Diagnostic challenges of high-grade myeloid malignancies with partial plasmacytoid dendritic cell differentiation: report of two cases with literature review.

The diagnosis of myeloid neoplasms with plasmacytoid dendritic cell (pDC) differentiation can be challenging due to immunophenotypic overlap requiring detailed characterization by flow cytometry and immunohistochemistry. We describe two patients with a history of myeloproliferative neoplasm (MPN) and myelodysplastic/myeloproliferative neoplasm (MDS/MPN) who presented years later with leukocytosis, lymphadenopathy, splenomegaly, and cachexia, with rapid clinical deterioration and death. Lymph node biopsy specimens revealed involvement by myeloid sarcoma with prominent pDC differentiation. Furthermore, concomitant bone marrow aspiration and biopsy showed involvement by the underlying myeloid neoplasm but no parallel expansion of pDC, as seen in the lymph node specimens, suggesting that pDC differentiation is fostered in the lymph node microenvironment. These two cases could represent the "myeloid sarcoma" counterpart of the recently described acute myeloid leukemia with pDC differentiation (pDC-AML). Although patients with pDC-AML have an inferior outcome when treated with conventional therapies, the recognition of a pDC component in these neoplasms potentially expands the therapeutic options.