Nontender Mass on the Right Side of the Neck

Awoman inher50swasadmitted toourhospitalwitha5-weekhistoryofanontendermassontherightsideof theneck.Nohuskyvoice, dysphagia,orodynophagiawerereported.Aphysicalexaminationrevealed a2 × 2-cm tumorover the right sideof her neck at level III. Bilateral vocal fold movements were normal. The free thyroxine level was0.94ng/dL, and the thyroid-stimulatinghormone levelwas 1.33 mIU/mL. (To convert thyroxine to picomoles per liter, multiply by 12.871.) Fine-needle aspiration (FNA) was performed, and cellular changes suspected to bemalignant neoplasmwere reported. Computed tomographic images of the neck showed enlargement of the right side of the thyroid glandwith retrosternal extension, and total thyroidectomywas arranged (Figure, A, arrowhead). During the operation, the tumorwas found to extend to the thoracic cavity and to be attached tightly to the right carotid artery. The surgical specimen showed a 6.5 × 6.0 × 2.8-cmdark brown and elasticmass replacing mostof therightsideofthethyroid.Thetumorpossessedaroughsurface, and focal hemorrhages were also noted. The thyroid tumor appeared to be composed of broad anastomosing islands of tumor cells separated by desmoplastic stroma (Figure, B). No lymphovascular invasion or extrathyroidal extensions were identified. The tumor cell nests were well demarcated, with smooth borders in some areas, and a syncytial pattern could also be observed with irregular borders in other areas.Microscopically, the tumorwas composed of confluentnestsandlobulesofepithelioidandspindle-shapedcellssurroundedbyfibroussepta.Thetumorcellshadvesicularnucleiandeosinophiliccytoplasm.Markedanisonucleosiswithirregularnuclearcontourswerealsonoted.Thenuclear-cytoplasmic ratioswereveryhigh, andthepleomorphictumorcellnucleialongwithasmallnucleoliwere evident. Increased lymphocyteandplasmacell infiltrationaroundand within the tumor cell nests was seen. Immunohistochemical studies revealedthat thetumorcellswerepositive forcytokeratin (Figure,C), CD5 (Figure, D), andp63butwere negative for thyroid transcription factor-1. What is your diagnosis? A B