Prostate basics
| March 1, 2008
In-Depth Report
Prostate basics
How can a gland that weighs only about an ounce cause so much trouble? To answer this question, one must look at its position in the body. The prostate is located in front of the rectum and just below the bladder. It wraps around the upper part of the urethra, the tube that carries urine from the bladder out of the body (see Figure 1). Its position near urinary and sexual organs means that certain prostate problems can affect urination and sexual function. The prostate consists mostly of connective and glandular tissues, and it produces a thick, milky-white fluid that forms part of the semen, the liquid ejaculated during sexual activity. To function properly, the prostate requires adequate amounts of certain hormones, including testosterone, produced by the testicles, as well as others from the pituitary and adrenal glands.
Figure 1: Locating the prostate gland
The prostate is located just below the bladder, in front of the rectum, and wraps around the upper part of the urethra. Prostate enlargement blocks the flow of urine from the bladder by narrowing the urethra. |
Your prostate examination
Doctors can use several approaches to evaluate the prostate. The most common are as follows.
Digital rectal examination
In this test, the doctor inserts a lubricated, gloved finger into the rectum and feels the surface of the prostate to determine whether it is swollen or has any lumps or abnormally textured areas (see Figure 2). This exam also helps doctors screen for diseases of the rectum, such as rectal cancer. When combined with a chemical test to determine the presence of blood in the stool (known as a hemoccult or fecal occult blood test), the digital rectal examination (DRE) is extremely effective in screening patients for a variety of gastrointestinal disorders, including stomach cancer, peptic ulcer disease, polyps, and hemorrhoids. The American Cancer Society advises that all men ages 50 or older have a yearly DRE. Studies have produced conflicting results, but one conducted by the Mayo Clinic demonstrated that men who get DREs are less likely to die from prostate cancer.
Although this exam generally takes less than a minute, some men find it uncomfortable and, most of all, embarrassing, so they postpone it or avoid going to a physician altogether. But the discomfort is mild and lasts only a few moments, and the procedure causes no physical injury.
The DRE is a useful screening test, but it isn't foolproof. Its accuracy depends on the skill of the physician conducting and interpreting the test. Moreover, early cancerous tumors are often too small to detect during a DRE, and some are located in areas a doctor's finger can't reach. For these reasons, clinicians who use DRE alone to screen for prostate cancer sometimes miss the smallest and most treatable tumors. On the other hand, small tumors that can't be felt by the physician may be less likely to cause future problems.
Figure 2: Digital rectal exam
To perform a digital rectal examination (DRE), the doctor inserts a lubricated, gloved finger into the rectum and presses against the rectal wall to determine the size of the prostate and feel for irregularities. Early-stage cancer may be felt as a small lump on the surface of the prostate. |
PSA test
This blood test measures the level of a protein called prostate-specific antigen (PSA), which is manufactured by the prostate. Doctors use the test to detect prostate cancer, but it does not provide a definite diagnosis. That's because an above-normal PSA level — more than 4 nanograms per milliliter (ng/ml) of blood — doesn't always indicate the presence of cancer. Likewise, levels of 4 ng/ml or less don't always mean the prostate is cancer-free. For this reason, some experts adjust for a man's age or consider a normal level to be 2.5 ng/ml. As doctors learn more about how to use the PSA test, they are less likely to use it as a simple, one-time indicator and, instead, to observe how a man's PSA level changes over time and with age.
The PSA test can provide clues to other conditions besides cancer. Indeed, most men who have mildly elevated PSA levels don't have cancer. Elevated PSA levels can accompany the noncancerous conditions known as benign prostatic hyperplasia (see "Prostate enlargement") and prostatitis (see "Inflammation of the prostate"). Complicating things further, an elevated PSA may simply mean that a man's prostate naturally releases more of the protein into the blood. Moreover, some studies suggest that ejaculation can elevate PSA levels for up to 48 hours. In addition to using the PSA test to identify problems, doctors also use it for follow-up after prostate cancer treatment and to predict which patients with BPH are likely to develop troublesome symptoms.
