Optimized immunohistochemical panel to differentiate myeloid sarcoma from blastic plasmacytoid dendritic cell neoplasm

Myeloid sarcoma (MS) and blastic plasmacytoid dendritic cell neoplasm (BPDCN) can be difficult to distinguish morphologically, even with the use of extensive immunohistochemical studies. Three new research markers, myxovirus A (MxA), CLA/CD162, and CD303/BDCA-2, have been reported to be positive in BPDCN, but their clinical utility has never been tested. We compared these markers to other antibodies that have been used traditionally to distinguish MS from BPDCN to assess the utility of these newer antibodies in differential diagnosis. Formalin-fixed, paraffin-embedded tissue sections of 23 MS and 17 BPDCN cases were assessed using immunohistochemical analysis for CD4, CD14, CD33, CD43, CD56, CD68, CD123, CD163, myeloperoxidase, lysozyme, terminal deoxynucleotidyl transferase (TdT), T-cell leukemia 1 (TCL-1), MxA, cutaneous lymphocyte-associated antigen (CLA)/CD162, and blood dendritic cell antigen 2 (BDCA2)/CD303. We identified antibodies with a high predictive value of ≥90% and used these markers to develop an approach to classification using specific staining criteria. Diagnostic classification criteria were based on staining patterns of one or more of the seven markers. BPDCN was associated with positive staining for CD56, TdT, or TCL1, or negative staining for lysozyme. MS was associated with positive staining for lysozyme or myeloperoxidase, or negative staining for CD56, CD123, myxovirus, or TCL1. The immunohistochemical staining patterns observed using a panel that includes MPO, CD56, CD123, TCL1, TdT, and MxA, are predictive of MS or BPDCN. In this study, neither CD162 nor CD303 had good predictive value in distinguishing MS from BPDCN.