Answer: Papilloma with lobular carcinoma in situ

Histology: see below

Discussion: The section shows a papilloma with focal areas of sclerosis mimicking low grade invasive ductal carcinoma. In addition, an area of necrosis is seen within the papilloma. The latter can actually be found in benign papillary lesions and by itself does not indicate a malignant process. Within the papillary lesion, there is diffuse cellular areas raising the differential of usual hyperplasia, atypical ductal hyperplasia or lobular neoplasia involving the papilloma. Although mostly the epithelial proliferation shows streaming and cellular overlapping and would be diagnosed as usual hyperplasia, in one area however there is a proliferation of monotonous cells with round nuclei. These cells are negative for E-cadherin by immunostains and therefore would be diagnosed as lobular carcinoma in situ involving a papilloma.