(A) Low power photomicrograph (40×; H&E stain) demonstrates tumor (*) compressing left atrium (LA). (B) The tumor is comprised of nests of fairly uniform cells with moderate eosinophilic, granular cytoplasm (200×; H&E stain). (C) Marked cytologic atypia with pleomorphic nuclei and prominent nucleoli was focally present (400×; H&E stain) in addition to (D) increased number of mitoses (arrows) (400×; H&E stain). (E) Scattered foci of coagulative tumor necrosis (N) were also noted (200×; H&E stain). (F) The tumor exhibited diffuse, strong cytoplasmic positivity for synaptophysin immunostain (400×). (G) S100 immunostain highlights sustentacular cells surrounding nests of tumor cells (400×).