A cancer screening programme reports a 40% reduction in disease-specific mortality between screened and unscreened groups. However, re-analysis accounting for 'overdiagnosis bias' reduces this to 15%. Overdiagnosis bias in cancer screening refers to:

• A Detection and treatment of cancers that would never have caused symptoms or death in the patient's lifetime ✓
• B Detection of cancers at an earlier stage that would have progressed to death regardless
• C Misclassification of benign lesions as malignant due to poor test specificity
• D Length bias from screening detecting slow-growing cancers more often

Explanation

Overdiagnosis is the detection and treatment of cancers (or precancers) that would never have become clinically apparent or caused death in the patient's lifetime — they would have remained dormant or regressed. This leads to apparent survival benefit without true mortality reduction. Length-time bias is a related but distinct phenomenon where screening preferentially detects slow-growing tumours with long pre-clinical phases. Overdiagnosis has a quantitative impact on the apparent mortality reduction in screening trials.

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

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Written and medically reviewed by the StethoPrep medical team.