INTRODUCTION

This category of neoplasms represents a diverse group of morphologically similar but biologically heterogenous soft tissue and bone neoplasms characterized by an epithelioid or polygonal cell morphology. Numerically, this is the least common group of neoplasms with only granular cell tumor and synovial sarcoma occurring with significant frequency. Synovial sarcoma represents approximately 5–10% of all soft tissue sarcomas and granular cell tumor is a relatively common benign neoplasm.

The characteristic or predominate cell type in each of these neoplasms has an ovoid or polygonal shape in smear preparations. In most of these neoplasms, a similar shape is demonstrable in H&E stained histologic sections yielding a superficial resemblance of these cells to epithelial cells. In the case of synovial sarcoma, the polygonal or ovoid shape seen cytologically appears to be a result of smearing artifact and a more spindle shape characterizes synovial sarcomas histologically. Some members of this cytomorphologic category demonstrate an accepted direction of differentiation such as granular cell tumor which shows neural differentiation and Langerhans cell histiocytosis which demonstrates histiocytic differentiation. Other lesions such as synovial sarcoma, epithelioid sarcoma, and extrarenal rhabdoid tumor represent sarcomas of uncertain type and lack a definitive direction of differentiation. Despite lack of a clear direction of differentiation for some of these neoplasms, characteristic immunohistochemical and molecular profiles exist for synovial sarcoma, clear cell sarcoma of soft parts, Langerhans cell histiocytosis, granular cell tumors, and extrarenal rhabdoid tumors.