Diagnosis and Screening for Colorectal Cancer

Colorectal cancer is common and readily treated in many cases, making it an ideal cancer to try to prevent using screening strategies. Because almost all colorectal cancers arise from adenomatous polyps, one might conclude that the vast majority of deaths from CRC would be preventable if screening tests were used to their maximum benefit. Yet, in general, less than 30% of persons for whom screening is recommended have been screened. In 1997, a large population-based study of 52,754 persons age 50 or older assessed whether people had had fecal occult blood testing (FOBT) and/or sigmoidoscopy within the recommended 1 or 5 year intervals, respectively. Only 19.8% said they had FOBT within the prior year and 30.4% had sigmoidoscopy. In that light, it is incumbent on the medical profession and those concerned with the public’s health to better identify the barriers to screening so that its maximum benefit can be achieved.

There are a variety of methods available for the diagnosis of CRC, each having its own associated controversy. In addition, there are several currently accepted screening guidelines being employed for the purpose of detecting CRC in otherwise asymptomatic patients with no additional risk factors. Although similarities exist, new information regarding the utility of endoscopic and radiographic techniques may change significantly those established recommendations.

Beginning at age 50, the U. S. Preventive Services Task Force recommends annual FOBT and flexible sigmoidoscopy (periodicity unspecified).

The American Cancer Society recommends one of five screening options:

  • Yearly FOBT
  • Flexible sigmoidoscopy every 5 years
  • Yearly FOBT plus flexible sigmoidoscopy every 5 years
  • Double contrast barium enema every 5 years
  • Colonoscopy every 10 years

Digital Rectal Examination
Digital rectal examination has traditionally aided in the diagnosis of many rectal cancers. Today, however, the proximal shift in the distribution of CRCs makes a greater number of lesions undetectable with digital rectal examination alone. As digital rectal examination at best can reach only the most distal 5 to 7 cm of the rectum, its diagnostic virtue lies only in its simplicity and low cost but in no way should be considered a substitute for a more thorough examination.

Fecal Occult Blood Testing
Fecal occult blood testing continues to be the subject of significant study. Its allure remains its relatively low cost, the potential for mass screening, and most importantly the detection of asymptomatic cancers. Most currently available guaiac tests rely on a peroxidase-like reaction to the heme component of blood. Despite the flaws associated with FOBT, including failure to detect 75% of adenomas and 30% to 50% of colorectal carcinomas in association with an overall false-negative rate of 40%, most reports on their use emphasize that a relatively high percentage of the cancers detected are early-stage lesions. The 33% reduction in mortality observed in individuals who underwent annual FOBT screening in the Minnesota Colon Cancer Control Study strongly supports FOBT as a useful diagnostic approach. Dietary precautions prior to collection of the FOBT can decrease the likelihood of false positives. Patients should be instructed to avoid aspirin and nonsteroidal antiinflammatory drugs (NSAIDs) for 1 week prior to collection of specimens, and red meat and vitamin C for 3 days prior. Current recommendations are for obtaining a total of six samples (two from each of three specimens).

Barium Enema
The air contrast barium enema (ACBE) has proved to be a valuable study for the diagnosis of CRC. Its advantages include its sensitivity for the detection of small polyps, its detection rate for rectal lesions, and its efficacy in diagnosing early inflammatory bowel disease as well as relatively low cost and lower complication rates when compared with colonoscopy.6 Disadvantages of ACBE are the need for more patient cooperation and the radiation exposure compared to that incurred with colonoscopy. ACBE can still be of significant value to physicians with limited access to colonoscopy.

Flexible Sigmoidoscopy
Sigmoidoscopy in asymptomatic persons appears to be a highly sensitive method for early detection of both cancer and adenomatous polyps of the distal colon and rectum. In trained hands, a 60-cm flexible sigmoidoscope can reach the descending colon and detect two to three times more neoplasms than the rigid sigmoidoscope. Sigmoidoscopy has a relatively low cost, requires no sedation, is sensitive, specific, safe, easy to perform, and is a useful means of reducing both the incidence of cancer and its associated mortality. However, the value of flexible sigmoidoscopy has recently been questioned due to its inability to detect lesions in the proximal colon. Two studies demonstrated that sigmoidoscopic screening will fail to detect a substantial proportion of asymptomatic CRCs or polyps associated with a high risk of cancer.

