The colitic patient faces a higher risk of large-bowel cancer than does a member of the general population. Opinions as to how large this risk is differ greatly between different authors. This is hardly surprising, however, as there have been differences in the method of collection of materials, and the duration of the colitis has not always been taken into account [1, 2]. In facing the problem of cancer development in longstanding ulcerative colitis, many important issues have to be considered.

Are all patients subject to the same risk from large-bowel cancer, or can any one group of patients be singled out as being especially at risk? If such a group can be identified, at what stage of their disease are they particularly liable to develop the cancer? If the patient's risk group can be defined with reasonable precision, is it possible to predict the sort of cancer risk which an individual patient faces? How should these patients be followed up and how should the surveillance programme be designed? Another crucial issue is whether the risk of cancer is sufficient to warrant the use of prophylactic surgery, and if so, when should it be recommended, and with all options present today, what form should the prophylactic surgery take?

The cumulative probability of cancer for all patients with ulcerative colitis, regardless of disease extent, has been demonstrated to be 2 % at 10 years, 8% at 20 years and 18% at 30 years, while the overall prevalence of colorectal cancer in any patient has been demonstrated to be 3-4% [3]. However, there is convincing evidence that there are colitis patients who have an especially high risk of developing cancer. Investigations into the relationship between cancer and colitis have focussed upon the identification of a "high-risk" group of patients and the one single factor which does exert the most influence regarding cancer risk in colitic patients concerns the extent of the patient's colitis. It seem to be quite clear that patients who are especially at risk for development of carcinoma are those with total or near-total involvement of the colon (i.e. involvement of the whole left half of the colon and the transverse colon as well). There is indeed a remarkable unanimity of opinion on this point between the various workers who have studied the problem [4, 5, 6]. There is, additionally, convincing evidence to show that the duration of col-itic symptoms has a strong impact on the incidence of cancer in those high-risk patients. In other words, the longer the history of colitis, the greater the risk of carcinoma.

An idea of the risk of cancer facing the individual patient with total involvement can be gained from the many studies that have been undertaken over the years. Remarkably little attention has been paid to the variation in the duration of the disease between different patients when the incidence of cancer in UC has been calculated. In published reports, the crude incidence is often used. However, the use of crude incidence to evaluate the risk of carcinoma in UC is misleading, as it fails to correct for differences in duration between individual patients. The figures may thus underestimate the true incidence of carcinoma in UC. The time factor is essential and by employing the life-table concept much more sophisticated and accurate results can be obtained. While there is evidence to show that the cancer risk for a patient with total involvement is low during the first 10 years of colitic symptoms, being around 0.4% per annum, the annual risk of cancer starts to rise sharply after this initial decade, and after 20 years of colitis, the individual with total involvement faces an annual risk of cancer of over 5% for the remainder of his life.

However, the yearly risks accumulate, so that the cumulative risk during a period of 5, 10, 20 or even 25 years is far higher. The cumulative risk of cancer in such patients rises slowly at first to around 4% after 10 years of colitic symptoms. However, after 10 years of colitis, the cumulative risk increases more and more sharply to reach a startling level of 41-48% after 25 years of colitic symptoms [5, 6]. Recently, similar results have been presented, while others demonstrate that those with total or extensive colitis (extending proximal to the splenic flexure), colitis of 8 years or more, a family history of colorectal cancer, primary sclerosing cholangitis and an early age of onset of colitis have the greatest risk of developing cancer [3, 7]. Primary sclerosing cholangitis (PSC) occurs in about 2.5-6% of patients with UC, adding considerable risk of cancer compared with UC in general [7]. The mean interval from diagnosis of PSC to dysplasia or cancer is only 2.9 years. Colorectal cancer associated with PSC is more likely to be proximal, to be diagnosed at a more advanced stage, and to be fatal.

While patients with extensive colitis may have an almost 20 times higher risk of cancer compared with the general population of patients, those with left-sided colitis may only have a four times higher risk, and for an individual patient with a distal proctocol-itis, the risk is very small, probably about the same as that in the general population of the same age and sex [4, 8].

The age of the patient at the onset of colitis is another factor which has inspired debate. It has often been claimed that colitic patients whose disease began in childhood may have a particular tendency to develop cancer [9]. One explanation that has been put forward is that total involvement - a strong risk factor - is far more common in patients under 20 years of age than in elderly patients. Another important reason why patients who develop ulcera-tive colitis as children are particularly at risk from carcinoma, is that they are the only patients who will live long enough to develop cancer! Patients who develop total involvement at the age of 50 or later already have a curtailed life expectation and will probably not live long enough to develop cancer. Therefore, the main factor which undoubtedly exerts an effect upon the cancer risk for colitic patients is the extent of the patient's colitis.

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