Length-time bias in cancer screening occurs because screened populations are more likely to detect:

• A Aggressive rapidly progressive cancers with short sojourn time
• B Slow-growing cancers with long pre-clinical detectable phase ✓
• C Interval cancers that present symptomatically between screening rounds
• D Cancers in high-risk individuals who are more likely to participate in screening

Explanation

Length-time bias occurs because slow-growing tumours with a long pre-clinical detectable phase (long sojourn time) are disproportionately detected by periodic screening compared with fast-growing tumours. Since slow-growing tumours inherently have better prognosis, screened populations appear to have improved survival — not because screening saves lives but because it selectively identifies indolent disease. Aggressive cancers with short sojourn times are missed between screening rounds and present as interval cancers (option C), which is a distinct issue from length-time bias.

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

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Written and medically reviewed by the StethoPrep medical team.