Answer: Lobular carcinoma in situ (LCIS) involving sclerosing adenosis

Histology: One of the cores has sampled a localized lesion which on the non-transected surface has a rounded border. At higher magnification, there are rounded nests and smaller cords containing a proliferation of bland cells with monotonous nuclei. The cells are fairly loosely cohesive. The cell type is typical of lobular morphology. The presence of nests which are rounded containing the lobular cells is more typical of LCIS. The other smaller cords and nests of cells are set within a hyalinized sclerotic stroma which is typical of sclerosing adenosis. The overall lobular growth pattern of the lesion is also consistent with an underlying lesion of sclerosing adenosis.

Discussion: Infiltrating lobular carcinoma would not have a circumscribed growth pattern as seen at low magnification. It would also lack the rounded nests filled with lobular cells typical of LCIS. Also, lobular carcinoma lacks dense hyalinized sclerotic stroma seen within sclerosing adenosis. Papillomatosis may also involve the lobules. However, in papillomatosis the cells are not as round and as monotonous and loosely cohesive as seen in LCIS involving sclerosing adenosis. Within papillomatosis involving lobules, one would see solid proliferations of cells with slit-like spaces with nuclei having a more oval appearance and overlapping growth pattern. Infiltrating duct carcinoma as with infiltrating lobular carcinoma would have a more infiltrative growth pattern and show tubular formation. Infiltrating duct carcinoma tends to have a greater degree of cytologic atypia than lobular carcinoma. The stroma associated with infiltrating duct carcinoma is that of a desmoplastic stromal reaction with a greater degree of cellular reactive fibrous tissue in contrast to the hyalinized connective tissue seen within sclerosing adenosis.