Laboratory and Other Diagnostic Tests
Laboratory findings that suggest the presence of colonic neoplasm include hypoalbuminemia, iron deficiency anemia, and occult blood in the stool. Elevated alkaline phosphatase concentration suggests colonic cancer metastatic to liver or bone. In liver metastases, bilirubin concentration remains normal until late in the course, and transaminase levels are usually normal or only slightly elevated.
Radiography and endoscopy are the primary means of establishing the diagnosis of large bowel cancer. Radiologists can detect fine mucosal details and small lesions with the air-contrast barium enema technique. It is a useful procedure for the identification of small polypoid lesions, although colonoscopy seems to be more sensitive.
Colonoscopy is used instead of barium enema or to confirm the presence of possibly cancerous lesions seen on barium enema and to document their nature by biopsy. With this information, appropriate therapy can be carried out with more assurance. Flexible sigmoidoscopes, essentially short colonoscopes, can be used with similar purpose for rectosigmoid lesions. For the most part, however, a full colon examination is preferred. Synchronous colonic carcinomas that are not evident on barium enema examination can be detected by colonoscopy; they occur in 1% to 5% of patients with colonic carcinoma. In addition, up to 40% to 50% of patients have synchronous adenomatous polyps.
Digital examination of the rectum may reveal a mass lesion. The digital rectal examination is an important part of the complete physical examination for many reasons and generally should not be omitted without reason. Abdominal mass lesions may be palpable at times. If there is metastatic spread to the liver, hepatomegaly may be noted, and the liver may feel hard and nodular and be slightly tender. Peritoneal metastases may cause ascites.
Endoscopic ultrasonography has been used to stage colorectal cancer, particularly rectal cancer. Locoregional staging with endoscopic ultrasound (EUS) for depth of tumor invasion and regional lymph node metastases has proved more accurate than pelvic computed tomography (CT) for rectal cancer staging. CT is still the best modality for the evaluation of distant metastases, such as to the liver.
Screening for Colorectal Cancer
The rationale for this approach is based on evidence that reduction in colorectal cancer mortality can be achieved through detection and surgical removal of early stage cancer, and by the removal of adenomatous polyps. Widely endorsed guidelines for colorectal cancer screening have been developed based on the division of the population into average risk and increased risk groups (Fig. 343-1 [Electronic Rights Not Granted]). Those at increased risk include patients with first-degree relatives (parents, siblings, children) who have had colorectal cancer or adenomatous polyps, with a family history of familial adenomatous polyposis (FAP) or HNPCC, or with a personal history of adenomas or colorectal cancer. The screening tests used include fecal occult blood tests, flexible sigmoidoscopy, barium enema, and colonoscopy at varying onsets and intervals depending on the assessment of risk.
Genetic testing of populations for the APC gene is not feasible, since many different mutations of the gene have been found in different families. It is useful to screen family members within an affected kindred, but the significance of a negative test is uncertain. Similarly, the genetic mutations related to HNPCC are not useful in population screening but may have value in genetic identification within families at risk.
There has been recent interest in searching for mutated oncogenes in fecal DNA as a screening test for colonic neoplasia. Improved fecal occult blood tests are also under development, as is “virtual colonoscopy,” an examination of the colon from images obtained during helical CT scanning. These tests have promise but have not reached a point of proven clinical utility.
The serum immunoassay for carcinoembryonic antigen (CEA) was developed to provide a means of early detection of colonic cancer. However, it is too insensitive and nonspecific to be a useful screening test. CEA concentration may also be elevated with other cancers and with benign conditions such as inflammatory bowel disease, alcoholic liver disease, and pancreatitis. The test is most useful in assessing the results of surgery. If CEA concentration is elevated preoperatively and falls to normal after curative resection, a second rise is a good indication of recurrence and may develop long before the cancer can be detected clinically. Surgical “second look” procedures have been advocated in this circumstance, but their value remains to be proved. Also, when initially high, CEA concentration has been used to gauge patient response to chemotherapy for metastatic disease.