The prostate is a small gland, about the size of a walnut, found only in men.
It surrounds the first part of the tube (urethra) which carries urine from the bladder to the penis.
The prostate produces a thick white fluid called semen which mixes with the sperm produced by the testes. It also produces a protein called prostate specific antigen (PSA) which turns the semen into liquid.
The growth of prostate cells and the way the prostate gland works is dependent on the male sex hormone, testosterone, which is produced in the testicles.
Prostate cancer generally affects men over the age of 50.
It differs from most other cancers in the body, because small areas of cancer within the prostate are very common and may remain unchanged for some time before they begin to grow.
About one third of men over the age of 50 have some cancer cells within their prostate and nearly all men over the age of 80 have a small area of prostate cancer.
These cancers grow very slowly and so, particularly in elderly men, may never cause any problem.
In other cases, the cancer can grow rapidly and spread to other parts of the body, particularly the bones.