Colon cancer screening and prevention

Dayna S. Early, M.D.

Colon cancer is an important health problem in the United States. By the time colon cancer becomes symptomatic, it can be advanced and may have metastasized to other organs. Therefore, efforts are focusing on ways to detect colon cancer at an early stage, at a premalignant stage (colon polyp) or to prevent it entirely.

Nearly all colon cancers develop from colon polyps. Several genetic mutations occur in the colonic mucosa that promote the progression from benign to malignant tissue. If polyps are detected before they become malignant, they can be removed and colon cancer can be prevented.

Colon cancer prevention is being addressed in many ways at Ellis Fischel Cancer Center. Cancer Screening Services for men and women offers a colon cancer screening program that follows the recom-mendations of the American Cancer Society.

In addition to screening services, Ellis Fischel is researching cancer prevention with two ongoing studies. The first study is addressing whether aspirin affects the rate at which colon polyps develop. A second study is examining what, if any, relationship exists between an individual's risk of developing colon polyps or cancer and their selenium status.

Colon cancer screening methods
Current methods of colon cancer screening include fecal occult blood testing (FOBT) and flexible sigmoidoscopy. Ellis Fischel's Cancer Screening Services follows American Cancer Society guidelines that recommend FOBT every year and a flexible sigmoidoscopy exam every three to five years. These recommendations apply to asymptomatic individuals who are older than 50 and have no personal or family history of colon cancer or colon polyps. Individuals with a positive screening test should undergo further evaluation, which generally includes a full colonoscopy.

Studies show sigmoidoscopy and FOBT can reduce the incidence of colon cancer if individuals with positive test results receive further evaluation. The main disadvantage of flexible sigmoid-oscopy is that it can only detect lesions in the distal one fourth to one third of the colon. On the other hand, FOBT can detect occult bleeding from a polyp or cancer anywhere in the colon, but results may be negative if lesions are not bleeding at the time of the test.

Molecular pathology in cancer surveillance
Applying molecular pathology to colon cancer screening may overcome some disadvantages of flexible sigmoidoscopy and FOBT. Many genetic mutations that cause benign colon polyps to become cancerous have been identified. If these genetic mutations can be identified in patients, it would be possible to identify individuals who are at risk for developing colon cancer and target them for more intense colon cancer screening.

Recent reports in medical literature describe a technique in which stool samples are collected and analyzed for genetic mutations -- similar to the way stool samples are collected and analyzed for occult blood. Small studies show good correlation between the genetic mutations identified in DNA from stool samples and the genetic mutations present in the actual tumor tissue. Similar molecular techniques have been applied to other gastrointestinal cancers, such as pancreatic cancer, gastric cancer and esophageal cancer.

While these techniques are still investi-gational, it seems likely that molecular pathology will play an important role in cancer surveillance in the near future.

Aspirin and colon cancer prevention
A large study sponsored by the American Cancer Society showed that aspirin reduces colon cancer deaths. To address whether aspirin prevents colon polyps, Ellis Fischel is participating in a study sponsored by the Cancer and Leukemia Group B. Patients eligible for the study must:

Have a history of Duke's A or B1 colon cancer or
Have Duke's B2 or C colon cancer and have been disease free for at least five years

Participants are randomized to receive aspirin or a placebo. They begin taking the study medication shortly after receiving a routine surveillance colonoscopy that removes all identified polyps. Three years later, participants undergo a follow-up colonoscopy to assess the incidence of adenomatous colon polyps in each group.

Selenium and colon cancer prevention
In addition to studying preventive medication, there is significant interest in the role of nutrition in cancer prevention. Much of this attention centers on anti-oxidant molecules, such as vitamin E, beta carotene, zinc and selenium. Anti-oxidant molecules may protect tissues from injury that could occur from unstable molecules produced during normal physiological processes at the cellular level. A deficiency of selenium, an antioxidant micronutrient, has been well characterized, but results of studies designed to detect any cancer preventive effect of selenium have been controversial.

An ongoing study at Ellis Fischel is examining the levels of certain selenium- containing proteins in different subsets of patients. In particular, this study is aimed at determining whether individuals without colon polyps or cancer have different selenium-containing protein levels than individuals with colon polyps and cancers.

Individuals presenting for colonoscopy are eligible for the study if they have no current diagnosis of cancer, if they are not taking regular vitamin supplementation and if they have no obvious evidence of nutritional deficiency. Those found at colonoscopy to have adenomatous polyps or colon cancer will be studied.

For more information regarding colon cancer screenings in average-risk or high-risk individuals, or to refer a patient for the aspirin study or selenoprotein study, please call Early at (573) 882-6163.