Virtually all excisional biopsy specimens obtained after needle biopsy or localization procedures contain foci of hemorrhage in the breast stroma (1). Blood within the lumens of ducts and lobules not involved by the pathologic process as well as in epithelial structures in the lesional area is a frequent manifestation of a prior needling procedure (2) (Fig. 28.1). Fragments of epidermis may be dislodged into the breast if a small skin incision is not made before inserting the needle. Epidermoid inclusion cysts may be formed from displaced skin epithelium (Fig. 28.1) (3). The healing needle core biopsy site typically forms scar tissue. The maturity of the scar depends on the interval that has elapsed. Some of these reactive foci have stromal mitoses and myofibroblastic hyperplasia, which should not be mistaken for a neoplastic process (4). Gel foam or similar material is often injected into the biopsy site after the tissue has been removed to improve hemostasis.

Disruption of the epithelium in the lesion may result in displacement of epithelial cells into the needle track and into stroma in the lesional area. This can produce a pattern that simulates invasive carcinoma (1,2,5,6) (Figs. 28.2, 28.3). Displacement of benign or carcinomatous epithelium is suggested by finding scattered, isolated fragments of epithelium in artificial spaces within the breast stroma. At the needle biopsy site, the displaced epithelium is accompanied by hemorrhage, inflammation, hemosiderin-laden macrophages, or granulation tissue. Displaced epithelium that is not in the biopsy site may not elicit a stromal reaction. This can lead to the mistaken diagnosis of invasive carcinoma in a benign lesion (Fig. 28.4). It is not unusual to find fragments of sloughed epithelium in the lumens of ducts and cysts that resemble displaced epithelium in the stroma. Displaced epithelium in vascular spaces is indistinguishable from intrinsic lymphatic or vascular invasion in routine H&E slides (2,5) (Figs. 28.5, 28.6, 28.7). Epithelial displacement might be suggested if p63-positive myoepithelial cell nuclei were demonstrated at the periphery of an intravascular epithelial cell cluster. The extent to which breast needling procedures contribute to the hematogenous or lymphatic dispersal of tumor cells has not been determined.

The p63 immunostain for myoepithelial cells can be helpful for confirming the presence of epithelial displacement if p63-positive nuclei can be demonstrated around the extra-lesional epithelial cells (Fig. 28.8). Coincidentally, it must be possible to detect p63-positive myoepithelial cells in the lesion that was sampled by the core biopsy. Myoepithelial cells are more likely to be present if the primary lesion is benign, but they can also be found in in-situ carcinomas. Failure to find p63-positive nuclei in association with the extra-lesional epithelium is not, by itself, diagnostic of invasive carcinoma, even if the lesion contains p63-positive myoepithelium, because only a minority of epithelial cells in a proliferative lesion are contiguous to myoepithelium, which can accompany displaced epithelial fragments. Hence, when p63-positive cells can be demonstrated as evidence of epithelial displacement, they are usually associated with a minority of the displaced epithelium.

Stromal epithelial displacement was found in three of five surgical biopsy specimens from patients with intraductal carcinoma and in three of seven papillary carcinomas studied by Boppana et al. (5). Lee et al. (6

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