Laboratory Studies
No laboratory test can determine definitively whether a given patient has a colonic polyp. A stool occult blood test can detect a fraction (20-40%) of colonic polyps that are larger than 10 mm in diameter but, it may also indicate other causes of gastrointestinal blood loss.
Anemia is not specific for colonic polyps but can be an indication of their presence.
A patient with a family history of familial adenomatous polyposis (FAP) may inherit a mutation in the APC gene. A blood test may detect this heterozygous state. In addition, because most APC mutations involve truncations of the protein, an in vitro protein truncation assay has been developed by Powell et al. [13] This assay amplifies segments of APC messenger RNA (mRNA) and expresses the protein parts in vitro to readily detect the truncated products. A positive test only indicates susceptibility, not the actual presence of a colonic polyp.
Genetic testing of blood samples can also detect most cases of hereditary nonpolyposis colorectal cancer (HNPCC)/Lynch syndrome. Despite the name, patients with HNPCC have multiple polyps but much fewer than that seen in patients with the APC syndrome.
A potential precolonoscopy screening diagnostic marker for colorectal cancer is fecal glycolytic pyruvate kinase isoenzyme type M2 dimer (M2PK), but more investigation is needed. [14] In a study comprising 20 patients with colorectal cancer, 20 with adenomatous colorectal polyps, and 20 normal subjects, fecal M2PK was the most sensitive and specific test for differentiating between colorectal cancer and adenomatous colorectal polyps and the control subjects. The study also assessed complete blood cell count, erythrocyte sedimentation rate, carcinoembryonic antigen, cancer antigen 19-9, fecal occult blood test, abdominopelvic ultrasonography, colonoscopy, and histopathology. [14]
Imaging Studies
Air contrast barium enema
An air contrast barium enema study can detect larger colonic polyps, but it can also miss smaller ones. This imaging modality has a low false-positive rate. In one study, air contrast barium enema detected only about 50% of colonic polyps larger than 1 cm in diameter.
Virtual colonoscopy
Virtual colonoscopy is performed by computed tomography (CT) scanning (or magnetic resonance imaging [MRI]), and it has shown promise in research studies, detecting more than 80% of large polyps. In a large, multicenter trial, however, a disappointing sensitivity of only 55% was obtained for colonic polyps 10 mm or larger in diameter. Another trial found a detection rate for CT scanning that was comparable to colonoscopy, although some methodological issues have been raised regarding this study. [15]
Virtual colonoscopy is beginning to be performed for screening outside of research settings on limited numbers of patients. Relatively recent data from such screening procedures suggest that virtual colonoscopy results in the detection of far fewer polyps that are smaller than 1 centimeter in diameter than standard optical colonoscopy. [16] Most small polyps have very benign characteristics at the time of removal and may be clinically insignificant. However, some are presumably precursors to advanced polyps. Therefore, their clinical significance, particularly for screening intervals, needs better definition.
The main drawback of virtual colonoscopy is that a second procedure, an optical colonoscopy, is required to remove the detected colonic polyps. Whether the radiation exposure from CT scanning may be significant is debated. [17, 18]
In most methods, a thorough colon preparation is required. Methods are under development to label stool with barium meals, obviating the need for cathartic bowel prep.
Procedures
Bowel cleansing
Adequate bowel cleansing is necessary prior to many procedures.
Several preparations are marketed for bowel cleansing (eg, polyethylene glycol 3350 [GoLYTELY, NuLYTELY, HalfLYTELY], magnesium citrate [Citroma], senna [X-Prep]) in preparing patients for gastrointestinal procedures, such as colonoscopy and barium x-ray studies. More recent experience suggests that splitting the preparation in half, with one portion taken the day before the procedure and the second portion taken early on the day of the procedure (eg, completed at least 4-6 h before the procedure) yields more effective cleansing of the right colon.
Bowel cleansing preparations may be used with various dietary modifications (eg, clear liquid diet the day before surgery or procedure) and are convenient to administer on an outpatient basis. Nonetheless, distaste for or discomfort from the preparation remains a complaint of some patients.
