Acute myeloid leukemia arising in treated chronic lymphocytic leukemia/small lymphocytic lymphoma: a challenge in analysis of lineage relationship.

Herein, we report a 75-year-old female with a history of chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/SLL) post obinutuzumab and venetoclax therapy, presenting with CD4+/CD56+/CD123+ acute myeloid leukemia (AML) with histiocytic morphology. The unusual morphology triggered a preliminary diagnosis of CLL transdifferentiation to histiocytic sarcoma (HS); however, a comprehensive workup led to the correct diagnosis of de novo AML. This case showcases the utility of ancillary studies in the work up of secondary myeloid neoplasms post B-cell lymphomas and underlines pitfalls to consider for accurate characterization of these complex malignancies.