Symposium on colorectal cancer. 1. Pathology of colorectal cancer.

Most colorectal cancers arise from pre-existing adenomas (the adenoma-carcinoma sequence). The histology, cytology and malignant potential of the adenoma group of tumours are described. From a study of the life history of the adenoma-carcinoma sequence in cases of both familial polyposis and isolated adenoma it would appear that most colorectal cancers evolve over a long period. The adenoma is important as a marker for increased risk of colorectal cancer, in clinical practice and in epidemiologic studies. Local excision for carefully selected early colorectal cancers is justified on the basis of the results of a long-term prospective study.