Overdiagnosis is the discovery of tissue that shows all the pathological features of cancer on biopsy, but which would have caused no symptoms or death if unrecognised. In addition to frequent false-positive results from screening tests that result in invasive tests and procedures, overdiagnosis itself is now recognised as an important additional harm of screening.1
A retrospective cohort study recently published in the Annals of Internal Medicine attempted to quantify this risk. To do this, doctors at the Yale Cancer Centre compared the cumulative incidence of breast cancer in 54,635 women who had recently undergone mammography with women who did not continue to participate in screening. Women with a history of cancer were excluded.
In women aged 70 to 74 years, the adjusted cumulative incidence of breast cancer was 6.1 cases per 100 screened women compared to 4.2 cases per 100 unscreened women. In this population, an estimated 31 per cent of breast cancer cases found in the screened women were overdiagnosed. For women aged 75 to 84 years, the figure was 47 per cent and for women over 85 years, 54 per cent of diagnoses.
This higher rate of cancer diagnosis among screened women suggests that a substantial proportion of breast cancers detected in this way would not cause symptoms or otherwise manifest themselves in women who were not screened, the authors conclude. They also report that they did not find a statistically significant reduction in breast cancer-specific deaths associated with screening.
These figures make it clear that we need better methods to find out who might benefit from screening and which findings are unlikely to progress to symptomatic cancer so that we can avoid overtreatment, explained main author Prof Ilana Richman from the Yale Cancer Center in an interview.2 "Overdiagnosis is particularly relevant for older women. There is a correlation between overdiagnosis and life expectancy," said Prof Richman.2