Answer: Invasive ductal carcinoma arising in papillary carcinoma

Histology: The majority of this lesion has a rounded border to it, and features a monotonous epithelial proliferation supported by fibrovascular cores. The proliferation projects into large, partially cystic spaces. These are the features of papillary carcinoma. At the edges of the lesion, one sees the same neoplastic epithelial cells intermingling with the stroma and fat in an irregular fashion, indicative of invasion.

Discussion: Intraductal papilloma with DCIS would feature myoepithelial cells surrounding the lesion as well as within the fibrovascular cores.A papillary ductal carcinoma in situ features myoepithelial cells at the periphery of the lesion, but typically there is an absence of myoepithelial cells within the fibrovascular cores.Papillary carcinoma is a term which is used for cystic or solid papillary lesions which are cytologically carcinoma, but which have a rounded contour suggestive of intraductal growth.However, these lesions lack myoepithelial cells within their fibrovascular cores or at their periphery, raising the possibility that these lesions represent a low grade, pushing form of invasive carcinoma as opposed to ductal carcinoma in situ lacking peripheral myoepithelial cells.

In the current case, the irregular nests of carcinoma at the periphery of the lesion are diagnostic of invasive carcinoma on morphologic grounds.Most observers would not regard the papillary carcinoma component in the center of the lesion as invasive for staging purposes.