A teenage boy presented with a 3-month history of a left preauricular mass. His medical history was notable for an atypical teratoid rhabdoid tumor of the left temporoparietal lobe diagnosed at age 3 years, which had been resected and treated with chemoradiation. Physical examination demonstrated a 2-cm firm, nontender, mobile mass without overlying skin invlvement. There was no palpable lymphadenopathy, butweakness of the frontal branch of the left facial nervewasnoted.Magnetic resonance imaging (MRI) of the neck revealed a 1.0 × 1.7 × 1.9-cmmildly enhancingmass lateral to the left subcondylar mandible, which was inseparable from the anterior parotid gland (Figure, A). A superficial parotidectomy was performed. The superior division of the facial nerve was intimately associated with the parotid mass, and sacrifice of the upper division of the facial nerve branches with a cable nerve graft was required. The cut surface of the surgical resection specimen of the superior parotid revealed a 1.7-cm reddish tan variegated mass. Microscopic sections showed a circumscribed tumor with microcystic architecture (Figure, B). The tumor was composed of monotonous amphophilic cells with granular cytoplasm and round to ovoid nuclei (Figure, B and C). An immunohistochemical panel showed notably positive tumoral staining for the proteins S-100 and vimentin and negative staining for DOG1 and p63 antibodies. Special stains for mucicarmine and periodic acid–Schiff diastase (PASD) were performed. Mucicarmine highlighted abundant extracellular (luminal) mucin, but intracellular mucin was absent. PASD confirmed the absence of zymogen granule formation. Fluorescence in situ hybridization (FISH) was positive for translocation of the ETV6 gene. What is your diagnosis? A B
Read full abstract