Myeloid sarcoma: Clinical, Pathologic, and Molecular Findings.

Myeloid sarcoma is defined as a neoplasm of myeloid blasts that involves an extramedullary site as a mass and effaces tissue architecture. Myeloid sarcoma is considered clinically to be equivalent to acute myeloid leukemia (AML) and requires appropriate therapy. As currently defined, extramedullary AML that does not form a mass is inconsistent with the diagnosis of myeloid sarcoma and the management of patients with extramedullary AML is less well defined. Furthermore, the pathogenetic relationship and clinical importance of extramedullary AML versus myeloid sarcoma is under explored. Recent studies have shown the importance of the MAPK/ERK pathway in pathogenesis of myeloid sarcoma and extramedullary AML. These studies also have shown that extramedullary sites of myeloid disease often evolve independently from medullary AML, likely related to microenvironmental clonal selection pressures and immune evasion mechanisms at extramedullary sites that differ from bone marrow disease. Here we review the clinicopathologic, immunophenotypic and molecular findings of myeloid sarcoma as currently defined. We also discuss the differential diagnosis of myeloid sarcoma and its relationship to non-mass forming extramedullary AML.