A rare case of malignant peritoneal mesothelioma with EWSR-ATF1 fusion transcription and unusual immunophenotype

Malignant peritoneal mesothelioma (MPM) is a rare, aggressive, and fatal neoplasm of the abdominal mesothelium with a very abysmal survival if left untreated. MPM with EWSR1-ATF1 fusion is an uncommon entity recently recognized by molecular studies. We report the first case of MPM with EWSR1-ATF1 fusion and aberrant CK20 expression. The patient was a 38-year-old man who presented to our institution with three months history of abdominal pain and distention, a significant weight loss (>30 lb), and failure to thrive. He was working in a construction company with 20 year-history of questionable asbestos exposure. Imaging studies demonstrated radiographic patterns suggestive of extensive peritoneal carcinomatosis. However, serum tumor markers were within the normal limit (CA 19–9, CEA, AFP, and βHCG). Subsequently, he underwent a biopsy of the peritoneal nodules. Biopsy showed a proliferation of epithelioid/round cells with ample cytoplasm and occasional vacuoles, displaying papillary architecture. Immunohistochemistry showed tumor cells were strongly and diffusely positive for WT1, AE1/3, CK7, CK20, desmin, and CD99, focally positive for calretinin, D2-40, and CK5/6, while negative for BerEp4, MOC-31, trypsin, TTF-1, P40, GATA3, CDX2, and PAX8. P16 and BAP1 were retained. Fluorescence in situ hybridization studies showed EWSR1 rearrangement, and the NGS fusion panel revealed EWSR1-ATF1 fusion. A diagnosis of MPM with EWSR1-ATF1 fusion was rendered. Unfortunately, the patient passed away within a month of diagnosis. Pathologists should be aware of this entity, especially when faced with tumors displaying mesothelioma morphology but showing atypical immunoprofile (co-expression of mesothelial markers with strong CK20).