Blastic plasmacytoid dendritic cell neoplasm (BPDCN) is a rare, clinically aggressive tumor derived from the precursors of plasmacytoid dendritic cells (PDCs). The patient was diagnosed with BPDCN showing blast in peripheral blood smear, and the blast lineage could not be assigned using flow cytometry.

We identified a 75-year-old male with BPDCN, who had a high number of blasts and significant reduction in white blood cell count. Due to the lack of a distinct lineage identified in the flow cytometry analysis, we first classified this acute leukemia as ’Acute leukemia, unclassifiable, with BPDCN’. Nevertheless, further genetic test results, such as next-generation sequencing (TET2 mutation) or chromosomal studies (complex karyotype), provide indications for the connections between myelodysplastic changes.

This case presents the challenging diagnosis of unclassifiable acute leukemia accompanied by BPDCN. In the absence of a proven treatment for BPDCN, chemotherapy regimens for acute myeloid leukemia or lymphoma are now being used. The lineage assignment in our case was not possible without further genetic test results, such as next generation sequencing and chromosomal analysis, to identify the characteristics of the leukemic blast. In patients with BPDCN who exhibit leukemic manifestation, this case underscores the importance of additional genetic testing.