Concurrent Bone Marrow Acute Undifferentiated Leukemia and Mediastinal T-Lymphoblastic Lymphoma With Identical SET::NUP214 Fusion and PHF6 and EZH2 Mutations.

Acute undifferentiated leukemia (AUL) is a rare hematologic malignancy lacking lineage-specific markers. Concurrent, clonally related AUL and T-lymphoblastic lymphoma (T-LBL) has not been reported previously. Here we describe a patient who was diagnosed with AUL in the bone marrow and T-LBL in the mediastinum after a thorough immunophenotyping by flow cytometry and immunohistochemistry. Despite their immunophenotypic differences, the AUL and T-LBL showed identical genetic alterations: SET::NUP214 fusion, PHF6, and EZH2 mutations. The patient achieved and remained in complete remission after chemotherapy and stem cell transplantation. This case underscores the value of comprehensive immunophenotyping and genetic analysis in rare hematologic malignancies.