– Most colorectal tumors (95%) are adenocarcinomas, which originate as a benign, adenomatous polyp in the rectum or colon.
Causes and Incidence
No definitive etiologic factors have been identified, but risk factors include a familial history, inflammatory bowel disorders and bowel polyps, and a high-fat, low-fiber diet. The incidence of colon cancer in the United States ranks behind breast and lung cancer. Each year colon cancer is diagnosed in more than 156,000 people, and it is the second leading cause of cancer deaths. In 93% the disease is diagnosed after age 50, and the incidence is equally distributed across gender lines, although women more often have cancer of the colon and men cancer of the rectum.
Over a period of 5 years or longer, the adenomatous polyps degenerate into malignant tumors, which are most often located in the rectum or lower colon. The tumor spreads by direct extension through the bowel wall and by intraluminal, hematogenous, and regional lymph node metastases. The liver and lungs are common sites of distant metastasis.
Cancer of the bowel is largely asymptomatic during the early stages. The most common presenting sign is rectal bleeding on defecation. Changes in bowel patterns, excessive gas, bloating, and cramping may also occur. Pain is unlikely until advanced stages of the disease.
The chance of survival falls below 50% with regional node involvement, and more than half of these individuals have node involvement at the time of diagnosis. Bowel obstruction or perforation, paralytic ileus, hemorrhage, and liver failure occur with advancing disease.
A history of risk factors, positive result on occult fecal blood test, or palpable lesion on rectal examination indicates a need for follow-up. Visualization of a lesion by colonoscopy or barium enema examination or an elevated carcinoembryonic antigen level indicates the need for tissue biopsy, which is the only definitive mode of diagnosis.
Excision of welldifferentiated rectal tumors; resection of the colon around the tumor with removal of the associated lymphatic drainage system; colostomy; laser or bypass surgery for inoperable obstructing tumors.
Adjunct systemic chemotherapy of 5-fluorouracil (5-FU) and levamisole for colon tumors; 5-FU and metronidazole for rectal tumors.
Radiation plus chemotherapy when four or more positive nodes are found and for palliation; instruction in ostomy care, ostomy support groups; counseling for altered body image.