MLL rearranged acute lymphoblastic leukaemia presenting as a maxillary sinus mass with a discordant immunophenotypic profile from the bone marrow.

We describe an unusual case of pre-B lymphoblastic leukaemia presenting with a unilateral maxillary sinus mass in which biopsies of the primary mass and the bone marrow demonstrated conflicting immunophenotyping results. The extramedullary mass was consistent with a precursor B-cell malignancy, while the bone marrow was initially reported as a possible mature B-cell malignancy. The treatments for the two are fundamentally different, which necessitated a delay in the initiation of his chemotherapy until a clear diagnosis was made. Mixed lineage leukaemia gene rearrangement was confirmed by fluorescence in situ hybridisation in both the primary mass and bone marrow, which unified the diagnosis as pre-B acute lymphoblastic leukaemia given the common cytogenetic feature.