The phenomenon of 'length-biased sampling' in cancer screening leads to an overestimation of screening programme benefit by preferentially detecting which type of cases?

• A Rapidly progressing, aggressive cancers with a short detectable pre-clinical phase
• B Cancers that are clinically symptomatic at time of screening
• C Cancers with high population incidence regardless of growth rate
• D Slow-growing tumors with a long detectable pre-clinical phase (longer sojourn time) ✓

Explanation

Length-biased sampling is a bias unique to periodic screening: tumors with a long detectable pre-clinical phase (slow-growing, better prognosis) have a greater probability of being 'caught' during a screening window than fast-growing tumors (which are symptomatic between screens). Consequently, screen-detected cancers are disproportionately slow-growing, giving the false impression that screening cured fast-aggressive cancers when in reality those cases are simply rarer in the screened sample. Length bias inflates apparent survival benefit without reflecting true mortality reduction, which is why randomized controlled trials measuring mortality are the gold standard for evaluating screening programmes.

Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.

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Written and medically reviewed by the StethoPrep medical team.