Microscopic appearance of the index malignant OFMT case (OFMT#1) showing monomorphic ovoid spindle cells with an infiltrative growth within the muscle (A, 200×), arranged in solid sheets (A, 200×) and cords within a hyalinized stroma (B, 100×). Higher power shows cohesive epithelioid tumor cells with nuclear atypia and increased mitotic activity (C, 400×). Immunohistochemistry shows focal expression of S100 (D, 100×) and diffuse staining for cytokeratin (E, AE1:AE3, 100×) and TFE3 (F, 200×)