It's often the last part of the physical exam, and no one likes it. If only there were a blood test or an easy and accurate X-ray image that could be taken, as is done with mammography for breasts.

But there isn't, at least not yet. The gold standard for determining the health of the prostate is the digital rectal exam. By inserting a gloved finger into a man's anus, the physician can feel the prostate through the wall of the rectum. Cancer feels like a hard nodule, a tiny marble, in what should be a fleshy gland about the size of a walnut.

Some men find the examinations degrading. Some physicians skip them, but they shouldn't. All men over 40 should get yearly prostate exams, according to the American Cancer Society. A timely prostate exam can make the difference between life and death, between a cure and detecting the cancer after it spreads to other parts of the body, when it's too late.

The symptoms of prostate trouble of any kind are:

Frequent urination,

Difficulty in urination,

Blood in the urine, and

Lower back pain.

Eight times out of 10, these symptoms signal benign enlargement of the prostate, which is treated by inserting a small tube through the urethra to cut away excess prostate tissue.

But if a cancer has reached the symptom-causing stage before it is detected, there is a good chance that it has already spread. That is why the digital exam is so important.

The rectal exam, however, has its limitations. The finger can only feel half the gland and, consequently, half the cancers.

"The cancer is always greater than what you feel in the rectal examination," said Dr. Thomas Stamey, chairman of urology at Stanford University Medical Center. "You are feeling, in general, the tip of the iceberg."

Physicians have been struggling to find better ways to spot the rest of the iceberg. A number of technologies, including ultrasound and a new blood test, have converged on this problem in the last couple of years, although their use for screening remains controversial.

Ultrasound may one day become the prostate equivalent of a mammogram. Dr. Fred Lee, a radiologist at St. Joseph Mercy Hospital in Ann Arbor, Mich., believes it is already.

"The most important thing is early detection," Lee said. "We found that our sensitivity, compared with the digital rectal exam, was twice as sensitive. We were approximately 80 percent correct; the rectal exam was 40 percent correct."

An ultrasound probe is placed in the rectum and bounces high-frequency sound waves into the prostate. The different densities between normal prostate tissue and cancer show up as shadows on the ultrasound image.

"I would advocate that once a man hits 50, he not only have a digital rectal exam but that he have a baseline ultrasound and a PSA" -- a blood test that indicates the amount of prostate tissue, including cancer, in the body, Lee said.

Lee also argues that it is cost effective to use ultrasound as a screening technique. It cost about $7,000 to find each cancer in Lee's study; another study said it costs $25,000 for every breast cancer found by mammography.

Ultrasound has helped with another advancement -- a quick, reasonably painless way to biopsy the prostate to determine whether a lump is actually cancerous. Ultrasound images allow the surgeon to guide a spring-loaded gun that fires a needle through the wall of the rectum and into the prostate to capture crumbs of potentially cancerous tissue. The needle is withdrawn so the sample can be analyzed.

"The gun fires the needle so rapidly that it is painless," Stamey said, though he admitted that "5 to 10 percent of our patients feel a little ache when the gun is fired." But it is so uncomplicated that "the patient gets off the table and walks home. No anesthesia. No catheter."

In the past, prostate biopsy was a complicated procedure requiring anesthesia and had a significant complication rate because the prostate, and sometimes the bladder, was injured when the surgeon blindly guided the needle over a finger in the rectum. The earlier biopsy technique missed about one third of the cancers, Stamey said.

Besides the imaging systems, researchers discovered that prostate specific antigen (PSA), a protein found on all prostate cells, including cancer cells, can be found in blood. The higher the PSA levels, the greater the probability that prostate cancer cells are multiplying and producing more PSA.

Stamey says his group has been able to correlate the concentration of PSA in the blood with the size of prostate tumors. The protein marker also is used to follow up patients who had their prostate surgically removed. After surgery, PSA levels should fall to zero. If they do not, or if the levels rise, then cancer cells remain in the body, possibly spreading the disease.

The role of these technologies for screening, however, has been hotly debated. "It remains to be proved with the current state of the art that ultrasound can really distinguish a cancerous portion of the prostate from the benign enlargement that occurs in all men's prostates after the age of 45 or so, with the efficiency to make it a screening test," said Dr. William Fair of Memorial-Sloan Kettering Cancer Center in New York City. "It will lead to more prostatic biopsies, maybe more than are really warranted. Whether it leads to more cures is another question."

Despite the debate over technology, there is a basic problem, said Fair: "Men are not getting the digital rectal exam."