(a) Suspected DCISM: here, a cluster of three cells is in
the stroma adjacent to a duct containing DCIS (*). Although suspicious, this
focus is not definitely diagnostic of microinvasion, and could represent
prominent endothelial cells in a small periductal vessel. Work-up with
appropriate myoepithelial and/or epithelial markers may help to resolve the
differential diagnosis in similar cases. (b) Definite DCISM: here,
the microinvasion comprises a cluster of 15–20 neoplastic cells devoid of
myoepithelium, spanning <1 mm in the lymphocyte-rich stroma between two
ducts (*) harboring DCIS. DCISM ductal carcinoma in situ with
microinvasion, DCIS ductal carcinoma in situ