CD31 is a surrogate marker for blastic plasmacytoid dendritic cell neoplasm: a case of a hematologic skin tumor immunohistochemically false negative for CD45

Round cell tumors of the skin in the adult constitute one of the diagnostic challenges for histopathologists, because their differential diagnosis covers a broad spectrum of tumors of epithelial as well as non-epithelial origins. As the first step towards the correct diagnosis, immunohistochemical screening of the tumor sample by CD45 (leukocyte common antigen, LCA) antibody is thought to be useful to examine whether the tumor is derived from hematopoietic lineages. Here, we report a diagnostically challenging case of CD31-positive malignant skin tumor with false immunonegativity for CD45, which turned out to be blastic plasmacytoid dendritic cell neoplasm (BPDCN). The tumor was composed of middle to large-sized round cells, which was supposed to be hematologic malignancy, probably of lymphoid or myeloid origins. However, immunohistochemistry of the tumor cells showed that they seemed to be CD45-negative. We considered the result to be false negative because immunohistochemistry of another CD45 antibody from a different supplier revealed that the tumor cells were positive. It turned out that CD45 antibodies from different suppliers were derived from different clones with presumably distinct specificity. On the other hand, CD31 immunopositivity of the tumor cells suggested the possibility of vascular origin of the tumor. However, ERG immunohistochemistry of the tumor cells showed weaker staining than that of vascular endothelium, enabling us to reach the correct diagnosis of BPDCN, a CD31-positive hematologic malignancy. We also discuss how to avoid these pitfalls and potential implications of CD31 expression as a surrogate marker for hematologic malignancy.