This study will evaluate the safety and effectiveness of a drug called amifostine in reducing the bowel side effects of radiation treatment for prostate cancer. Amifostine is a 'radioprotector' medicine that to protects normal tissue from radiation damage. This study will determine whether placing amifostine in the rectum during radiation treatment for prostate cancer can decrease common side effects of treatment, including diarrhea, painful bowel movements, bleeding, and gas.