Cytological, histological and ultrastructural nuclear features of monster cells in a canine carotid body carcinoma.

A 7-year-old female Shih-tzu dog was presented with severe dyspnoea. A large mass was palpated in the left cranial neck. Cytological examination of an aspirate sample revealed cells with marked anisokaryosis, giant elements and many bare nuclei. Scattered intact giant cells showed scant, granular cytoplasm and intranuclear inclusions. Histologically, neoplastic cells were subdivided into lobules by fine collagenous trabeculae. Numerous pleomorphic giant, or 'monster', cells were observed, showing a highly indented nuclear envelope, intranuclear cytoplasmic pseudoinclusions (ICPs) and 'ground-glass' nuclear appearance. Neoplastic emboli were present, but no distant metastases were detected grossly. Immunohistochemically, the neoplastic cells expressed synaptophysin and had variable expression of neuron-specific enolase and vimentin. The cells were negative for pan-cytokeratin, CAM 5.2, glial fibrillary acidic protein and S100. Nuclear abnormalities and cytoplasmic neurosecretory granules were noted ultrastructurally. These features were consistent with a diagnosis of carotid body carcinoma (chemodectoma). Monster cells with ICPs have not been documented previously in canine chemodectoma.