Nasal CD56 positive small round cell tumors. Differential diagnosis of hematological, neurogenic, and myogenic neoplasms.

CD56-positive nasal and nasal-type natural killer (NK)/T-cell lymphoma is now a well-defined disease entity. Rare cases of blastic NK-cell lymphoma positive for CD56 have been recently reported. However, CD56 expression is also identified in several types of non-hematopoietic small round cell tumors in which lymphoma is included as a differential consideration. Here, we present nine cases of CD56+ small round cell tumors of histological origin unrelated to nasal NK/T-cell lymphoma. Eight of the nine cases presented as solid tumors of the sinonasal region. Clinical, histological, ultrastructural, and immunohistochemical examination and gene analysis for T-cell receptor (TcR) and immunoglobulin heavy chain (IgH) genes and in situ hybridization (ISH) for Epstein-Barr virus (EBV) were performed. Two cases presented with features consistent with blastic NK-cell lymphoma or lymphoblastic lymphoma of NK-cell phenotype. These cases showed features of lymphoblastic lymphoma, phenotypes of sCD3-, cCD3+, CD45+, CD56+, TdT+, and human leukocyte antigen (HLA)-DR+, germline of IgH and TcR genes, and EBV negative reactivity. One case had myeloid/NK-precursor acute leukemia/lymphoma with a phenotype of CD13+, CD33+, CD34+, CD56+, and MPO-. Three cases were neurogenic, including one case of olfactory neuroblastoma and two of primitive neuroectodermal tumors (PNET). It was difficult to differentiate CD56+ PNET from blastic NK-cell lymphoma, especially when only paraffin-embedded sections were available. Myogenic markers, such as HHF35, alpha-sarcomeric actin, and desmin, were positive in three cases of rhabdomyosarcomas. Our findings suggest that as CD56 is used more routinely as a marker in immunohistochemical staining, the differential diagnosis of extranodal lymphohematological malignancies and small round cell tumors will become more complicated.