Prostate cancer begins when cells in the prostate gland start to grow out of control. The prostate is a gland found only in males. It makes some of the fluid that is part of semen. The prostate is below the bladder (the hollow organ where urine is stored) and in front of the rectum (the last part of the intestines).
https://www.cancer.org/cancer/prostate-cancer/about/what-is-prostate-cancer.html