[Malignant epithelioid neoplasm with EWSR1::CREM fusion in soft tissue: a clinicopathological analysis].

<b>Objective:</b> To investigate the clinicopathological features, immunophenotype and molecule characteristics of EWSR1::CREM fusion malignant epithelioid neoplasm in soft tissue. <b>Methods:</b> The clinical and pathological data of 2 cases of EWSR1::CREM fusion malignant epithelioid neoplasm in soft tissue diagnosed at the Department of Pathology, Beijing Jishuitan Hospital, Beijing, China from May 2023 to May 2024 were analyzed. Immunohistochemical study, fluorescence in situ hybridization (FISH) and next generation sequencing (NGS) were performed. Relevant literature was reviewed. <b>Results:</b> There were one male and one female patients, aged 35 and 29 years, respectively. The tumors developed in the somatic soft tissue, including calf and chest wall, and were 6.0 and 6.2 cm in size, respectively. The imaging studies suggested space-occupying lesions in muscle tissue. Case 1 did not involve the bone, while Case 2 showed fracture of the 8th rib. Microscopically, a fibrous pseudocapsule surrounded by lymphocytes and plasma cells was identified. The tumors were composed of small to medium-sized round and short spindle-shaped cells, showing nodular or sheet-like pattern. The tumor cells showed round nuclear outline, coarse chromatin with prominent nucleoli. Immunohistochemically, tumor cells showed diffuse positivity of ALK (D5F3), MUM1 and Syn, focal or patchy positivity of CKpan, EMA, S-100, NSE, WT-1 and SMA, and a high Ki-67 index (20%-30%). FISH demonstrated break-apart signals of EWSR1 gene in the 2 cases. NGS revealed EWSR1::CREM gene fusion. Case 2 showed an ATRX gene mutation. The two patients were free of recurrence or metastasis at the 10-month and 1-month follow-up, respectively. <b>Conclusions:</b> EWSR1::CREM fusion malignant epithelioid neoplasm is rare and lacks distinctive morphological and immunohistochemical features. FISH and NGS can help make a definitive diagnosis.