What is prostate cancer?

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. The size of the prostate varies with age. In younger men, it is the size of a walnut, but it can be much larger in older men. The tube that carries urine (the urethra) runs through the center of 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 because the prostate can press on the urethra. BPH is a problem that often must be treated, but it is not cancer.

Prostate 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.
Some prostate cancers can grow and spread quickly, but 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).
A prostate biopsy might also show a change called atypical small acinar proliferation (ASAP). It is sometimes just called atypia. In ASAP, the cells look like they might be cancer when seen under the microscope, but there are too few of them on the slide to be sure. If ASAP is found, there's a high chance that cancer is also present in the prostate.
If you have had a prostate biopsy that showed high-grade PIN, ASAP, or certain other changes, there is a greater chance that there are cancer cells in your prostate. For this reason, you will be watched carefully and may need another biopsy.