Microinvasive lobular carcinoma associated with intraductal spread arising in a mammary hamartoma

A 53 year old woman presented with a lump in the inner lower quadrant of the left breast. Histological examination of the breast tumour confirmed that the lesion was a mammary hamartoma. Carcinoma with foci of microinvasion was observed in the lobules of the hamartoma concomitant with the intraductal spread of lobular carcinoma. Immunohistochemically, the cancer cells were negative for β-catenin, which generally stained normal breast ducts and ductal carcinomas. This is only the sixth case of breast carcinoma arising in a mammary hamartoma to be reported and, moreover, the fourth case of lobular carcinoma occurring within a hamartoma. Despite the apparent rarity of this case, pathologists should be aware of the possibility of carcinomas arising within mammary hamartomas.