Overdiagnosis in breast imaging

The main harm of overdiagnosis is overtreatment. However a form of overdiagnosis also occurs when foci of cancer are found by imaging in addition to the symptomatic lesion when this leads to additional treatment which does not benefit the patient. Even if overtreatment is avoided, knowledge of the diagnosis can still cause psychological harm. Overdiagnosis is an inevitable effect of mammographic screening as the benefit comes from diagnosing breast cancer prior to clinical detectability. Estimates of the rate of overdiagnosis at screening are around 10%. DCIS represents 20% of cancers detected by screening and is the main focus in the overdiagnosis debate. Detection and treatment of low grade DCIS and invasive tubular cancer would appear to represent overdiagnosis in most cases. Supplementary screening with tomosynthesis or US are both likely to increase overdiagnosis as both modalities detect predominantly low grade invasive cancers. MRI causes overdiagnosis because it is so sensitive that it detects real tumour foci which after radiotherapy and systemic therapy do not, in many cases go on and cause local recurrence if the women had had no MRI and undergone breast conservation and adjuvant therapy with these small foci left in situ.