Flow cytometric detection of isolated central nervous system relapse in acute myeloid leukemia under FLT3-targeted therapy.

Central nervous system (CNS) relapse in acute myeloid leukemia (AML) is rare in adults and infrequently occurs as an isolated manifestation following allogeneic hematopoietic stem cell transplantation. We report a 68-year-old man with FLT3-ITD-mutated AML with monocytic differentiation who achieved sustained morphological and molecular remission following induction chemotherapy, consolidation, allogeneic transplantation, and FLT3 inhibitor maintenance therapy. Approximately 18 months after diagnosis, the patient developed isolated CNS relapse despite continued bone marrow remission and complete donor chimerism. Cerebrospinal fluid (CSF) flow cytometry identified leukemic blasts, and PCR detected FLT3-ITD, confirming CNS involvement. Following intrathecal chemotherapy, systemic therapy, gilteritinib treatment, and a second transplantation, the patient developed recurrent CNS disease with optic nerve involvement and vision loss. Notably, repeat CSF studies demonstrated persistent leukemic infiltration detectable by flow cytometry despite repeated negative FLT3-ITD PCR results, suggesting immunophenotypic-molecular discordance under FLT3-targeted therapy. This case highlights the CNS as a sanctuary site in AML and underscores the critical role of CSF flow cytometry in the diagnosis and monitoring of CNS relapse, particularly when molecular studies are negative. This case provides a valuable teaching point highlighting the limitations of relying on a single diagnostic method.