The PSA test has grown popular since it became widely available in the late 1980s. But experts disagree about whether PSA testing should be routine for men over age 50. The death rate from prostate cancer in the United States has dropped, and some experts believe that the explanation is widespread PSA testing. Others, however, attribute the decline to other factors, such as the decreasing amount of fat in the American diet. They point out that prostate cancer mortality rates have also declined in England and Wales, where PSA screening is rare.
Because an elevated PSA can't differentiate cancer from BPH or prostatitis, it can cause needless worry — and may lead to costly and invasive procedures, such as biopsy, in order to determine if cancer is present. Conversely, the PSA doesn't detect all cancers, so a normal PSA level may offer a false sense of security. Many men with cancer confined to the prostate have normal PSA values. Even advocates of PSA testing doubt its value in men with less than a 10-year life expectancy (ages 75 or older, for men in average health) because these men are more likely to die of something else first.
The PSA can, however, help detect prostate cancers that are too small to feel during a DRE. Indeed, the test has been credited with finding many more cancers in early stages than had been found in the past. But it's not yet known if earlier detection actually improves people's chances of survival, because prostate cancer's slow growth means researchers need very long follow-up periods to see results.
PSA velocity. Doctors are increasingly using a measure called PSA velocity or series. This is the rate at which a man's PSA level increases over time. PSA scores tend to rise more rapidly in men with cancer than in those with BPH. A 2004 study in The New England Journal of Medicine showed that men with prostate cancer who have rapid increases in PSA were more likely to die from the cancer than those with slower-rising levels.
Free PSA. Doctors sometimes look for the level of "free" PSA. PSA protein circulates in the blood in two forms, either bound to other proteins or unbound (free). Several studies suggest that men with elevated PSA levels who have a very low percentage of free PSA are more likely to have prostate cancer than a benign prostate condition. The free PSA level doesn't give a definitive answer, but it may be useful when considering whether a biopsy is the appropriate next step. Still, even men with a low percentage of free PSA have only about a 10% chance of having prostate cancer. Most physicians find that PSA velocity is better than free PSA at predicting the need for biopsy. Therefore, if the patient has had several PSA tests over a period of years, doctors are less likely to test for free PSA.
The American Cancer Society advises doctors to offer their patients annual PSA tests beginning at age 50, or age 45 for men at high risk (see "Are you at high risk for prostate cancer?"). The American Urological Association likewise endorses this view.
On the other hand, the American College of Physicians and the American College of Preventive Medicine advise against routine screening. But they do recommend that physicians run through the potential benefits and drawbacks of the PSA test with their patients to help them decide whether to be screened (see "What you should know about PSA screening").
What you should know about PSA screeningProstate cancer is extremely common, yet only 3% to 4% of men die of it. Screening doesn't lower your risk of having prostate cancer; it increases the chance you'll find out you have it. PSA testing can detect early-stage cancers that a digital rectal examination (DRE) would miss. A normal PSA level of 4 ng/ml or below doesn't guarantee that you are cancer-free. A high PSA level may prompt you to seek treatment resulting in possible urinary and sexual side effects. Other conditions, like BPH and prostatitis, can also elevate your PSA level. |
Newer screening tests
Researchers are developing more screening tests for prostate cancer. Like the PSA test, they rely on biomarkers, such as antigens or proteins, which are elevated or may only be present in men who have prostate cancer. The hope is that these newer tests will better detect existing cancers (better sensitivity), and will not raise the alarm for cancer when it is not present (better specificity).
Prostate-specific membrane antigen (PSMA). This substance is found in all prostate cells. PSMA levels are higher in men with prostate cancer, but also increase as men get older. The PSMA blood test has been found to be highly sensitive, but as a screening tool, the PSMA has not proved superior to the PSA test. The PSMA test is currently used as part of an imaging scan to determine whether prostate cancer has spread to other areas of the body. It has also shown promise as a predictor of recurrent prostate cancer.
