Answer: Displaced epithelium in needle biopsy site

Histology: There is no evidence of typical duct carcinoma in-situ present within the specimen. Rather, there is a linear tract of cellular fibrosis consistent with a prior needle biopsy site. Within this needle biopsy tract are small nests and individual cells consistent with breast epithelium. Some of these nests appear to have a myoepithelial cell layer. Focally surrounding the biopsy site are foci of usual duct hyperplasia.

Discussion: Radial scars typically have entrapped tubules within hyalinized acellular fibrous tissue. At the perimeter of the radial scar, these tubules merge in with more typical papillomatosis and usual duct hyperplasia. The cellular fibrosis seen in the current case and the lack of usual duct hyperplasia at the perimeter of the entire lesion differs from that of a radial scar. Furthermore, radial scars have as the name implies a radiating appearance of fibrous tissue growing out from all directions whereas the current lesion has a linear low power appearance. The major differential diagnosis in this case is infiltrating duct carcinoma. This case would be unusual for infiltrating duct carcinoma in that the small tubules are restricted only to this linear fibrous tract. The tubules do not extend out into the adipose tissue as is typically seen with infiltrating duct carcinoma. Furthermore, the original biopsy showed only duct carcinoma in-situ without an infiltrating component. It would be untenable to postulate that all the infiltrating duct carcinoma is present within the needle biopsy site area on excision yet was not present on the initial needle biopsy. These findings are consistent with displaced epithelium resulting from the needle biopsy procedure. The epithelium that may be displaced may be either benign epithelium or duct carcinoma in-situ. The presence of a myoepithelial cell layer around some of these tubules suggest that at least some of the epithelium displaced in the current case represents displaced benign epithelium.