Colonoscopy
Colonoscopy is the preferred test to detect colonic polyps, obtain biopsies, and/or perform endoscopic resection of polyps. [2] Sensitivities for large colonic polyps in the 80%-90% range have been reported.
Although flexible sigmoidoscopy and stool tests for occult blood have been the mainstays of screening to prevent colon cancer, some clinicians now favor colonoscopy as a primary screening tool. [1, 3] Colonoscopy is the preferred tool in patients with defined polyposis or colorectal-cancer syndromes or in patients with a marked family history of colorectal cancer.
Flexible sigmoidoscopy
Flexible sigmoidoscopy is a good screening test and is the only procedure or imaging modality to be validated by studies that document a decrease in colorectal cancer mortality. [3] However, this procedure does not examine the entire colon. Studies indicate that the majority of large adenomatous polyps in women will be missed by using flexible sigmoidoscopy alone.
Screening is usually begun at age 50 years in patients who are at an average risk.
Randomized controlled trials have documented a reduction in mortality from colon cancer in populations screened by flexible sigmoidoscopy. However, studies suggest that about 40% of high-risk proximal adenomas remain undetected when this procedure is used as the primary screening modality.
Capsule endoscopy
An ingestible, camera-equipped capsule developed as a means of exploring the gastrointestinal tract was approved by the FDA in 2014 for the detection of colon polyps in patents who have had an incomplete optical colonoscopy. [4, 5] The device, a 12 x 33 mm capsule equipped with two miniature color video cameras, transmits images for approximately 10 hours. A study by Hagel et al demonstrated that the capsule had an 81.5% accuracy in detecting polyp carriers (per-patient analysis). [4, 19]
Diagnostic rates for colon capsule endoscopy in the detection of colonic polyps and colorectal cancer have improved, and this modality also has a role in completing the evaluation of colonic pathology in patients in whom colonoscopy was incomplete. [20]
Second-generation colon capsule endoscopy has a significantly higher sensitivity than the first-generation capsule in detecting colon polyps and is FDA approved as an option for colorectal cancer screening in those with a previous incomplete colonoscopy as well as in those with suspected lower gastrointestinal bleeding. [21] However, thorough, rigorous bowel preparation is mandatory for optimal effectiveness. [5]
This procedure is currently not endorsed for colon polyp screening. Van Gossum et al compared the efficacy of this device with that of colonoscopy in the detection of colorectal polyps, advanced adenomas, and cancer. [22] In a study of 328 patients with known or suspected colonic disease, the authors found that capsule endoscopy had a sensitivity and specificity of 64% and 84%, respectively, for the detection of polyps that were 6 mm or more in size. Sensitivity and specificity for the detection of advanced adenomas were 73% and 79%, respectively. Capsule endoscopy detected 14 of 19 cancers that had been found using colonoscopy.
The investigators also determined that in patients with good or excellent colon cleanliness, capsule endoscopy's sensitivity was higher for all lesions than it was in individuals whose colon cleanliness was fair or poor. Van Gossum and colleagues concluded that in comparison with colonoscopy, capsule endoscopy has a low sensitivity for the detection of colonic lesions.
Stool DNA studies
Tests have been developed that detect mutant, fragmented, and/or methylated DNA from exfoliated colon tumor cells in stool. These tests have shown the ability to detect a substantial fraction of tumors in clinical trials [23] but are expensive and appear to be less sensitive than colonoscopy.
Histologic Findings and Staging
Adenomatous polyps are of three different histologic types, as follows: tubular, villous, and tubulovillous. Adenomatous polyps may show changes of dysplasia, which distinguish them from hyperplastic polyps. The most common benign polyp is hyperplastic.
Colonic polyps are typically benign. Colonic polyps that contain high-grade dysplasia or microinvasive cancer confined to the mucosa are often termed carcinoma in situ.
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Familial adenomatous polyposis, total colectomy specimen. The colonic mucosa is studded with innumerable sessile and small pedunculated polyps, which involve the entire length of the specimen.
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Multiple large polyps in the colon. The polyp in the center of the image is situated on a stalk. (Endoscopic image).
