Background: It is widely accepted that the adenoma±carcinoma sequence represents the process by which most, if not all, colorectal cancers arise. The evidence supporting this hypothesis has increased rapidly in recent years and the purpose of this article is to review this evidence critically and highlight its clinical signi®cance.Methods: Medline searches were used to identify recent key articles relating to the adenoma±carcinoma sequence. Further pertinent articles were obtained by manual scanning of the reference lists of identi®ed papers.Results: The evidence supporting the adenoma±carcinoma sequence can be classi®ed as epidemiological, clinicopathological and genetic. The most recent and largest body of data relates to molecular genetic events and their cellular effects; however, many other approaches, such as cytogenetics, molecular cytogenetics and cytometry, have also yielded valuable information.Conclusion: Recent work continues to support the adenoma±carcinoma sequence, but there is a paucity of data on the interrelationship between different genetic mutations and on the relationship between molecular and other types of genetic abnormalities. The clinical utility of the observations described has yet to be fully realized and global genetic analysis of colorectal tumours may prove to be central in rational adenoma management. IntroductionColorectal cancer is the second leading cause of cancerrelated death in the Western world; in the UK there are currently around 30 000 new cases per annum and 17 000 related deaths 1 . In recent years our understanding of the cellular and molecular events underlying the development of colorectal cancer has improved immeasurably but, despite this and advances in surgery, radiotherapy and chemotherapy, the average 5-year survival rate remains around 40 per cent 1 . One of the most important fundamental concepts in colorectal cancer to emerge in recent years has been the adenoma±carcinoma sequence, a term that describes the stepwise progression from normal to dysplastic epithelium to carcinoma associated with the accumulation of multiple clonally selected genetic alterations. This concept not only provides an excellent model to study the genesis of invasive cancer, but also affords a means of preventing colorectal cancer by endoscopic removal of precursor lesions.The appropriate management of individuals with precursor adenomatous polyps (adenomas) is of the utmost importance. It is known that after removal of such polyps 30±35 per cent of patients will have further adenomas detected at 3±4 years 2±4 and this has led to a policy of endoscopic surveillance for all adenoma-bearers 5 . However, given that approximately 40 per cent of the Western population will develop adenomas 6±8 and only 3 per cent will go on to suffer from colorectal cancer, it is clear that only a small proportion of adenomas progress to malignancy. Unfortunately, there are no reliable criteria available that can predict adenoma progression or recurrence, and the important questions of which individuals...

The aim of the study was to determine whether clinical information alters the CT report. This prospective blinded study consisted of 50 consecutive patients who attended a Department of Radiology for CT. Each study was interpreted by two of three consultant radiologists, before and after knowledge of the clinical information. 19 reports were changed after clinical information was known. Clinical follow-up was available in 15 cases. In ten cases the reports were more accurate after clinical information and in five cases the reports were less accurate. In three of the five cases where accuracy was reduced, the clinical information was incorrect. It was concluded that clinical information affects the CT report. If the information is accurate it has a beneficial effect; if it is inaccurate it has a detrimental effect. The more complex the investigation, the more important the clinical information. There was a correlation between readers regarding the influence of clinical information. Correct clinical information therefore improves the radiology report. It is the responsibility of the clinician to provide this information in an accurate and legible form.

The level of error in radiology has been tabulated from articles on error and on "double reporting" or "double reading". The level of error varies depending on the radiological investigation, but the range is 2-20% for clinically significant or major error. The greatest reduction in error rates will come from changes in systems.

In a FOB test-positive screened population, lower gastrointestinal symptoms are common, but are not predictive of colorectal neoplasia.