The prostate is a gland found only in males. As depicted in the image below, the prostate is located slightly below the bladder and in front of the rectum. The size of the prostate can vary with age. In younger adult men, the prostate is the size of a walnut, but it can grow to be much larger in older men. The small tube that transports the urine (the urethra) runs through the center of the prostate gland. The prostate includes cells that make some of the fluid (semen) that protects and nourishes the sperm.
The prostate begins to develop before a person is born and keeps growing until a male matures. If male hormone levels are low, the prostate gland will not grow to its full size. In older men, however, the portion of the prostate surrounding the urethra may continue to grow. This causes BPH (benign prostatic hyperplasia) which will cause problems passing urine due to the prostate pressing onto the urethra. BPH is a problem that surely must be treated, but is not cancer.
(Photo courtesy of the American Cancer Association)
There are multiple types of cells in the prostate gland, however almost 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 will grow and spread quickly, but most of the time, prostate cancers grow rather slowly. Many autopsy studies show that many older men (and even younger men) who passed away from other diseases, also had prostate cancer that never caused a problem during their lives. These studies have shown that as many as 7 to 9 out of 10 men had prostate cancer by age 80.
Pre-cancerous changes of the prostate
Many medical professionals are of the opinion 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.