Unusual cytological appearance of a neuroendocrine carcinoma in a dog

A 13-year-old female neutered Boxer dog was presented with a history of chronic intermittent haemorrhagic diarrhoea. Cytological evaluation of fine-needle aspirate samples from an enlarged mesenteric lymph node revealed an individualised round to polygonal cell population with poorly defined cell margins and marked anisocytosis and anisokaryosis. The nuclei were round to oval with lacey chromatin and commonly nucleoli. The cytoplasm was lightly basophilic and rarely contained variable numbers of fine red to purple granules. The cytological interpretation was anaplastic neoplasia, most likely a poorly differentiated, high-grade mast-cell neoplasm. Histologic evaluation of the lymph node and a liver nodule revealed densely cellular, solid sheets and nests of pleomorphic round to polygonal neoplastic cells separated by a fine fibrovascular stroma. The cells had granular cytoplasm with no evidence of metachromatic staining using toluidine blue and Giemsa stains. Immunohistochemically, the neoplastic cells were positive for chromogranin A, neuron-specific enolase and synaptophysin, and negative for cytokeratin AE1/AE3 and S-100. A final diagnosis of neuroendocrine carcinoma was made.