Breast cancer pseudometastasis in a sentinel lymph node with cytokeratin-positive debris

We report extensive pseudometastasis detected by immunohistochemical (IHC) staining within a sentinel lymph node. An 83-year-old woman underwent simple mastectomy and sentinel lymph node biopsy (SLNB) for infiltrating ductal carcinoma. Intraoperative frozen section of the SLNB specimen appeared histologically negative for metastasis. IHC staining for cytokeratin in permanent sections, however, showed what was reported as micrometastasis in the subcapsular sinus. Since these cells did not resemble the primary tumor cells morphologically, and had actually been called histiocytes in the frozen section, further IHC staining was done. The subcapsular cells were negative for epithelial membrane antigen (EMA) staining, but they were positive for CD68, a macrophage marker. Thus the cytokeratin-positive cells were not metastatic breast tumor cells, but rather were histiocytes with phagocytized cytokeratin debris. This case report illustrates that IHC staining for cytokeratin in SLNB specimens for breast cancer must be supported by morphologic assessment and further appropriate staining before it can become the basis for treatment decisions.