A Wolf in Bear's Clothing

A right inferior orbitotomy was done for tissue diagnosis. Histological analysis of the tumor mass showed sheets of poorly differentiated, infiltrative cells with moderate eosinophilic to amphophilic cytoplasm, large nuclei and prominent nucleoli (Pathology Images 1-2). Immunophenotyping by flow cytometry showed expression of CD4, CD43, CD45RO, CD68, CD99, lysozyme, and myeloperoxidase (Pathology Image 3), diagnostic of myeloid sarcoma (MS). Bone marrow aspirate confirmed acute myeloid leukemia (AML) (Pathology Images 4-5), while CSF analysis did not show malignant cells. Genetic analysis showed rearrangement of the mixed lineage leukemia (MLL) gene (Pathology Images 6-7). Peripheral blood smear showed rare atypical circulating cells equivocal for AML. MS is a rare extramedullary complication of acute leukemia consisting of a solid tumor proliferation of myeloid blasts that can involve any part of the body including head, body cavities and subcutaneous tissue. It is most common in children under 10 years of age with primitive AML. 1, 2 Most of these children are of African, Asian, South Pacific and Latin American decent.3 Although rare, AML can present with orbital involvement in the form of MS before diagnosis of the underlying leukemia. 1,4 In such cases, the subsequent development of leukemia usually occurs within 5-12 months. A recent study by Johnston et al found that orbital MS patients had significantly higher overall survival than central nervous system (CNS) MS and patient with non CNS MS. 5 Leukemia represents 2-6% of orbital tumors in children, but eleven percent of children with proptosis will have some form of acute leukemia. 1 CT and MRI are important for distinguishing neoplasms from hematomas or abscess, but cannot distinguish granulocytic neoplasms from other tumors. Tissue diagnosis is often necessary. Leukemia is strongly suggested by the presence of elevated WBC and peripheral and medullar blasts, though these were not present in our patient.