Colorectal Adenoma

Overview
  • Colorectal Adenomas likely represent the neoplastic precursor lesion of colorectal carcinoma and as such have become an intense focus of study for the molecular pathogenesis of cancer. Although the vast majority of colorectal adenomas are benign, a small percentage, estimated to be 1% may evolve into a malignancy. The molecular evolution of colorectal adenomas into colorectal carcinomas is believed to be the result of a multi-step process involving both genetic and epigenetic changes which progressively derange cell cycle regulation. As this process progresses, small foci of cells within the adenoma may become dysplastic and ultimately malignant.
Morphology
  • Overview
    • Colorectal Adenomas appear as "Polyps", meaning grossly-visible masses derived from the large intestine mucosa that protrude into the lumen of the bowel. Based on their morphology, colorectal polyps can be divided into three basic subtypes as described below.
  • Tubular Adenomas
    • Tubular Adenomas are composed of a glandular architecture similar to that of the large intestine mucosa. These polyps can range in size from small to quite large (2-3cm) and tend to become pedunctulated as they grow. Foci of dysplastic cells are present in the polyp but are not as prominent as that observed in the villous subtype.
  • Villous Adenoma
    • Villous Adenomas are typically sessile and can grow to be quite large (up to 10cm). They extend long finger-like projections above the level of surrounding normal mucosa and appear like velvety patches on endoscopy. The dysplasia observed in villous adenomas tends to me more severe than that of the tubular subtype; correspondingly, the probability that villous adenomas harbor bona fide malignant cells is three times as great as that of tubular subtype.
  • Tubulovillous Adenoma
    • Tubulovillous Adenomas contain a mix of tubular and villous elements and thus an intermediate degree of dysplasia and probability of harboring a malignant cells.
Clinical Consequences
  • Colorectal Adenomas are almost always asymptomatic although in a small percentage of patients may cause occult lower GI bleeding. As expected, the primary concern is evolution of these lesions into full-blown colorectal carcinomas. The probability that a polyp contains malignant cells appears to be due to its size, morphology, and degree of dysplasia. Larger polyps and those of the villous subtype are more likely to contain foci of malignancy. For any type of polyp, the degree of dysplasia is proprotional to the probability that the polyp possess malignant cells.