Histologic Description: This is a myxoid lesion featuring epithelioid cells that have focally vacuolated cytoplasm. The neoplastic cells were negative for keratin, desmin, S100, and HMB45. The morphology was suggestive of gastrointestinal stromal tumor (GIST) but the CD117 stain being negative initially argued against this possibility. However, the neoplastic cells were diffusely positive for DOG1. Sequencing revealed a PDGFRA mutation. GISTs with the PDGFRA mutations often are epithelioid, have vacuolated cytoplasm and myxoid stroma, and are CD117 negative.
Differential Diagnosis: Myxoid liposarcoma would demonstrate a branching capillary vasculature as well as lipoblastic cells, and would be characterized by a t(12;16) translocation fusing the FUS gene with DDIT3. Low-grade fibromyxoid sarcoma may involve the gastrointestinal tract; however, these lesions are typically more fibrous than myxoid, and are composed of bland spindled cells. They demonstrate diffuse labeling for MUC4, and demonstrate fusions of either FUS or EWSR1 with CREB3L1 or CREB3L2. Myxoid melanoma would demonstrated immunoreactivity for S100 and HMB45, and typically demonstrate more pleomorphic cells.