‘Inside‐out’ p120 immunostaining pattern in invasive micropapillary carcinoma of the breast; additional unequivocal evidence of reversed polarity

Invasive micropapillary carcinoma (IMPC) of the breast is a distinct histologic variant of ductal carcinoma in which tumor cells display reverse polarity. Tumor cells form morula-like clusters and tubular structures suspended in a clear space, with a characteristic “inside-out” pattern.1 Although IMPC can be readily identified by its typical histological features, immunohistochemistry (IHC) for epithelial membrane antigen (EMA) and sialyl Lewis X can be employed to confirm this “inside-out” pattern, especially where the IMPC component is focal.2 By IHC, EMA highlights the apical pole of the tumor cell membrane, which faces the empty space and the stroma, confirming the reverse polarization. Immunohistochemistry for E-cadherin, a component of adherens-type junction, also demonstrates “inside-out” pattern in IMPC: Positivity in lateral cell borders with sparing of the apical portion of the cell, also referred to as a “cup-shaped” staining pattern.3 Since p120 catenin is bound to the juxtamembrane intracytoplasmic domain of e-cadherin, we proposed that in IMPC, as with E-cadherin, p120 catenin expression should be limited to lateral cell borders with no apical positivity. This article is protected by copyright. All rights reserved.