A practical approach to diagnose soft tissue myeloid sarcoma preceding or coinciding with acute myeloid leukemia.

Myeloid sarcoma involving soft tissue is rare and may present a pathologic diagnostic challenge, particularly when it precedes or coincides with hematological malignancies. Furthermore, it may mimic non-Hodgkin lymphoma, poorly differentiated carcinoma, melanoma, or round blue cell tumors, which is a potential diagnostic pitfall. In addition to a retrospective review of myeloid sarcoma (MS) cases seen at our institution, we describe differential diagnoses, diagnostic pitfalls, and practical approaches to diagnosing soft tissue MS preceding or coinciding with acute myeloid leukemia. Our institutional retrospective review (1999-2011) of MSs identified 12 cases of MS in which there was no known blood or bone marrow involvement at diagnosis. A panel of immunohistochemical stains and/or flow cytometry was reviewed; marker selection was subject to the pathologist's discretion. These tumors were consistently positive for CD117 (9/9), CD43 (7/7), myeloperoxidase (8/10), CD68 (4/5), and CD34 (5/9) by flow cytometry and/or immunohistochemistry. We also described a referral case, which had classic MS morphology and a myelomonocytic immunophenotype including positivity for CD45, lysozyme, and CD117 with supporting molecular information. Based on our institution's experience and review of the literature, we recommend that when the index of suspicion for MS is high, an immunohistochemical stain and/or flow cytometry panel should include CD43, lysozyme, CD117, CD68, CD33, Human Leukocyte Antigen DR (HLA-DR), and myeloperoxidase, in addition to thorough review of the patient's history, cytogenetic studies, and proper discussion with the clinician.