#### Summary points Ductal carcinoma in situ (DCIS) is a preinvasive (also termed non-invasive) breast cancer, where proliferations of malignant ductal epithelial cells remain confined within intact breast ducts (fig 1⇓). DCIS is a precursor lesion that has the potential to transform into an invasive cancer over a timescale that may be a few years or decades long. The development of its ability to invade and metastasise is as yet unquantifiable and is attributed to the accumulation of somatic mutations in premalignant cells. Treatment aims to prevent DCIS from progressing to invasive breast cancer. Fig 1 Difference between normal, ductal carcinoma in situ (DCIS), and invasive disease DCIS was rarely diagnosed before the introduction of national screening programmes but is now common, accounting for 20% of screen detected cancers in the United Kingdom.1 Treatment usually comprises surgery (mastectomy or wide local excision), with or without adjuvant radiotherapy. However, it is possible that a subset of these lesions would never progress to invasive breast cancer over the lifetime of the patient if left untreated, and in this (as yet undefined) population traditional management may represent overtreatment. Deciding on appropriate personalised treatment for individual patients diagnosed with DCIS is an ongoing challenge, because the optimum management remains …