Colorectal cancer and screening

The principle behind the FOBT and other stool-based examinations is that invasive carcinoma will cause bleeding, which can be detected using methods, such as guaiac staining, immunohistochemistry, and DNA.

Small polyps tend not to bleed, and larger lesions tend to bleed intermittently. As a result, these tests are subject to sampling error and are more likely to detect cancer rather than precancerous lesions.

FOBT is supported by randomized controlled studies, which demonstrate that its use leads to cancers being detected at an earlier and more curable stage compared with no screening, leading to reductions in colorectal cancer mortality of 15% to 33%.

The limitations of stool testing include the necessity for annual testing, low individual test sensitivity, and the need to follow any positive test with an invasive test. Furthermore, the sensitivity is highly variable and depends on hydration status of the feces, brand and style of the test, as well as chosen target lesion (i.e., adenoma vs. carcinoma).

FSIG is an optical endoscopic procedure that examines the most distal portion of the colon lumen.

FSIG is associated with a 60% to 80% reduction in colorectal cancer mortality for the area of colon within its reach. Overall decreased incidence compared with an unscreened group also has been demonstrated. Its principal advantages over colonoscopy are that it requires a less extensive bowel preparation than a full colonic examination, and because it is less invasive than colonoscopy, sedation is not required. It also allows for concurrent sampling and/or removal of detected lesions.

Like colonoscopy, FSIG can be complicated by patient discomfort and, more importantly, bowel perforation. Its greatest limitation, however, is its failure to evaluate the entire colon.

The U.S. Multi-Society Task Force considers that either 5- or 10-year intervals are acceptable but favors 10-year intervals.

Colonoscopy is an optical procedure that evaluates the entire colon. It is one of the most commonly performed medical procedures. Patients require an extensive colonic cleansing preparation, and the procedure is performed under sedation. Because it allows for direct optical visualization, biopsies are often performed.

The greatest advantage to colonoscopy is that usually the entire colon can be screened and suspicious lesions sampled in a single visit. The seminal paper by Winawer and colleagues demonstrated a reduction in incidence of colon cancer of 76% to 90%.

Although no prospective randomized controlled trial has been performed to evaluate colonoscopy, its health benefits are undisputed. A U.S. Department of Veterans Affairs study demonstrated a 50% reduction in mortality when colonoscopy was performed on a symptomatic population.

The limitations of colonoscopy include the need for colon cleansing and patient sedation. Because of sedation, a patient chaperone is needed. Controlled studies have shown the colonoscopy miss rate for adenomas 10 mm or larger to be 6% to 12%.

The additional risks of colonoscopy related to sedation and biopsy include cardiopulmonary events, such as arrhythmia and hypotension, as well as postpolypectomy bleeding and perforation.

Colonoscopy every 10 years beginning at age 50 years (45 years in African Americans) is recommended as a screening option for colorectal cancer.

CTC is a minimally invasive examination that uses CT to acquire images of the colon and two-dimensional (2D) and three-dimensional (3D) displays for interpretation. The mechanism and technique are explained in detail later in this chapter.

CTC evaluates the entire colon, including those segments that are particularly redundant and difficult to evaluate with colonoscopy. In addition, the examination can be performed without sedation. The examination can also detect extracolonic findings. In one study, 9% of the total patients had clinically important extracolonic findings. Metaanalysis of 33 studies on nearly 6400 patients demonstrated an 85% to 93% sensitivity and 97% specificity for polyps 10 mm and larger. The 96% sensitivity of CTC for invasive cancer was comparable to that of OC.

One of the greatest limitations of CTC is its availability. Because funding is not widely available for screening CTC, the professional capacity is markedly limited.

CTC requires a cathartic bowel preparation, although studies are underway to eliminate this need. The most notable limitation is that CTC is an imaging study only, and patients must be referred to OC for biopsy and/or removal if a significant colonic lesion is encountered.

CTC every 5 years for patients older than 50 years of age (45 years in African Americans) is an acceptable screening option for colorectal cancer.

Patient is instructed to drink clear liquids the day before CTC.

The main bowel-cleansing agents include cathartics, such as magnesium citrate and sodium phosphate and gut lavage solutions, such as polyethylene glycol (PEG).

Sodium phosphate is an oral saline cathartic and is referred to as a “dry prep” because it leaves behind little residual fluid in the colon.

Magnesium citrate is also a saline cathartic that causes fluid to accumulate in the bowel because of its osmotic effects and promotes peristaltic activity and bowel emptying.

PEG is an effective agent for cleansing the bowel, but results in excessive fluid retention in the colon. It is considered a “wet prep” and is not ideal for virtual colonoscopy.

Dual-positive oral contrast agents are used for tagging of the residual stool and luminal fluid after catharsis. Barium and/or iodine solutions are ingested with meals, usually in conjunction with the oral cathartics, 24 to 48 hours before imaging to allow adequate incorporation of the positive contrast material with colonic contents.

Tagged stool and residual fluid demonstrate higher attenuation and are easily discernible from the homogeneous soft tissue density of polyps and colonic folds.