Fig. 1: Microscopic features of peritoneal lesions with EWSR1–CREM fusion and co-expression of cytokeratin and WT1.

a–d (Case 1, 54/F, mesocolonic mass) Well-circumscribed neoplasm surrounded by a thick fibrous capsule associated with a dense lymphoid cuffing (a). The predominant architecture was solid (a, bottom). However, both macro- and microcysts were noted, the smaller cysts containing serous fluid (b). At higher power, the epithelioid cells had ill-defined borders, with eosinophilic cytoplasm, relatively round but slightly irregular nuclear membranes, vesicular chromatin, prominent nucleoli, and rare mitotes (b). The neoplastic cells showed diffuse immunoreactive for cytokeratin AE1:AE3 (c) and nuclear labeling for WT1 (d). e–h (Case 2, 10/F, rectovaginal pouch). This solid and cystic neoplasm was composed of epithelioid cells arranged in sheet-like pattern (e) or forming small tubular structures surrounding serous fluid (f). The cells were diffusely immunoreactivity for cytokeratin (g) and showed nuclear labeling for WT1 (h). i–l (Case 4, 9/M, adrenal) At low power, this epithelioid neoplasm was surrounded by a fibrous capsule and associated with a prominent lymphoplasmacytic cuff and abundant dystrophic calcification (i). At high power, the epithelioid cells showed eosinophilic cytoplasm, with forming focal microcysts containing serous fluid (j) and tubular structures within a hyalinized matrix (k). The neoplastic cells demonstrated patchy cytokeratin labeling (l) and nuclear labeling for WT1 (not shown).