Case Reports CD56-positive Small Round Cell Tumor: Osseous Plasmacytoma Manifested in Osteolytic Tumors of the Iliac Bone and Femora

Monoclonal gammopathy of undetermined significance does not overexpress cluster of differen-tiation (CD) 56, but plasma cell myeloma frequently overexpressed it. However, plasma cell leukemia and extramedullary plasmacytoma usually down-regulate CD56 expression. Plasmacytoma, especially ‘solitary plasmacytoma of bone’, is difficult to diagnose as plasma cell neoplasm, because it occasionally appears similar to other bone tumors, both clinically and pathologically, and is rarely accompaniedbymonoclonal protein in theserumorurine. Thepresent case was a patient with an osteolytic ‘small round cell tumor ’ of the iliac bone, which also invaded the femora. An immunohistopathological finding ofCD56expression played a key role inmaking a diagnosis.Thedefinitivediagnosisofplasmacytomawasmadebasedon theelectronmicroscopic findings. The plasmacells which infiltrated her sternumshowed the same restriction to kappa light chainexpression in their cytoplasmsas that of the iliac bone tumor cells, but did not expressCD56. Locally infiltrating osteolytic bone tumors should be examined for surface immunoglobulin light chains as well as CD56 expression when plasmacytoma is suspected. Key words: CD56 – osseous plasmacytoma – osteolytic bone tumor – plasmacytoma – SB