The use of magnetic resonance imaging to screen women with high risk mutations in the genes associated with breast cancer has raised debate on what constitutes sufficient evidence for the efficacy of new screening tests.1^–4 The gold standard is evidence from randomised trials that early detection reduces mortality, as is the case for mammography and breast cancer,5 but how should we evaluate new tests that might detect cancer earlier?

Showing that a new test is more sensitive than others suggests that it has promise as a possible screening test,6 but detecting more apparent cases does not necessarily mean that using the test routinely will lead to a further reduction in breast cancer deaths. To fulfil the criteria for an effective screening test, the additional cancers detected must include ones that would both progress during the patient's lifetime and be curable by earlier treatment. The extra cancers picked up by new tests may count as cancers histopathologically but might not progress to cause symptoms in the women's lifetime: thus, a new test might lead to more overdetection rather than improved outcomes (figure). Overdetection of …