OBJECTIVETo study the immunophenotype and molecular genetics of epithelioid sarcoma (ES), INI1 expression and its role in differential diagnosis.

METHODSTwenty cases of ES were retrieved from the archival files and selected for immunohistochemical study, DNA sequencing and fluorescence in-situ hybridization. The clinical and pathologic features were also reviewed.

RESULTSThe age of patients ranged from 16 to 75 years (mean = 40.2 years). The median age of patients in classic ES and proximal-type was 37.9 years and 42.0 years, respectively. The male-to-female ratio was 1.2: 1.0. Classic ES mostly occurred in the extremities while proximal-type ES often affected the perineum and external genitalia and trunk. Histologically, granuloma-like structures, consisting of aggregates of epithelioid and spindly tumor cells with central necrosis, were observed in classic ES. The epithelioid tumor cells contained abundant eosinophilic cytoplasm, merged with spindly cells at the periphery and admixed with collagen fibers. In proximal-type ES, the tumor cells showed prominent epithelioid and/or rhabdoid features, had marked cytologic atypia and grew in multinodular or diffuse patterns. In 2 cases of proximal-type ES studied, the "rhabdoid" tumor cells demonstrated a diffuse sheet-like growth pattern, mimicking malignant rhabdoid tumor. Immunohistochemical study showed that vimentin was positive in all cases. Pan-cytokeratin, CK8, CK7, epithelial membrane antigen and CD34 were expressed in 16, 15, 1, 18 and 13 cases, respectively. The staining for S-100 protein was focal and weak in 5 cases. None of the cases studied expressed CD31 and HMB45. Loss of INI1 was demonstrated in 10 of the 13 classic ES cases and 5 of the 7 proximal-type ES cases. The mutation of INI1 gene was detected in 1 of the 6 cases. Deletion of INI1 gene including heterozygous deletion, homozygous deletion and haploid was observed in 8 of the 11 cases.

CONCLUSIONSOwing to the histologic heterogeneity, pitfalls in diagnosis of ES sometimes are encountered. INI1 is lost in most cases of ES. Immunohistochemical study, including staining for INI1, provides useful clues in pathologic diagnosis. Instead of INI1 mutation, inactivation of INI1 gene related deletion is not uncommon.