Answer: Ductal carcinoma in situ

Histology: This is a very difficult and challenging case. The intraductal proliferation has a somewhat spindled and polymorphic appearance in areas, which suggest the possibility of florid usual ductal hyperplasia. However, several features argue against that possibility. First, the lesion is growing in intermediate and large size ducts, which is a typical pattern of ductal carcinoma in situ and not typical of usual duct hyperplasia. Second, while some areas are spindled and have nuclear overlap, other areas highlight the monotonous cytology of the lesional cells. Third, the cells show diffuse immunoreactivity for estrogen receptor, and loss of labeling for high molecular weight cytokeratin, a typical pattern of DCIS. Finally, though not shown on the slides, this extensive intraductal process was associated with areas of invasive carcinoma, support its precursor role. Hence, the diagnosis is that of ductal carcinoma in situ

Discussion: The features described above contrast with usual duct hyperplasia, which usually has patchy labeling for ER and more extensive labeling for high molecular weight cytokeratin. Atypical duct hyperplasia is typically a lobulocentric process, and does not usually involve large ducts to this extent. Atypical lobular hyperplasia can be excluded by the absence of discohesion and absence of signet ring cells in the proliferation.