Solid waste that is excreted from the body moves slowly down the intestines, and, under normal circumstances, the resultant stool exits through the rectum and then the anus. Damage to the rectum can result in bowel problems, including rectal bleeding, diarrhea, or urgency.
In prostatectomy it is very rare (less than 1%) for men to have altered bowel function after surgery. In rare cases of locally advanced prostate cancer where the cancer has invaded the rectum, surgery may result in rectal damage.
Radiation therapy is targeted to the prostate, but the rectum sits right behind the prostate. With modern radiation therapy (IMRT or IGRT), it is very rare to have moderate or severe bowel problems. During radiation therapy you may experience softer stools and, rarely, diarrhea (less than 10% of men report this side effect). These symptoms typically resolve within a few weeks of completing radiation therapy. With modern radiation, only 2% to 3% of men will have bothersome rectal bleeding that continues months or years after treatment. Be sure to discuss with your doctor the types of radiation therapy that are appropriate for you, as older forms of radiation therapy (called 3D conformal) can increase rectal side effects significantly.
Overall, it is more common with radiation therapy to have slightly lower rates of overall bowel function compared with surgery. This is temporary and largely resolves by 6 to 12 months post-treatment.
As of 2016, select centers have begun to use an approved device called SpaceOAR, a gel that is injected between the prostate and the rectum in men for whom there is major concern of rectal irritation. It has been shown to further reduce the chance of rectal side effects in some men.