What is the screening for cancer?
Cancer screening principles
Screening is the process whereby asymptomatic individuals are tested in order to detect a disease that has yet to be symptomatic. For this to be effective in a population, there are certain criteria that must be met by the disease in question, the screening test, and the screening programme.
- Its natural history is well understood.
- It has a recognizable ‘early’ stage.
- Treatment at an early stage is more successful than at a later stage.
- It is sufficiently common in the target population to warrant screening.
- Sensitive and specific.
- Adequate facilities for diagnosis in those with a positive test.
- High quality of treatment for screen-detected disease.
- Screening repeated at intervals if the disease is of insidious onset.
- Benefit must outweigh physical and psychological harm.
- Benefit must justify the financial cost.
It is crucial that treating the disease to be screened at an early stage is more effective than treating at a later stage. To justify a screening programme, one cannot compare the outcome of screen-detected disease with that of symptomatic disease, because three biases operate in favour of screen-detected disease.
- Lead time bias arises from the fact that, if early diagnosis advances the time of diagnosis of a disease, then the period from diagnosis to death will lengthen, irrespective of whether or not treatment has altered the natural history of the disease. If patients die of their cancer at the same age at which this event would have occurred without screening, no benefit has been afforded by screening. Screening will only be of value if it improves the survival curve of a screened population, compared with unscreened.
- Length bias operates, as slow-growing tumours are more likely to be detected by screening tests, when compared to fast-growing tumours, which are more likely to present with symptoms before a screening test can be applied or between tests. Thus, screen- detected tumours will tend to be less aggressive and associated with a relatively good prognosis.
- Selection bias results from the characteristics of individuals who accept an invitation to be screened. Such a person is more likely to be health-conscious than one who refuses or ignores screening and may therefore be more likely to survive longer, irrespective of the disease process.
The beneficial cancer screening
In screening, it is also important to have a target population to avoid large numbers of fruitless tests in individuals at low risk of cancer. In screening for the common cancers, where the incidence is highly age-dependent, the age range should be that in which the disease is relatively common and in which the patients are likely to be fit, enough for curative treatment.
There are other predictors of risk, and family history is becoming important in this respect, particularly as it is now possible to detect specific genetic mutations from blood samples and to use these to screen close relatives. Examples of this are mutations in the APC gene in FAP, in the DNA mismatch repair genes in HNPCC, and in the BRCA1 and 2 genes in familial breast and ovarian cancers.
A screening test must be acceptable and safe, so that it will be adopted by the target population. It must also be sensitive and specific. Sensitivity is the proportion of individuals with the disease who have a positive test, and specificity is the proportion of individuals without the disease who have a negative test.
When a screening programme is established, it is important that the diagnostic facilities are adequate. Similarly, treatment of early disease must be associated with minimal morbidity and mortality.
It must also be remembered that screening may cause psychological harm, and, along with any physical morbidity caused by investigation and treatment, this represents part of the cost of screening. The benefits gained through cancer screening must outweigh such morbidity, and society must make a decision whether or not the health gain justifies these and the financial costs.