Understanding colon cancer screening tests

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Understanding colon cancer screening tests
Understanding colon cancer screening tests

Judy Barnes is a 52-year-old, African-American woman with no chronic medical problems. During a routine exam, she says she saw Katie Couric's televised colonoscopy on the Today show. She asks: “Do I need a colonoscopy?”
Possible responses:

(a) “You don't have to worry about that. Colon cancer is mainly a man's disease.”

(b) “Yes, everyone should have a colonoscopy every year to check for colon cancer.”

(c) “I'll do a rectal exam today, and if there's no sign of blood, that's all you need.”

(d) “There are a number of different screening tests for colon cancer. Let's talk about them, then decide what's right for you.”

What would you tell Ms. Barnes?

Cancers of the colon and rectum combined comprise the third most common cancer diagnosis in U.S. men and women and the second leading cause of cancer death in the United States. Because preventive approaches, risk factors, and prognosis are similar for these diseases, they are commonly grouped under the broad heading of “colorectal cancer” (CRC).

The American Cancer Society (ACS) estimates that in 2004, there will be 146,940 new cases of CRC diagnosed and 56,700 deaths, accounting for 10% of all cancer deaths. New cases and deaths are equally distributed among men and women.

Risk factors

A number of factors increase an individual's risk of developing CRC. Chief among these are familial syndromes (familial adenomatous polyposis, hereditary nonpolyposis colon cancer, and Gardner's syndrome), inflammatory bowel disease and a personal or family history of adenomatous polyps or colon cancer.

Individuals with any of these risk factors require early and aggressive screening and monitoring. However, all of these conditions account for, at most, 20% of CRC cases. The remaining cases (approximately 120,000 annually) occur in individuals whose only identified risk is their age.

The incidence of CRC begins to rise rapidly around age 50 and continues to climb through the remaining years of life.

Approximately 90% of cases occur after age 50. Because of the high prevalence of the disease in that population, the existence of effective screening measures, and the high potential to prevent the disease and improve survival, the ACS and all other major U.S. guidelines-issuing organizations recommend routine CRC screening for average-risk individuals beginning at age 50. 

A significant body of evidence supports the concept that most CRCs arise from a nonmalignant lesion — the adenomatous polyp (adenoma). The progression from adenoma to cancer is thought to occur in most cases over a period of five to 10 years. Detection and removal of adenomas during this premalignant phase have been shown to markedly decrease the incidence of CRC.

All of the currently recommended screening tests for CRC also detect polyps, albeit at widely varying rates. Thus, screening for CRC holds a tremendous advantage over screening for most other cancers (i.e., breast or prostate cancer) — the potential to prevent cancer from developing.  

Outcomes from CRC vary markedly, depending on the disease stage at the time of diagnosis. When CRC is detected at an early, localized stage, the five-year survival rate is 90%. Unfortunately, only 37% of CRCs are currently diagnosed at this stage. With spread of the disease to adjacent organs and/or lymph nodes, five-year survival falls to 64%, and once distant metastasis occurs, this rate plunges to <10%.

Despite the wealth of evidence demonstrating the benefits of screening and unanimous endorsement by medical authorities, CRC screening rates remain disturbingly low. Reports from the CDC and the National Cancer Institute indicate that fewer than half of at-risk adults have ever been screened for CRC, and only about one third are up to date with their screening. This lags far behind the 75%-80% reported adherence rates for breast cancer screening.

In 2008, the United States Preventive Services Task Force People (USPSTF) recommended that clinicians cease CRC screening in patients older than 75 years who have been getting regular colon cancer screening since age 50 and who have had consistently negative screenings — no polyps (adenomas) or colon cancer.

The upper age limit was set after studies determined that the net benefit of screening after age 75 was small. However, the USPSTF guidelines do recommend colon cancer surveillance for people over age 75 who have an increased risk of colon cancer, such as family history, a previously diagnosed colon cancer or adenomatous polyps.Currently, the American Cancer Society and the American College of Gastroenterology do not specify an upper age limit beyond which colon cancer screening is no longer recommended.

Screening methods

There is currently no single “best” test to screen for CRC. Four methods are mentioned in the recommendations published by all major medical organizations: fecal occult blood testing, flexible sigmoidoscopy, colonoscopy, and double-contrast barium enema.

Each of the tests has unique requirements and performance characteristics. Patients also vary tremendously in their preference of testing method. Providers and patients must work together to make an informed choice. When considering the following options, the only “wrong” choice is choosing not to screen.

Fecal occult blood testing (FOBT): In the age of high-tech, gee-whiz medicine, FOBT seems often to be viewed as an outdated mode of CRC screening. In actuality, FOBT remains the most thoroughly researched approach to screening for CRC. It is the only one of the recommended methods that has been studied in large randomized, controlled trials and the only method proven to decrease both the incidence of CRC and death from the disease.

FOBTs look for the presence of blood in the stool. Currently marketed tests rely on one of two methods: guaiac and immunochemical. In the United States, a majority of FOBTs performed use the guaiac-based method. The tests consist of guaiac-impregnated cards that when treated with a developer containing hydrogen peroxidase give a color-coded result in the presence of the peroxidaselike activity of heme in the stool.

However, these tests can yield false-positive results if certain foods (meat, some raw fruits and vegetables), vitamins (vitamin C tablets or foods high in vitamin C) or medications (nonsteroidal anti-inflammatory drugs) are ingested in the days before the test.

An alternate approach to FOBT utilizes an immunochemical reaction to detect the globin portion of the hemoglobin molecule. Since globin does not survive passage through the upper GI tract, the test is specific for occult bleeding in the colon and rectum. No dietary or medication restrictions are required prior to testing. A number of published reports suggest that immunochemical tests offer enhanced specificity in CRC compared with guaiac-based testing.

FOBT is effective because cancers and polyps often bleed. Since this bleeding is usually intermittent, the tests need to be repeated to minimize the possibility of missing blood in the stool. FOBT derives its benefit not from onetime use, but by its use in a program of annual screening.

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