Screening for other forms of Cancer
Screening has been defined as the systematic application of a test or inquiry to identify individuals at sufficient risk of a specific disorder to benefit from further investigation or direct preventive action, among persons who have not sought medical attention on account of symptoms of that disorder. Before a screening test can be introduced it is necessary to be able to demonstrate that the test not only distinguishes people who will develop the disease from those who will not, but that a remedy is available to individuals who are judged to be screen-positive that can significantly improve their health compared with not screening, and treating the disease on clinical presentation in the usual way.
In assessing screening tests for cancer, a large randomised trial is usually necessary. This would compare mortality from the specific cancer in a group that has been screened and treated with the corresponding mortality in an unscreened group that received treatment only after clinical presentation. In screening programmes that aim to detect cancer lesions at an early stage it is impossible to determine the proportion of all of the cancers in question that would have presented clinically over a specified period that are detected by screening, because cancers cannot be detected without then intervening. The outcome of screening trials is therefore expressed as a proportional reduction in mortality from the specific cancer and then a judgment made as to whether this is worthwhile.
Table 11 shows the cancers and screening methods that have been shown to be worthwhile, those that are of unknown value and those that are known to not be worthwhile. Breast cancer screening by mammography in women aged over 50 years can reduce mortality from the disease by about 30%. Screening for colorectal cancer by FOBT can reduce mortality from the disease by about 15%. Both rates were shown using randomised trials. Screening for cancer of the cervix by cervical cytology has been judged to be worthwhile ( about 80% reduction in mortality from this disease), though without evidence from randomised trials.
A difficulty with screening is that some cancer screening programmes have been introduced in the absence of evidence that they are worthwhile, for example, prostate cancer screening, and breast cancer screening in women aged under 50 years. A difficulty with screening is that some cancer screening programmes have been introduced in the absence of evidence that they are worthwhile, for example, prostate cancer screening, and breast cancer screening in women aged under 50 years. It is important that health authorities resist the temptation to introduce population screening programmes until there is firm evidence of efficacy, as judged by a reduction in mortality from the cancer in question. The presumption of benefit should not be sufficient grounds for introducing large scale programmes. Sometimes an effective screening test (e.g. prostate-specific antigen (PSA) for prostate cancer) has led to the introduction of screening programmes in the absence of trial results showing evidence of benefit in terms of disease prevention. Once such services are in place they can be difficult to stop. Then existing data should be used to try to evaluate efficacy, albeit in a less than ideal manner.
There is a general need to continually evaluate screening services to ensure that the performance expected from the results of randomised trials and other relevant research can be achieved in practice. Service provision will depend on available resources and the burden of disease from the cancer in question in the absence of screening.
The following cancer screening programmes should be made generally available:
- Screening for breast cancer by 3 yearly mammography examinations for women from the age of 50 years
- Colorectal cancer screening by FOBT every 2 years from the age of 50 years.
- Cervical cancer screening by 5 yearly cervical smear examinations for women from the age of 25 years.
Others should not be offered as services at all or should be part of research programmes designed to determine their value. There are screening tests available and being evaluated for stomach cancer, oral cancer, nasopharynx cancer and neuroblastoma. Screening for prostate cancer and screening for lung cancer are, however, the subject of much recent research.