Screening

British Medical Journal, Oct 21, 2000 by John H Scholefield

Several large randomised studies have shown that screening with faecal occult blood testing is feasible, and two studies have shown that such screening reduces the mortality from colorectal cancer. In a study in Nottingham, for every 100 individuals with a positive test result, 12 had cancer and 23 had adenomatous polyps. The cancers detected at screening tended to be at an earlier stage than those presenting symptomatically (Dukes's A classification: 26% screen detected v 11% in controls). The disadvantage of screening with faecal occult bloods is its relatively low sensitivity--a third to a half of cancers will be missed on each round of screening. The Nottingham data suggest that screening every two years detects only 72% of cancers. This could be improved by testing annually and using more sensitive immunologically based faecal occult blood tests.

Who should be screened?

Although about 20% of the population will develop adenomatous polyps, only 5% of these will develop colorectal cancer. This equates to a I in 20 lifetime risk for colorectal cancer. The cancer occurs most often in the age group 65-75 years, but for adenomas the peak incidence is in a slightly earlier age group (55-65 years). Thus population screening for colorectal cancer should target both these age groups.

In addition, some people inherit a much higher susceptibility to colorectal cancer. Some inherit a well recognised single gene disorder, such as familial adenomatous polyposis or hereditary non-polyposis colon cancer, whereas most inherit an undetermined genetic abnormality. These people tend to develop colorectal cancer before the age of 50, and therefore screening in this high risk population needs to be tailored to each individual's risk pattern. They may also be at risk for cancers at other sites, and screening for ovarian, breast, and endometrial cancers may be appropriate in some of these cases. The advice of clinical geneticists in these cases can be invaluable.

Cost effectiveness of screening

If screening for colorectal cancer is to be acceptable to healthcare providers it must be shown to be cost effective. Estimates of the cost of screening for colorectal cancer range from 1000 [pounds sterling] to 3000 [pounds sterling] per life year saved, depending on the screening technique used. The cost of using faecal occult blood testing would be the lowest--similar to estimates for breast cancer screening.

Cost estimates are associated with several unknown factors. The factors that cause greatest concern to those considering funding screening programmes are the cost of cancers missed and the potential damage caused to asymptomatic individuals by invasive procedures such as colonoscopy.

Potential harm from screening

Although it has been suggested that considerable anxiety and psychological morbidity may be caused by inviting populations to participate in screening for colorectal cancer, little evidence exists to substantiate this. Indeed in the Nottingham trial no longstanding psychological morbidity from the screening programme was found. Similarly, no evidence exists that screening for colorectal cancer leads to false reassurance from negative tests.