[263] Cellular Spindled Histiocytic Pseudotumor Complicating Mammary Fat Necrosis: A Potential Diagnostic Pitfall

Andrew P Sciallis, Beiyun Chen, Andrew L Folpe. Mayo Clinic, Rochester, MN

Background: Fat necrosis (FN) of the breast is a relatively common reactive/reparative process that may be either primary, often following trauma, or secondary to prior surgery or therapeutic irradiation. Primary mammary FN may closely mimic breast neoplasia, both clinically and radiographically, and is thus frequently biopsied. Recently, in consultation, we have seen a number of cases of mammary FN complicated by a cellular, spindled proliferation of macrophages, mimicking various spindle cell neoplasms of the breast. Herein we report our experience with these distinctive pseudotumors.
Design: Our institutional and consultation archives for the period 1994-2011 were searched for cases of mammary FN, yielding a total of 161 cases. For consultation cases, only those in which the submitting pathologist's differential diagnosis included a spindle cell neoplasm were included. For institutional cases, only those presenting as a mass lesion and showing a cellular spindle cell histiocytic proliferation were included. Nineteen cases met these criteria and comprised the final study group. All available routinely stained and immunohistochemical slides were re-reviewed.
Results: For consultation cases, suggested diagnoses included spindle cell metaplastic carcinoma, cellular fibroepithelial lesion with stromal overgrowth, desmoid-type fibromatosis, angiosarcoma and “atypical spindle cell proliferation”. The morphologic features of all cases were similar, showing a moderately cellular, fascicular proliferation of mitotically active, normochromatic spindled cells, surrounded by more typical features of FN, including non-spindled, lipid-laden macrophages, inflammatory cells, degenerating adipocytes, calcification, and myofibroblastic and capillary proliferation. When performed, immunohistochemical studies showed the spindled cells to have a histiocytic phenotype, with expression of CD163 and CD68, and absent expression of various cytokeratins, S100 protein, p63, and beta-catenin.
Conclusions: We have presented 19 cases of a distinctive cellular proliferation of spindled histiocytes, arising in the setting of mammary FN. This presumably represents an exaggerated, unusual morphological manifestation of the normal response to FN in the breast. Awareness of this distinctive pseudotumor should help to prevent its misdiagnosis as various other spindle cell neoplasms that may involve the breast.
Category: Breast

Monday, March 19, 2012 8:45 AM

Platform Session: Section B, Monday Morning


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