Prostate cancer. Coloured magnetic resonance imaging (MRI) scan of an axial (horizontal) section through a man's pelvis, showing cancer of his prostate gland (green, centre). The prostate is seen between the bones of the pelvis (white), with the rectum (dark red) at lower centre. Muscle appears blue/brown. The prostate, which lies below the bladder, produces a fluid that forms part of semen. Prostate cancer mainly affects elderly men, causing obstruction to the neck of the bladder and impairing urination. It may also spread to other parts of the body. However, the outlook is good if the cancer is discovered at an early stage and the prostate is surgically removed.