The Pre-Malignant Lesion: The Adenomatous Polyp

There is abundant evidence that virtually all colorectal carcinomas (cancers) begin as adenomatous polyps.  In a number of studies, colonic polyps have been left and some disappear, but many increase in size or progress to a cancer.

It is not uncommon for carcinomas (cancers) to have associated adenomatous (polyp) tissue.  In a study of post mortem examinations of the colon, the population with the highest proportion of adenomas was observed in the area with the highest incidence of colonic cancer.

The change from benign adenoma (polyp) to colorectal cancer is thought to often take between 10 and 35 years.  It is estimated that the annual conversion rate of a polyp to a cancer is approximately 0.25%.  But in some patients polyps become more likely and faster to grow into cancers, but there exists a window of opportunity to prevent the cancer rather than treat it.

The Adenoma / Carcinoma Sequence

Genes inherited from parents contribute to risk of progression from polyp to cancer.  It is thought that it is the accumulation of changes in genes (DNA) that is critical to the development of colorectal cancer (Figure 1).

  Figure 1. Genetic model of colorectal carcinogenesis

The lifetime risk of colorectal cancer for people in Great Britain is one in twenty, but this becomes one in ten if there is a first degree relative less than 55 who had colorectal cancer.  It is interesting to note that it has been demonstrated that polyps also run in families, and in the USA colorectal cancer is often considered a prevented disease if a screening colonoscopy is carried out at age 50.