BPDCN

Blastic plasmacytoid dendritic cell neoplasm (BPDCN)

Morphologically the primitive cells may resemble monocytic AML, or may have less easy to define or even lymphoic morphology. The cells typically lie in the "blast area" - with dim to moderate CD45 expression with low side scatter. A skin rash is present in most cases.

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Potential diagnostic difficulties

BPDCN is derived from myeloid cells and shares some morphological and some immunophenotypic characteristics with typical AML:

• Features of primitive phenotype: CD7 and TdT are frequently expressed but CD34 is most often absent
• Lineage-defining markers should not be expressed: MPO, cCD3, and CD19 (as well as CD20, cCD22, CD79a) should not be expressed
• Myeloid lineage-associated features are often found: expression of CD33 is relatively common (x%), CD117 may also be expressed (x%), CD13 may be found (x%). Therefore cases may fit the criteria to assign myeloid lineage
• It is therefore important that specific features of BPCDN are actively sought, particularly in cases with skin rash, absent CD34, and where diagnosis is based on expression of two myeloid markers

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Diagnostic criteria for BPDCN:

Bright expression of: CD56 is generally accompanied by CD36, CD38, CD43, CD71, and HLA-DR, To confirm the diagnosis look also for plasmacytoid dendritic cell (PDC) associated markers: CD123, CD303, CD304, TCF4, TCL1 .

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MAJOR CRITERIA MINOR CRITERIA EXCEPTIONS

WHO diagnostic criteria allow BPDCN to be diagnosed:

• in the presence of CD123 and one other PDC-associated marker in addition to CD4 and/or CD56,
• in the presence of three PDC-associated markers and the absence of CD34 and other cell type-specific markers including CD3, CD14, CD19, lysozyme, and MPO.

Given the presence of other hematologic malignancies with similar phenotypes, correlation with histomorphology, clinical information and IHC studies is always necessary to make a definitive diagnosis of BPDCN.