Overview of Flexible Sigmoidoscopy Technique
After informed consent is obtained, the patient is placed in the left lateral supine position after bowel preparation with two Fleet-type enemas. Perform a digital exam, and then insert the lubricated tip of a flexible 60- to 70-cm sigmoidoscope into the rectum about 10 cm. The scope is held with the left hand and the fingers of the left hand will operate the scope levers, allowing it to turn various directions. The shaft is advanced with the right hand. The goal is to insert the scope maximally, with examination for pathology occurring primarily during withdrawal of the scope. Air needs to be insufflated to expand the bowel cavity, but too much air will lead to cramping and spasm, making the examination more difficult. Do not advance if there is resistance or if there is a “red out” where the scope is in direct contact with the bowel wall. If the bowel appears not to move as you advance the scope, the scope is likely “telescoping,” and will cause discomfort to the patient. The scope should be retracted until the bowel moves away from the field of view. The scope can then be advanced again to allow for a fuller length of the bowel to be examined. Another way to maximize the length of bowel examined is by torquing of the scope about 30 degrees against a mucosal fold and retracting about 5 cm. This technique will allow the bowel to bunch up on the scope, permitting a longer section of the bowel to be examined. After maximal insertion or if patient is not able to tolerate any further advancement, begin to slowly withdraw the scope. Note any diverticula, masses, or other abnormalities and document your findings. Biopsy can be performed through a biopsy port on the scope. Avoid biopsy of vascular areas and be sure all bleeding is stopped after the biopsy before completing your examination.

Examination of the entire colon by colonoscopy remains the gold standard for visualization, biopsy, and when possible removal of colonic neoplasms. Colonoscopy is an expensive procedure involving sedation and has a higher complication rate than other diagnostic modalities. In some reports, just over 17% of colonoscopies failed to reach the cecum. In a recent Veterans Administration (VA) study, 3121 asymptomatic men with a mean age of 62.9 years underwent a screening colonoscopic examination; 37.5% had one or more neoplastic lesions detected, with invasive cancer diagnosed in 1% of the patients. In this study, 52% of the 128 patients with advanced proximal colon lesions had no distal adenomas that a sigmoidoscopy would detect. The authors also found that hyperplastic polyps had an odds ratio of 1.5 for advanced proximal disease.

The selection of a particular screening technique poses several challenges to the family physician. The gold standard for screening is currently colonoscopy due to the definitive nature of the procedure’s complete visualization of the large intestine. However, the procedure is associated with risks of perforation, anesthetic complications, and patient discomfort. The high cost of colonoscopy can be a deterrent either to the individual or payer if not a covered benefit for the purpose of screening the low-risk asymptomatic patient. In the most capable hands, a flexible sigmoidoscope will reach only the first part of the descending colon, missing lesions of the right and transverse colon that may be present in asymptomatic patients. The Minnesota Study documents the effectiveness of FOBT as a screening technique with its inherent limitations. Computed tomography (CT)-guided colonography, also known as virtual colonoscopy, holds a great deal of promise as a noninvasive approach to screening that eliminates several of the risks of colonoscopy. While promising, CT-guided colonography costs have yet to be delineated, it has yet to be proven an effective substitute for colonoscopy, and it also lacks the capability for biopsy afforded during endoscopic colonoscopy.

The challenge for the family physician is selecting the method most appropriate for our patients that may be determined by cost, availability of technology, and the ability of a patient to tolerate a procedure. There are courses available to teach colonoscopy with biopsy to nongastroenterologists. Following such training, a preceptorship is needed to develop both technical and diagnostic expertise in colonoscopy. The key issue is having sufficient volume in one’s practice to maintain the acquired skill and a supportive environment (staff and facilities) to maximize patient care and minimize the risk of complications.

Gregory L. Brotzman and Russell G. Robertson


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