When South Korean doctors launched a nationwide thyroid cancer screening programme, diagnoses shot up 15 fold. Yet the death rate from thyroid cancer didn’t budge. More patients were being created than lives were being saved.

It is a clear illustration of a problem that is quietly reshaping how doctors think about cancer: overdiagnosis. Not misdiagnosis but the accurate detection of tumours that would not actually harm the patient.

Modern cancer screening is rightly celebrated as one of medicine’s great achievements. Finding cancer early saves lives. But as technology has become ever more sensitive, are we sometimes doing more harm than good?

A cancer doesn’t spring from a single rogue cell flicking a switch. It develops through multiple steps, and many clusters of abnormal cells never complete that journey.

Some sit quietly in the body for decades. Only a fraction ever become life threatening. The problem is that once an abnormality is detected and labelled as cancer, it triggers a chain reaction – anxiety, aggressive........

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