Chapter 64. Colon Cancer

A 72-year-old man reports rectal bleeding with bowel movements over the past several months and the stool seems narrower with occasional diarrhea. He has a history of hemorrhoids but at this time is not experiencing rectal irritation or itching, as with previous episodes. His medical history is significant for controlled hypertension and a remote history of smoking. On digital rectal examination, his stool sample tests positive for blood but anoscopy fails to identify the source of bleeding. On colonoscopy, a mass is seen at 30 cm (Figure 64-1). A biopsy was obtained and pathology confirmed adenocarcinoma.

Colon cancer is a malignant neoplasm of the colon, most commonly adenocarcinoma. With the establishment of screening programs and treatment improvements, there has been a slow decline in both the incidence and mortality from colon cancer, although it is still a leading cause of cancer death.1

• Colon cancer is the third most common cancer in both men and women in the United States, second only to lung cancer as a cause of death.1
• The incidence and mortality have been slowly but steadily declining in the United States for the past decade.2 The American Cancer Society estimated there were 142,570 new cases of colorectal cancer (102,900 colon cancer cases) and 51,370 deaths in 2010.3
• Onset is after age 50 years and peaks at around age 65 years.
• Proximal colon carcinoma rates in blacks are higher than in whites.4

• Colon cancer appears to be a multipathway disease with tumors usually arising from adenomatous polyps or serrated adenomas; mutational events occur within the polyp including activation of oncogenes and loss of tumor-suppressor genes.1,5
• The probability of a polyp undergoing malignant transformation increases for the following cases:1
  • The polyp is sessile, especially if villous histology or flat.
  • Larger size—Malignant transformation is rare if smaller than 1.5 cm, 2% to 10% if 1.5 to 2.5 cm, and 10% if larger than 2.5 cm.
• Important genes involved in colon carcinogenesis include adenomatous polyposis gene mutations, KRAS oncogene, chromosome 18 loss of heterozygosity leading to inactivation of SMAD4, and DCC tumor-suppression genes (deleted in colon cancer). Other mutations in genes, such as MSH2, MLH1, and PMS2, result in what is known as high-frequency microsatellite instability (H-MSI), which is found in cases of hereditary nonpolyposis colon cancer and in approximately 20% of sporadic colon cancers.4

See References 1 and 6.

• Ingestion of red and processed meat.
• Hereditary syndromes—Polyposis coli and nonpolyposis syndromes (40% lifetime risk).4
• Inflammatory bowel disease.
• Bacteremia with Streptococcus bovis—Increased incidence of occult tumors.
• Following ureterosigmoidostomy procedures (5% to 10% incidence over ...