A breast cancer screening programme using mammography detects cancers at a median tumour size of 10 mm, while symptomatic diagnosis occurs at 25 mm. The 5-year survival from the screened cancers is 95% vs 60% in unscreened patients. Before concluding the screening programme is effective, the evaluation must account for which bias that particularly affects survival-based evidence?
- A Volunteer bias alone
- B Length-time bias and lead-time bias together ✓
- C Overdiagnosis bias only
- D Recall bias
Correct answer: B. Length-time bias and lead-time bias together
Explanation
Lead-time bias inflates apparent survival by advancing the time of diagnosis without changing the date of death — earlier detection simply means patients 'survive longer' with the diagnosis. Length-time bias occurs because screening tends to detect slower-growing, less aggressive tumours (overrepresented in screen-detected cases), giving the false appearance of better prognosis. Both biases must be corrected when using survival as a screening outcome; mortality (deaths per 100,000) is the preferred endpoint for evaluating screening effectiveness.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
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Written and medically reviewed by the StethoPrep medical team.