Villous adenoma of the colon

Morphologically, villous adenomas of the colon are generally sessile and papilliferous - frond-like - lesions that tend to secrete mucus.

Villous adenoma of the colon presents with blood and mucus in the stool. 80% occur in the recto-sigmoid.

30% are actively invasive and 30% have atypical cellularity.

Rarely, they may cause hypokalaemia.

The epithelial element of these adenomas is more dysplastic than that seen in tubular adenomas and consequently these have, in general, greater potential for malignant change.

Notes:

• approximately 70% of polyps removed at colonoscopy are adenomas
  • from 70% to 85% of these are classified as tubular (0-25%, villous tissue), 10-25% are tubulovillous (25-75%, villous tissue), and <5% are villous adenomas (75%-100%, villous tissue)
    
    
• most colorectal cancers arise from neoplastic adenomatous polyps (adenomas)
  • adenomas are monoclonal derivatives of a mutated epithelial stem cell. Simple small (<1 cm) tubular adenomas are extremely common and have a low risk of becoming malignant. Only a few acquire the additional genetic alterations that make them grow, develop advanced histological features, and turn to cancer. Advanced adenomas are those that are larger (>1 cm) or that contain appreciable villous tissue or high-grade dysplasia
  • generally accepted that most (>95%) colorectal cancers arise from benign, neoplastic adenomatous polyps (adenomas). Although this adenoma-carcinoma sequence can probably never be proved directly, persuasive data exist indicating that colorectal neoplasia progresses through a continuous process from normal mucosa, to benign adenoma, to carcinoma
    
    
• some degree of dysplasia exists in all adenomas
  • high-grade dysplasia includes the histological changes previously called 'carcinoma in situ, 'intramucosal carcinoma,' or 'focal carcinoma.'
  • approximately 5-7% of patients with adenomas have high-grade dysplasia, and 3-5% have invasive carcinoma at the time of diagnosis
  • increasing dysplasia and, presumably, malignant potential correlate with
    • increasing adenoma size
    • villous component
    • and patient age
  • likelihood of invasive carcinoma also increases with increasing polyp size

Reference:

• 1) Bond JH. Polyp guideline: diagnosis, treatment, and surveillance for patients with colorectal polyps. Practice Parameters Committee of the American College of Gastroenterology. Am J Gastroenterol. 2000 Nov;95(11):3053-63.