The adenoma-carcinoma sequence.

The adenoma-carcinoma sequence refers to the development of malignant change in a precursor focal dysplastic polypoid lesion, the adenoma. In the intestinal tract of man this progression has been most clearly documented in the large bowel and although the evidence is largely circumstantial, it would appear that the majority of malignant tumours at this site arise in this way. By contrast this course is unusual in the stomach, the other major site of gut malignancy, where adenomas are uncommon. Small intestinal adenocarcinoma is rare but there is evidence that a significant proportion of these tumours develop from a precursor adenoma. In the large bowel, adenomas are very common lesions in those populations where there is a high incidence of colorectal cancer. The likelihood of malignancy developing in an adenoma is related to increasing size, a villous as opposed to a tubular growth pattern, and the severity of cytological and architectural abnormalities (dysplasia) of the epithelium.