Screening for Cancer

Screening tests are used to detect the possibility that a disease is present before symptoms occur. Screening tests usually are not definitive. Results are confirmed or disproved with further examinations and tests.

Diagnostic tests are done once a doctor suspects that a person has cancer (see also Diagnosis of Cancer).

Doctors determine whether a particular person is at special risk of cancer—because of age, sex, family history, previous history, or lifestyle factors—before they choose to do screening tests. The American Cancer Society has provided cancer screening guidelines that are widely used. Other groups have also developed screening guidelines. Sometimes recommendations vary among different groups, depending on how the groups' experts weigh the relative strength and importance of available scientific evidence.

Although screening tests can help save lives, test results can be falsely positive or falsely negative:

• False-positive results: Results that suggest a cancer is present when it actually is not

• False-negative results: Results that show no hint of a cancer that is actually present

False-positive results can create undue psychologic stress and can lead to other tests that are invasive or expensive. False-negative results can give a false sense of security, even though a person has a cancer. For these reasons, there are only a small number of screening tests that are considered reliable enough for doctors to use routinely.

In women, widely used screening tests are the Papanicolaou (Pap) test and tests for high-risk human papillomavirus (HPV) subtypes to detect cervical cancer and mammography to detect breast cancer. Both screening tests have been successful in reducing the death rates from these cancers in certain age groups.

In men, prostate-specific antigen (PSA) levels in the blood may be used to screen for prostate cancer. PSA levels are often high in men with prostate cancer, but levels also are elevated in men with noncancerous (benign) enlargement of the prostate. As such, the main drawback to its use as a screening test is the large number of false-positive results, which generally lead to more invasive tests such as a prostate biopsy. And doctors are now realizing that not all prostate cancers found on biopsy will go on to cause problems. Whether the PSA test should be used routinely to screen for prostate cancer is unresolved, with varying recommendations from different groups. Men should discuss the PSA test with their doctor.

Several tests may be used to screen for colon cancer, people should discuss which test to use with their doctor. A common screening test for colon cancer involves checking the stool for blood that cannot be seen by the naked eye (occult blood). Finding occult blood in the stool is an indication that something is wrong somewhere in the digestive tract. The problem may be cancer, although many other disorders, such as ulcers, hemorrhoids, diverticulosis (small pouches in the colon wall), and abnormal blood vessels in the intestinal walls, can also cause small amounts of blood to leak into the stool. In addition, taking an aspirin or another nonsteroidal anti-inflammatory drug (NSAID) or even eating red meat can temporarily cause a positive test result. Another test checks for abnormal DNA in stool that has come from a colon cancer. Outpatient procedures such as sigmoidoscopy, colonoscopy, and a special type of computed tomography (CT) of the colon (CT colonography) are also often used for colon cancer screening.

Lung cancer screening with a lung CT is available for people who currently smoke cigarettes or smoked within the past 15 years. The risks and benefits of lung cancer screening should be discussed with a doctor.

Routine self-examination for signs of cancer has sometimes been recommended. However, except possibly for testicular cancer, home screening with self-examinations has not been proved to be effective in identifying cancer, so even if people do examinations at home it is important to also follow recommendations for screening tests.

Some screening tests can be done at home, such as checking the stool for blood by putting a small amount of stool on a special card and mailing it to a laboratory to be processed. An abnormal result should prompt a visit to the doctor for confirmation.

Tumor markers are substances secreted into the bloodstream by certain tumors. It was first thought that measuring levels of these markers would be an excellent way to screen asymptomatic people for cancer. However, tumor markers are often present to some extent in the blood of people who do not have cancer. Finding a tumor marker does not necessarily mean a person has cancer, and tumor markers have a very limited role in cancer screening.