma.

Some prostate cancers grow and spread quickly, but most grow slowly. In fact, autopsy studies show that many older men (and even some younger men) who died of other causes also had prostate cancer that never affected them during their lives. In many cases, neither they nor their doctors even knew they had it.

Possible pre-cancerous conditions of the prostate

Some research suggests that prostate cancer starts out as a pre-cancerous condition, although this is not yet known for sure. These conditions are sometimes found when a man has a prostate biopsy (removal of small pieces of the prostate to look for cancer).

Prostatic intraepithelial neoplasia (PIN)

In PIN, there are changes in how the prostate gland cells look when seen with a microscope, but the abnormal cells don’t look like they are growing into other parts of the prostate (like cancer cells would). Based on how abnormal the patterns of cells look, they are classified as:

  • Low-grade PIN: The patterns of prostate cells appear almost normal.
  • High-grade PIN: The patterns of cells look more abnormal.

Low-grade PIN is not thought to be related to a man’s risk of prostate cancer. On the other hand, high-grade PIN is thought to be a possible precursor to prostate cancer. If you have a prostate biopsy and high-grade PIN is found, there is a greater chance that you might develop prostate cancer over time.

PIN begins to appear in the prostates of some men as early as in their 20s. But many men with PIN will never develop prostate cancer.

For more on PIN, see Tests to Diagnose and Stage Prostate Cancer.

Proliferative inflammatory atrophy (PIA)

In PIA, the prostate cells look smaller than normal, and there are signs of inflammation in the area. PIA is not cancer, but researchers believe that PIA may sometimes lead to high-grade PIN, or perhaps directly to prostate cancer.

The American Cancer Society medical and editorial content team

Our team is made up of doctors and oncology certified nurses with deep knowledge of cancer care as well as journalists, editors, and translators with extensive experience in medical writing.

Yang XJ. Interpretation of prostate biopsy. UpToDate. 2019. Accessed at https://www.uptodate.com/contents/interpretation-of-prostate-biopsy on March 15, 2019.

Yang XJ. Precancerous lesions of the prostate: Pathology and clinical implications. UpToDate. 2019. Accessed at https://www.uptodate.com/contents/precancerous-lesions-of-the-prostate-pathology-and-clinical-implications on March 15, 2019.

Zelefsky MJ, Morris MJ, Eastham JA. Chapter 70: Cancer of the Prostate. In: DeVita VT, Lawrence TS, Rosenberg SA, eds. DeVita, Hellman, and Rosenberg’s Cancer: Principles and Practice of Oncology. 11th ed. Philadelphia, Pa: Lippincott Williams & Wilkins; 2019.

Last Medical Review: August 1, 2019 Last Revised: August 1, 2019

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