Cancer screening describes the systematic testing of a population in order to detect a cancer before it causes symptoms. The World Health Organization has identified key features that are required for successful screening (Table 16.1). These relate to the tribe (population), test, treatment, tumour and treasury (money). Unfortunately, there are few cancers for which population screening is available.
The utility of a screening test depends on the ability to detect a true positive (sensitivity) and the ability to reject a true negative (specificity) (Table 16.2).
Successful screening
For screening to be successful the detection test should be easy to use and should ideally detect a pre-cancerous condition that allows treatment before the development of an invasive cancer. There are few cancers that have a natural history that allows detection of a pre-cancerous condition. There should be a clearly defined target population and increasingly this will be defined on the basis of molecular abnormalities such as BRCA1 gene status for breast and ovarian cancer, or APC gene status for colorectal cancer. In such target groups the higher risk is matched by more intensive screening, or alternatively the opportunity to intervene to reduce risk using prophylactic surgery with mastectomy, oophorectomy or colectomy.
Successful detection has to be coupled to an intervention that changes the natural history of the cancer and offers extension of lifespan. The more advanced the cancer at the point of detection, the smaller the magnitude of intervention that will impact on survival. For some patients there is a resistance to attend for screening, and education of the population about the benefits is key.
Lead time bias
The introduction of screening can detect tumours at an earlier (presymptomatic) stage, therefore when compared with a symptomatic cohort the subsequent survival is spuriously prolonged, even if earlier treatment does not actually increase lifespan. For screening to be beneficial, the earlier detection and treatment must impact on mortality, increasing lifespan.
Length time bias
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