A population-based mammography programme identifies a higher proportion of slowly growing, non-aggressive breast cancers than would be expected from clinical series. Rapidly progressive tumours present symptomatically between screening rounds and are missed. Which phenomenon explains this over-representation of indolent cases among screen-detected cancers?

• A Lead time bias
• B Length bias ✓
• C Detection bias due to volunteer effect
• D Verification bias

Explanation

Screening samples the detectable preclinical phase, and slow-growing tumours spend more time in that phase, so they are disproportionately captured while aggressive tumours surface between rounds. This is length bias, and it inflates the apparent survival benefit of screening. Verification bias refers to selective application of a gold standard test, which is unrelated to growth rate.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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