Transcription factor E3 protein-positive perivascular epithelioid cell tumor of the appendix presenting as acute appendicitis: a case report and review of the literature.

Perivascular epithelioid cell tumors (PEComas) are a group of mesenchymal tumors that coexpress melanocytic and smooth muscle markers; their exact origin remains unknown. This group includes renal angiomyolipoma, clear cell sugar tumor, and lymphangioleiomyomatosis, although the term perivascular epithelioid cell tumors is currently used for lesions that exhibit a similar morphologic and immunohistochemical profile throughout the human body. Recently, a distinct subset of PEComas has been shown to harbor transcription factor E3 gene (TFE3) fusions. We report, for the first time, a unique case of TFE3-positive PEComa presenting as acute appendicitis in a 24-year-old woman. Microscopically, the tumor was composed of benign-appearing epithelioid cells with clear and eosinophilic cytoplasm, and arranged in nested and alveolar patterns. Immunohistochemical studies showed diffuse strong positivity for neuron-specific enolase, TFE3, and progesterone receptor and focal strong positivity for human melanoma black-45 (HMB-45) and melanocyte differentiation antigen (Melan-A) in the tumor cells. Although rare, PEComa should be included in the differential diagnosis of mesenchymal tumors of the appendix.