EWSR1::CREM rearranged intra-abdominal malignant epithelioid neoplasm: two new cases of an emerging entity with clinicopathological characteristics and histological pitfalls.

The EWSR1::CREM rearranged intra-abdominal malignant epithelioid neoplasm is an emerging tumor, with only a few publications describing it to date. Here, we report two new cases of this highly aggressive tumor, primarily involving the peritoneal surface. The tumors presented as a widespread diffuse peritoneal lesion associated with a 4-cm pelvic mass in a 28-year-old woman (Case 1) and as a 10-cm intra-abdominal mass infiltrating the stomach with multiple hepatic metastases in a 53-year-old woman (Case 2). The tumors shared predominant epithelioid morphology with minimal nuclear polymorphism. One of them additionally harbored spindle and rhabdoid cell populations. Both tumors displayed immunoreactivity for pan-cytokeratins, EMA, and CD99, and variable positivity for MUC4, progesterone and estrogen receptors, pan-NTRK, and synaptophysin. This misleading histology and immunophenotype give rise to a wide spectrum of differential diagnoses and highlight the crucial role of RNA sequencing in diagnostic accuracy and thus in appropriate therapeutic approaches.