Screening for Colorectal Cancer
Colonoscopy is an endoscopic process using a lens that allows a physician to visualize the mucosa from the rectum to the start of the colon (ileo-cecal junction). Removal of adenomatous polyps during colonoscopy has been proven to lower the risk of colorectal cancer.49,50
Screening colonoscopies are recommended beginning at age 50, but those with any risk factors and/or a family history should consider screening at an earlier age.
How a colonoscopy is performed and by whom may influence whether or not adenomas or cancers are detected. During a 15-month period, analysis of 7,882 colonoscopies performed by 12 experienced gastroenterologists found that the time it took to withdraw the colonoscope influenced detection rates. Gastroenterologists who took less than 6 minutes to withdraw the scope were much less likely to detect cancer than those who withdrew the scope more slowly (up to over 16 minutes.). Even advanced cancers were more likely to be missed when the scope was withdrawn more quickly.51
The time of day the colonoscopy is performed may also influence its reliability. In a chart review of a total of 2,087 colonoscopies at Metro Health Medical Center in Cleveland, Ohio, those done in the afternoon had a significantly higher failure rate compared to those done in the morning.52 The "failure" of a colonoscopy means that the scope could not reach the start of the colon (the cecum). This incomplete look at the colon often necessitates repeating the scoping procedure or undergoing further imaging, such as a CT scan.
The rate of incomplete colonoscopies may be influenced by who performs the procedure. In a study designed specifically to look at factors that lead to incomplete colonoscopies, the elderly, females, and those that have had prior abdominal or pelvic surgeries are more likely to have an incomplete colonoscopic evaluation. In this same study, the researchers found that having the colonoscopy done in an office rather than hospital setting tripled the risk of new or missed colon cancer in men and doubled it in women.53
Computer Tomographic Colonoscopy
Computer tomographic colonoscopy (CTC) is sometimes referred to as a "virtual colonoscopy." It involves the use of CT imaging the colon. Preparation for CTC is much like a traditional colonoscopy with the use of laxatives to create an empty bowel. Carbon dioxide or air is infused through the rectum to create a smoother surface to assess. CTC's are useful for larger polyps but may not pick up smaller or flattened polyps as well as traditional colonoscopy. If any polyps or suspicious areas are seen on CTC, the patient must then undergo a colonoscopy to visually assess and/or remove the polyps.
CTC is limited in some extent relative to a traditional colonoscopy in that if a polyp is detected, it cannot be removed during the procedure. This is a disadvantage as the patient will then need to undergo a traditional colonoscopy following the CTC to remove the polyp. Another disadvantage of virtual colonoscopies is the high levels of radiation needed to perform the procedure.
Fecal Occult Blood Test
Fecal occult blood test (FOBT) Occult blood in the stool can be detected with a simple test and is recommended as routine screening for colorectal cancers. Long before blood can be seen by the naked eye, minute quantities may signify the presence of cancer. The association of a positive FOBT with actual colorectal cancer, however, is fairly low, only 10%.54 This is because occult blood more often comes from benign conditions, such as minor hemorrhoids; a FOBT may even detect bleeding associated with the upper gastrointestinal tract.
The FOBT is about 70% sensitive to the detection of colorectal cancer, while a colonoscopy performed by an experienced gastroenterologist is roughly 95% sensitive.55,56
Indirect Tests for Colon Cancer and Emerging Techniques
Colon Cancer Specific Antigens (CCSAs): A blood-based means of detecting colon cancer may be right around the corner. CCSAs are nuclear matrix proteins that are unique to colon cancer cells. When circulating, these CCSAs serve as a "fingerprint" indicating that either colon cancer or a premalignant adenoma is likely present.57 Circulating levels of several of the CCSAs, including CCSA-2, CCSA-3 and CCSA-4 have all been independently shown to be both sensitive and specific to colon cancer or premalignant adenomas.58,59 While this test is not commercially available yet, ongoing research is looking at optimizing combinations of the different CCSAs to predict the likelihood of colon cancer with great accuracy. In the future, this blood test may be used to gauge the urgency for colonoscopy screening.
Calprotectin in the stool has been used as a marker for IBD, and is a useful tool in determining the possibility of adenoma or colorectal cancer.60,61 Fecal calprotectin is a product of granulocyte formation, a hallmark of chronic inflammation, and as such is not specific to the cancerous process but indicates that inflammation is present. In one study, of the patients referred for colonoscopy due to abdominal symptoms, elevated calprotectin was found in 85% of those with colorectal cancer, 81% of those with IBD and only 37% of those with normal findings.62
Molecular markers in the stool. Since precancerous adenomas and colon cancer arise in the lining of the colon, the cells involved are shed with the stool on passing. With advances in technology and molecular biology, examining the stool for unique DNA changes that signify cancer is an area of interest.
The next generation of stool testing for colon cancer involves the stool DNA (sDNA) test, which was able to detect 64% of precancerous adenomas greater than 1 cm and 85% of colon cancers, and the fecal immunochemical test (FIT).63 A patented stool DNA test called PreGen-Plus is approximately 65% sensitive to the detection of colorectal cancers,64 but the high cost of this test may limit its utility for many consumers.
These non-invasive tests remain less sensitive than a colonoscopy, and have advantages and disadvantages that should be discussed with a healthcare provider.49