Spatial Lineage Divergence at Diagnosis in a Pediatric KMT2A -Rearranged Leukemia Presenting with Concurrent AML and B-LBL

We describe a 1-year-2-month-old boy with KMT2A -rearranged leukemia presenting with concurrent acute myeloid leukemia in bone marrow and B-lymphoblastic leukemia/lymphoma in a right calf mass. Both lesions harbored an identical KMT2A::MLLT10 fusion, confirming common clonal origin. This represents the first documented case of spontaneous “spatial lineage divergence” at diagnosis, distinct from therapy-induced lineage switch. The finding highlights the potential need for multi-site biopsy and molecular profiling in extramedullary disease.