The diagnosis of prostate cancer is usually confirmed by performing a needle biopsy of the prostate gland. During a prostate biopsy, a needle is inserted through the rectum and advanced into the prostate gland. Ultrasound is used to guide the needle during the biopsy. A small tissue sample is collected, and then studied under a microscope to look for cancer cells. Usually, the biopsy is performed as an outpatient procedure, under local anesthesia.