Cancer screenings are tests that look for early signs of cancer in healthy people with no symptoms. The aim is to find as many early cancers, or pre-cancers, as possible so people can be treated when the chance of cure is highest. The goals of cancer screening are prevention of cancer or detection at a very early stage.
Screening for some cancers is now common practice in many western countries. Screening is still somewhat controversial and some of the benefits are hotly debated. To be able to screen for cancer, we must have a test that can find that particular cancer early. We cannot screen for most cancers as we do not yet have reliable screening tests.
Once a test is found, researchers have to make sure there is low risk of the test saying you have cancer if you don’t – a false positive result; this often causes understandable anxiety. Patients may then go on to have unnecessary tests to try to find the “cancer” (which they actually don’t have). The test must not miss too many cancers – false negative results; these give false reassurance. The test must also be easy to do and not unpleasant or harmful for the people being screened. Finally the financial aspect cannot be ignored and the screening test must be cost effective. This involves large clinical trials to test if new screening methods are effective, acceptable and affordable.
Many western countries have established national screening programmes for breast cancer, cervical cancer and bowel cancer. There are effective tests for these types of cancer; mammograms, smear tests and faecal occult blood tests.
There are no effective national screening tests for other types of cancer, although research is looking into screening tests for prostate cancer and ovarian cancer. Some people who are at a high risk of developing certain types of cancer can have specific tests, such as colonoscopies for colon cancer or genetic blood tests for inherited breast cancer.
When considering screening tests, the benefits should always outweigh the harms. In 1968, Wilson and Jungner of the World Health Organisation developed 10 principles, that are still valid today, which a national screening programme should meet;
- The condition should be an important health problem
- There should be an accepted treatment for patients, and treatment should be better at an earlier stage
- Facilities for diagnosis and treatment should be available
- There should be a recognisable latent or early symptomatic stage
- There should be a suitable test or examination
- The test should be acceptable to the population
- The natural history of the condition should be adequately understood
- There should be an agreed-upon policy on whom to treat as patients
- The cost of case-finding should be economically balanced in relation to possible expenditure on medical care as a whole
- Case-finding should be a continuing process and not a “once and for all” project.
Although screening tests for breast, cervical and bowel cancer satisfy these principles. Until there is clear evidence to show that a national screening programme brings more benefit than harm, there will not be a programme of prostate cancer screening for men with no symptoms.
Evidence from a prostate cancer screening trial in Europe showed screening reduced mortality by 20 per cent. However, this was associated with a high level of over treatment. To save one life, 48 additional cases of prostate cancer needed to be treated.
By Dr Michael Roger
MB BCh (Liverpool), MRCGP (UK)