Early prostate cancer antigen (EPCA-2). This test is for a protein that is present in the nucleus of cancerous cells. Small amounts of EPCA-2 leak into the bloodstream, so EPCA-2 can be measured with a blood test. In April 2007, a study in the journal Urology reported that the EPCA-2 test is highly sensitive for prostate cancer. What's more, the test was able to accurately distinguish between cancer that was confined to the prostate and cancer that had spread beyond the prostate. On the downside, eight of the 35 men in the study with BPH (but no cancer) were identified as having elevated levels of EPCA-2, suggesting that false positives may be a problem. Over all, the data were preliminary, but promising. The EPCA-2 test is still in clinical trials and is not yet available to the public.
Autoantibodies. Sometimes the amounts of proteins produced in cancerous cells are too small to measure in a blood test. But these proteins can function as antigens, triggering the body's immune system to produce relatively large amounts of antibodies that can be measured in blood samples. Antibodies that attack the body's own proteins are called autoantibodies. Researchers at Harvard and the University of Michigan have identified an autoantibody signature (essentially a fingerprint) of autoantibodies produced against prostate cancer proteins. The autoantibody signature test holds great promise, but the research is still in the very early stages.
Ultrasound
Transrectal ultrasonography (TRUS) is a way of creating an image of the prostate gland using sound waves. In conventional ultrasound procedures, a probe placed against the skin sends painless, ultra-high-frequency sound waves into the body. As the waves strike internal organs, they produce echo patterns that a computer converts into images (sonograms) on a video screen.
Ultrasound tests are much more sophisticated than they used to be. For example, in TRUS, the doctor places a probe, called the ultrasound transducer, into the rectum. Painless sound waves scan the prostate gland in two planes. The resulting pictures (see Figure 3) often serve as a guide for a biopsy of the prostate, helping to pinpoint suspicious areas.
Doctors may recommend TRUS when they suspect prostate cancer based on an abnormal DRE or an elevated PSA. However, TRUS is costly, and it rarely detects prostate cancers that a DRE or PSA cannot find. As a result, although TRUS is commonly used to guide a biopsy, it's not recommended for routine screening.
Figure 3: Transrectal ultrasonography
In this procedure, the patient lies on his side and a small probe (the ultrasound transducer) is placed in the rectum. Sound waves are beamed at the prostate, and the waves that are reflected back are transformed by a computer into images on a video screen. In the sonograph above, the irregular dark area within the prostate is a cancer. |
Magnetic resonance imaging (MRI)
This imaging exam is not used for routine screening, but it can be useful in determining the stage of prostate cancer and also whether lymph nodes might be positive for cancer. For prostate imaging, MRI can be combined with a related imaging technology called magnetic resonance spectroscopy for more accurate results.
Biopsy
This procedure is useful when a DRE or PSA test raises suspicions about cancer. It entails removing bits of tissue from the prostate to be examined in the laboratory for cancerous cells. The procedure takes very little time.
While you lie on your side, the doctor inserts an ultrasound probe into the rectum and scans the prostate. A spring-loaded device rapidly inserts a needle through the rectal wall into the prostate and retrieves a tiny tissue sample. The doctor uses the ultrasound image as a guide in taking biopsies from specific areas. Usually, six to 12 samples are obtained. The doctor may inject an anesthetic into the prostate after the ultrasound probe is introduced and before the biopsies are taken, again guided by the ultrasound image. Although you may feel a pinch, no additional anesthesia is necessary.
Taking an antibiotic before and after the procedure reduces the risk for infection. Avoiding aspirin and similar drugs, such as ibuprofen (Advil, Motrin) and naproxen (Aleve), for a week before the biopsy protects against excessive bleeding, which is a possible complication.
Review Date: 2008-03-01
Harvard Medical School does not endorse products or services.
preview