Better Science Screening for Colon Cancer By Beverly Rockhill, PhD In the middle of this century, infectious diseases waned as major determinants of morbidity and mortality in the US, and chronic diseases became prominent. In the 1940s and 1950s, epidemiologists began to shift attention to the new, major, health menaces: cardiovascular diseases and the major cancers, including cancers of the lung, colon, and breast. Decades of epidemiologic research have repeatedly shown that these diseases are multicausal in nature, and that risk of these diseases is directly affected by behaviors that individuals have control over. In a series beginning in this newsletter, I will review key behavioral characteristics that exert a strong influence on health: tobacco use, alcohol consumption, weight/weight gain, diet, physical activity, and disease screening behavior. For this first installment in the series on what individuals can do to lower their risk of common and potentially deadly diseases, I focus on screening for colorectal cancer. Screening in General Screening asymptomatic persons to detect preclinical disease has become an important part of public health. Screening for preclinical disease makes sense only if treatment initiated earlier in the disease process reduces suffering and risk of death from the disease; there is obviously no value to simply living longer with a diagnosis if actual length of life is not extended. Although certain screening tests can be highly effective in reducing morbidity and mortality, others are of unproven benefit (for instance, screening for prostate cancer with the PSA, or prostate-specific antigen, screening test), and some which had long been believed by scientists to be of benefit are now being questioned (i.e., screening for breast cancer with mammography). I will talk about these screening tests in future columns. Choosing a Screening Test The selection of appropriate screening tests for a given individual depends primarily on the individual's age and sex. In addition, consideration of individual risk factors, such as family history of disease and certain lifestyle factors, is often used to determine appropriate screening tests and appropriate frequency of testing. In 1984, the U.S. Public Health Service commissioned the U.S. Preventive Services Task Force. This panel was charged with developing recommendations for clinicians on the appropriate use of preventive interventions, including screening. In 1989, the first Guide to Clinical Preventive Service was published. The second edition was published in 1996, and is on-line at http://odphp.osophs.dhhs.gov/pubs/guidecps/. For anyone interested in reading about the scientific evidence on different screening tests and clinical interventions, and how groups of physicians arrive at a consensus about guidelines for testing and counseling individual patients, this book is very useful. Screening Test Efficacy Requirements According to the U.S. Preventive Services Task Force, a screening test must satisfy two major requirements to be considered effective for use in a population: the test must be able to detect the target condition earlier than without screening and with sufficient accuracy to avoid producing large numbers of false-positive and false-negative results (a false-positive is someone who is told they have an abnormal screening test and need further diagnostic work-up, even though they really do not have the condition being screened for; a false-negative is a person who really does have the underlying disease being screened for, but who has a normal result on a screening test); and Screening for and treating persons with early disease should improve the likelihood of favorable health outcomes (e.g., reduced suffering and risk of death) compared to treating patients when they present with signs or symptoms of the disease. In other words, if there is nothing to be gained by catching a disease early, because nothing can be done anyway, there is little point to screening for preclinical disease. Colorectal Cancer Screening Colorectal cancer is the second-most common form of cancer in the United States, after lung cancer, and is the second leading cause of cancer death. Each year, about 140,000 new cases are diagnosed, and 55,000 persons die of the disease(4). About half of these cases occur in women; colon cancer is not only a man's disease, as some women may believe. The lifetime risk of dying of colorectal cancer in the US is estimated to be about 2.6%--this means that out of every 100 people who live to be 85 years of age, approximately 3 of them will get colon cancer sometime in their lifetime. It is important to catch colorectal cancer early, as survival is good when the disease is caught before it has spread. The estimated five-year survival rate is 91% in persons with localized disease (disease that has not spread beyond the immediate area of the tumor), 60% in persons with regional metastases (where the tumor has spread to regions that are relatively close to the area of the original tumor), and a very low 6% in those with distant metastases-this means that for those who are diagnosed with colorectal cancer that has already spread throughout their body, only 6% will be alive in 5 years. If everyone was screened for colorectal cancer, theoretically there would be no diagnoses of late-stage colorectal cancer; all cases would be picked up in the early stages.   Almost two-thirds of persons newly diagnosed with colorectal cancer have regional or distant metastases. The average patient who dies of colorectal cancer loses 13 years of life, and in addition to the mortality associated with this disease, its treatment can produce significant pain and discomfort. Screening for early-stage colorectal cancer as well as for polyps that have not yet become cancerous has the potential, therefore, to significantly reduce morbidity and mortality associated with colorectal cancer. If everyone was screened for this disease, theoretically there would be no diagnoses of late-stage colorectal cancer; all cases would be picked up in the early stages. Some colorectal cancer screening tests can serve two purposes at the same time, unlike many other screening tests-they have the ability to detect, and, in the case of sigmoidoscopy and colonoscopy, to remove precancerous polyps and early-stage cancerous tumors. Two Types of Colorectal Cancer Tests The principal tests for detecting polyps and early-stage cancer in healthy people who have no symptoms of disease are the fecal occult blood test (FOBT) and flexible sigmoidoscopy. Fecal occult blood test (FOBT) — FOBT is based on the premise that precancerous polyps and early stage cancers will bleed more than normal colonic mucosa. If an FOBT test comes back positive, indicating occult (not visible) blood in the stool, the doctor will recommend a full examination of the colon with a colonoscopy (described below). It is important to keep in mind, though, that there are various conditions that can lead to occult blood in the stool, including aspirin use, diets high in red meat, stomach ulcers, and so forth. Thus, the main problem with the FOBT test is that it leads to a lot of false-positives-there are a lot of people who test positive on the FOBT test, and who then go on to get a colonoscopy, who do not have any signs of colorectal cancer. Flexible sigmoidoscopy — involves the insertion of a flexible fiberoptic scope into the colon. There are two lengths of scopes, 35 cm and 60 cm. The longer scope can view a greater portion of the colon and can therefore detect more polyps and tumors. Unlike FOBT, flexible sigmoidoscopy is both a screening and diagnostic intervention tool; any polyps detected can be biopsied and removed during the procedure. When a sigmoidoscopic examination shows polyps or tumors, a colonoscopy (an examination with a similar type of scope that visualizes the entire length of the colon) is performed. Who Should be Screened? Screening for colorectal cancer is recommended for all persons aged 50 years and older by a variety of groups, though there is no consensus on whether FOBT or sigmoidoscopy or a combination of the two produces the greatest benefit. For persons with a family history of colorectal cancer, screening is recommended to begin at an earlier age, particularly if the family member was diagnosed with colorectal cancer at a young age. For persons with a family history of hereditary syndromes associated with very high risk of colorectal cancer, and those with a previous diagnosis of high-risk polyps or colon cancer, regular screening with colonoscopy (at least once a year) is part of routine management. For more detailed information on these screening tests, visit the Guide to Clinical Preventive Services at http://odphp.osophs.dhhs.gov/pubs/guidecps/

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