[Cytokeratin (pan)-positive EWSR1/FUS::CREB fusion malignant tumor in abdominopelvic cavity: a clinicopathological and molecular analysis of four cases].

Objective: To investigate the clinicopathological and molecular genetic characteristics of pan cytokeratin (CKpan)-positive EWSR1/FUS::CREB fusion malignant tumors in abdominopelvic cavity. Methods: Four cases of malignant tumor with CKpan-positive EWSR1/FUS::CREB fusion were selected from January 2019 to July 2024 in the Department of Pathology, Tianjin Medical University Cancer Hospital, Tianjin, China. Their clinical, pathological, and immunohistochemical characteristics were examined. Their molecular genetic characteristics were analyzed using fluorescence in situ hybridization (FISH) and next-generation sequencing (NGS). Results: Among the 4 patients, there were 2 males and 2 females, aged 44, 44, 48 and 66 years, respectively. The tumor sites included 1 case located between the stomach and transverse colon, 1 case on the serous surface of the gastric wall, 1 case in the transverse mesocolon, and 1 case in transverse mesocolon and small mesentery. The clinical manifestations were mostly abdominal distension and abdominal pain. The maximum diameter of the tumor in the surgical resection specimen was 3.5-8.5 cm. The tumor's cut surface was grayish-white and gray-yellow in color, with medium consistency. Microscopically, the tumor cells were mainly composed of epithelioid tumor cells, and 2 of the tumors showed that tumor cells arranged in a solid sheet or multinodular pattern, and the cytoplasm of the tumor cells was abundant, lightly stained, and the boundaries were unclear, accompanied by the formation of capsules or microcapsules, and lymphocyte and plasma cell sleeves were seen. In one case, the pseudopapillary arrangement was present, and the tumor cells were radially distributed around the fibrovascular axis. In another case, it was arranged in a pseudoacinar pattern, and the nest was surrounded by slender reticular fibers. Immunohistochemistry showed that tumor cells expressed CKpan (4/4) and WT1 (4/4, including 1 focal positive). Vimentin, CK8/18, D2-40 and S-100 were expressed in various intensities, while Calretinin was locally positive or negative. FISH showed that 2 cases had EWSR1 break-apart and 2 cases had FUS break-apart. NGS confirmed the presence of EWSR1::CREM fusion (1 case) and FUS::CREM fusion (2 cases), respectively. Except for 1 recently diagnosed case, 3 cases were followed up: 1 patient died due to tumor recurrence and metastasis (overall survival was 33 months), and 2 patients survived (1 case had recurrence 58 months after surgery, and 1 case had no recurrence or metastasis after surgery). Conclusions: CKpan-positive EWSR1/FUS::CREB fusion malignant tumor is a rare malignancy tumor with undetermined classification that tends to occur in the abdominopelvic cavity and often involves the gastrointestinal tract. Molecular testing such as FISH and NGS is helpful for a definitive diagnosis.