Intra-articular Epithelioid Sarcoma Showing Mixed Classic and Proximal-type Features: Report of 2 Cases, With Immunohistochemical and Molecular Cytogenetic INI-1 Study

Epithelioid sarcoma, a rare sarcoma with epithelial differentiation, most often occurs in the distal extremities; however, it may occur in essentially any location. With the recent recognition that the loss of expression of the tumor-suppressor gene INI-1 may be associated with epithelioid sarcoma, it has become clear that epithelioid sarcoma may occur in previously unsuspected locations such as bone. Only 2 cases of intra-articular epithelioid sarcoma have been previously reported. We retrieved 2 intra-articular cases coded as epithelioid sarcoma from our archives. Both expressed cytokeratins (AE1/AE3 and OSCAR), CD34, vimentin, and epithelial membrane antigen, and showed complete loss of expression of INI-1. Fluorescence in situ hybridization was performed on formalin-fixed, paraffin-embedded sections by using a laboratory-developed dual-color probe containing INI1 (CTD-2511E13 and CTD-2034E7) (22q11.2) (OR) and PANX2 (RPCI3-402G11) (22q13.33) (GR) probes as control. Both cases occurred in a clearly intra-articular location in the knee. Case 1 was that of a 19-year-old man with a long-standing history of pain and limited joint function. This patient was disease free after amputation. Case 2 was that of a 60-year-old woman. Follow-up information available for this patient showed bilateral subpleural metastases. Morphologically, case 1 showed features of proximal-type epithelioid sarcoma, whereas case 2 showed mixed features of classic and proximal-type epithelioid sarcoma. Immunohistochemistry showed complete loss of INI-1 protein in both cases; fluorescence in situ hybridization analyses were negative for INI-1 gene deletion. Herein, we have reported 2 cases of intra-articular epithelioid sarcoma, showing morphologic and immunohistochemical features identical to those of epithelioid sarcoma in conventional locations, including loss of INI-1 expression. Intra-articular epithelioid sarcoma should be distinguished from malignant pigmented villonodular synovitis and from carcinoma metastatic to the synovium. Improved recognition of this rare clinical presentation should allow for better understanding of its unique features.