Acute Myeloid Leukemia with an Unusual Cytokeratin Expression Mimicking Metastatic Carcinoma

Abstract

Immunohistochemistry is an indispensable diagnostic modality, particularly in malignancies. There are instances in the literature where a panel of immunohistochemical (IHC) markers has shown unexpected expression in a tumor. Hence, understanding normal and aberrant IHC marker expressions, particularly their pattern, is an essential prerequisite for diagnosis. We report the case of an elderly woman evaluated for dyspnea and anemia. She had a past history of breast cancer treated by mastectomy. Her peripheral smear showed 22% blast cells. Bone marrow examination showed infiltration by immature cells with the pattern of an epithelial malignancy. However, since the peripheral blood had blast cells, this prompted us to perform a panel of IHC markers (CD 34 and CD 45) on bone marrow trephine biopsy sections. The infiltrating cells showed dot-like expression of cytokeratin (clone AE1/AE3), raising the suspicion of a metastatic carcinomatous deposit. CD 45 immunostaining also showed positive expression. Flow cytometry revealed acute myeloid leukemia. The expression of cytokeratin by the blasts initially misled the diagnosis as a metastatic carcinoma. Simultaneous flow cytometric evaluation pointed out the accurate diagnosis. Thus, it is essential that pathologists understand aberrant staining patterns that one can encounter in IHC.