Colon Cancer affects 1 of every 1000 Canadians, and on Prince Edward Island, the numbers are 1 for every 1526 people in rural areas and 1 for every 2024 people in urban areas (PEI Colorectal Cancer Fact Sheet 2011). Unfortunately women have more colon cancer than men on the island (PEI Colon Cancer Facts 2010). It is the third most common cancer diagnosis on PEI, and thankfully screening has been shown to reduce death from this cancer dramatically.
Ok, you got my attention, how do we screen for this? For folks of average risk (see below for definition of average risk), there are several options. The Canadian Association of Gastroenterology and the Canadian Digestive Health Foundation make the following recommendations for screening: Fecal Occult Blood Testing/Fecal Immunochemical Testing, Flexible Sigmoidoscopy every 5 years (with or without Occult Blood Testing), Double Contrast Barium Enema every years, or Colonoscopy every 10 years. Because the risk for cancer becomes 1 person out of 125 between the ages of 50-59 and 1 out of 50 in the 60 to 69 age group, these are the folks that get the focus of attention (Canadian Journal of Gastroenterology 2004 Feb;18(2):93). Let’s talk about each of these screening tools.
Fecal Occult Blood/Immunochemical Testing: This is the least invasive screening test that is currently available for colon cancer screening available today. In this test, you collect three small samples of stool and they are tested for the presence of blood in the stool. The rational behind this test is that colon cancers and polyps have a tendency to bleed, so looking for blood in the stool is a way to look for the needle in a haystack. Because the bleeding may not be continuous, there need to be three samples, and the test needs to occur every one to two years (Cochrane Library 2007 Issue 1:CD001216 or Ann Intern Med 2009 Feb 3;150(3):162).
Flexible Sigmoidoscopy: In this test, a flexible scope is inserted into the colon from the rectum up to the transverse colon. It requires the patient to perform a ‘clean out’ so that the colon can be seen through the scope. This is not the entire colon, but it represents 50% of the colon. About 10% of patients who undergo sigmoidoscopy will have an adenomatous polyp (that is a pre-cancerous polyp). It is estimated that a single screening flexible sigmoidoscopy will find cancer on 0.33-0.7% of the time and polyps 10-12% of the time (Lancet 2002 Apr 13;359(9314):1291 or Gut 1998 Apr;42(4):560). There is some disagreement over the frequency of screening when Flexible Sigmoidoscopy is used and so the current recommendation is once every 3-5 years. This screening is not without some risks, as there is a risk of injuring the colon by the scope during the procedures (0.88 per 1000 sigmoidoscopies BMJ 2003 Aug 23;327(7412):404).
Colonoscopy: In this test, a flexible scope is inserted into the colon from the rectum through the entire colon. Just like the flexible sigmoidoscopy, the test requires a patient to ‘clean’ the colon prior to the procedure. Unlike the flexible sigmoidoscopy, this test typically requires that the patient receive some sedation in order to tolerate the procedure. Because this test visualizes the entire colon, it is recommended once every 10 years. There is a risk of perforation with colonoscopy, a bit more than with Flexible Sigmoidoscopy (1.96 per 1,000 colonoscopies). The rates of adenomatous polyps found on Colonoscopy is about the same as flexible sigmoidoscopy (N Engl J Med 2001 Aug 23;345(8):555).
Which test is best? This is a harder thing to quantify. Remember, the purpose of a screening test is to help find the needle in the haystack without missing a single needle, or burning down the haystack. These tests are not designed to definitively identify cancer, they are designed to identify patients who need closer study, that is, those who are at higher risk. The stool testing is geared toward not missing anyone who might have a cancer, so it is geared toward potentially being positive when someone is healthy in order to avoid being negative when someone has a cancer. what this means in practical terms is that when you have a stool test that comes back positive, you have between a 6-40% chance of having a cancer or a large polyp when the diagnostic test is performed. Another way to look at it, for every 50 tests that are positive, one will be due to a colon cancer (Ann Intern Med 1997 May 15;126(10):811 or Am Fam Physician 1999 May 15;59(10):2714).
Who is considered high risk? Patients who have had a positive screening test are considered high risk until they have had a negative colonoscopy. Additionally, patients who have a first degree relative (mother, father, brother, sister) who has colon cancer are considered high risk and should undergo colonoscopy (J Clin Oncol 2009 Feb 10;27(5):686). Patients who have had an adenomatous or hyperplastic polyp on a previous colonoscopy are considered higher risk as well. Patients with a family history or Familial Polyposis have 2-3 times increased risk for colon cancer and should undergo colonoscopy for their screening test (J Clin Epidemiol 2006 Feb;59(2):114).
Remember, that once you have had a normal colonoscopy, you do not need a repeat colonoscopy for 10 years. If a polyp is found, then you need a repeat colonoscopy after 5 years.
For more information on screening check out these resources: