A 27-year-old woman underwent surgical management of a suspected loculated pleural effusion discovered during evaluation for flank pain. At the time of surgery, a thick pleural rind was encountered with tenacious adhesions, dense exudate, and trapped lung. H&E-stained sections of the pleural peel showed a variably cellular spindle cell neoplasm (Figure 1) comprising a relatively uniform population of atypical cells with centrally located oval shaped nuclei, inconspicuous nucleoli, and tapering bipolar eosinophilic cytoplasmic processes. Neoplastic spindle cells were predominantly arranged haphazardly within a pale staining basophilic myxoid stroma (Figure 2); however, focally, they were arranged in compact fascicles with collagenized stroma (Figure 3). Immunohistochemistry showed neoplastic cells were positive for calretinin (Figure 4) and SS18-SSX (Figure 5) with rare, isolated tumor cells staining for pancytokeratin cocktail (Figure 6); WT1 and claudin-4 were negative (not shown).

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