The presenting symptoms vary according to the location of the neoplasm in the colon. Cancers that occur in the more voluminous and distensible cecum and right colon, where bowel contents are liquid, usually do not cause obstruction. These cancers tend to grow to large size and ulcerate, producing a gradual chronic blood loss. In some cases enough hemoglobin is lost to produce melena. Passage of reddish maroon stool sometimes occurs, indicating the diagnosis.
Large cecal masses may involve the ileocecal valve and produce apparent small bowel obstruction. Sometimes tumors of the ascending colon partially block the lumen. If the ileocecal valve is competent, the cecum becomes painfully dilated with gas and air, creating a tender right lower quadrant mass. Patients may massage this area and find relief as they force the trapped gas past the obstruction.
Cancers in the left colon, where the lumen is narrower and less distensible and the fecal stream is solid, commonly obstruct. This causes constipation and cramping abdominal pain. Sometimes a paradoxical diarrhea develops as some colonic contents are forced past a partially obstructing cancer. Bleeding from left-sided lesions may be bright red but is generally not massive. Patients with rectal cancers in particular tend to have bright red bleeding as well as tenesmus and small-caliber stools. Colonic cancers that advance to the stage at which they invade through the bowel wall may perforate it, leading to peritonitis. Anorexia and weight loss are common in advanced colonic cancer. Anorexia is intensified by partially obstructing lesions that may cause cramping abdominal pain associated with meals.
Colonic cancers may metastasize via lymphatic or hematogenous routes or both. Metastases to regional lymph glands, with intraabdominal spread, are most common. Metastases to the liver are frequently found. Lung and bone are other sites of metastatic spread.
Several conditions must be differentiated from colonic cancer. Diverticulitis may cause an inflammatory mass and obstruction that may resemble cancer, particularly in the sigmoid colon. Barium enema may sometimes be insufficient to distinguishing between a perforated diverticulum and a perforated carcinoma. Colonoscopy with biopsy is valuable for distinguishing these disorders from one another once the infectious process has been controlled but can be difficult if muscle hypertrophy, spasm, and mucosal edema are present.
In patients with ulcerative colitis, inflammatory strictures may be difficult to distinguish from malignant strictures. Again, the colonoscope is not always able to pass through the narrowed area. Frequently, however, a stricture has a component of muscle spasm, and such areas may be evaluated although they appear on barium enema examination to be too narrow for the colonoscope to pass. A cytology brush may be passed through the narrowed area, enabling the physician to detect malignant cells. Biopsy specimens from the proximal end of the stricture may not reveal a carcinoma present in the midportion or distal end. If any doubt exists, the stricture should be considered malignant until proved otherwise.
Benign-appearing polypoid lesions of the colon must be differentiated from carcinoma if they are larger than 7 mm in diameter. Simple biopsy of polyps may be misleading because a cancerous area may be missed as a result of sampling error. Cytologic specimens from polyps are not useful in practice because they do not permit differentiation of invasive carcinoma from the presence of atypical surface cells. Thus the best way to evaluate a polyp is to remove all of it by cautery snare. If a polypoid cancer with an involved margin is removed, resection of the involved segment of bowel and mesentery is usually indicated. Polyps 7 mm or smaller are not usually malignant and may be removed by fulguration (“hot”) biopsy. In this technique the polyp is destroyed by electrocoagulation while a biopsy specimen is obtained.