The prostate

The prostate is a gland found only in men. As shown in the picture below, the prostate is just below the bladder and in front of the rectum. It is about the size of a walnut. The tube that carries urine (the urethra) runs through the prostate. The prostate contains cells that make some of the fluid (semen) that protects and nourishes the sperm.

The prostate begins to develop before birth and keeps on growing until a man reaches adulthood. Male hormones (called androgens) cause this growth. If male hormone levels are low, the prostate gland will not grow to full size. In older men, though, the part of the prostate around the urethra may keep on growing. This causes BPH (benign prostatic hyperplasia) which can lead to problems passing urine. BPH is a problem that must be treated, but it is not cancer.

There are several types of cells in the prostate, but nearly all prostate cancers start in the gland cells. This kind of cancer is known as adenocarcinoma. The rest of the information here refers only to prostate adenocarcinoma.

Most of the time, prostate cancer grows slowly. Autopsy studies show that many older men (and even younger men) who died of other diseases also had prostate cancer that never caused a problem during their lives. These studies showed that as many as 7 to 9 out of 10 men had prostate cancer by age 80. But neither they nor their doctors even knew they had it.
Pre-cancerous changes of the prostate

Some doctors believe that prostate cancer begins with very small changes in the size and shape of the prostate gland cells. These changes are known as PIN (prostatic intraepithelial neoplasia). Almost half of all men have PIN by the time they reach age 50. In PIN, there are changes in how the prostate gland cells look under the microscope, but the cells are basically still in place -- they don't look like they've gone into other parts of the prostate (like cancer cells would). These changes can be either low-grade (almost normal) or high-grade (abnormal).