Screening is not as good as prevention. It is also important to note that screening is not a diagnostic test. All results will need further tests to confirm a diagnosis, for example, a biopsy.
Cancer screening involves testing large numbers of people who have no symptoms of the disease. However, only a small number of individuals with the disease will benefit because their cancer is detected early.
When screening large numbers of people there are four potential outcomes true positive, true negative, false positive and false negative.
- A true positive result demonstrates the benefit of screening in reducing morbidity and mortality.
- True negative results give reassurance, which can be the motivation for screening.
- False positive results occur when the test is positive, with no disease. This can cause unnecessary anxiety and require further tests which lead to more costs (3).
- False negative results occur when the test fails to detect the presence of cancer. This can create a false reassurance and can lead to a late diagnosis and treatment
False positive results of mammography screening have been well studied. Kemp cites a study over a 10 year period: 2,400 women, 40 to 69 years of age, who had a median of four mammograms were evaluated. Over this period the rate of false positive results was 23.8%. Based on this it was estimated that after 10 screening mammograms, cumulative risk of false positive was 49% and 19% of women without breast cancer would undergo biopsy.
A further problem with screening for cancer is that the cancers detected may include low grade tumours that would never have presented clinically (4).
This was demonstrated in a randomised trial screening for lung cancer. There was an excess of 46 cancers diagnosed in the screening group all of which were early stage cancers that were resected, but despite 11 years of follow up their counterparts never appeared as more advanced cancer in the control group and there was no reduction in lung cancer mortality in the screened group (4).