Malignant neuroendocrine tumour in a horse.

Introduction: A gelding argentin horse, 25 years of age, was presented to a private veterinary clinician with a history of 3 months masticatory problems. The oral cavity examination revealed a large ulcerated mass 12 x 5 x 5 cm that affected the upper 2nd and 3rd premolar teeth, and involved as well the nasal cavity where it formed a bulging mass. Multiple samples were obtained from the mass and were submitted to histopathological and immunohistochemical (IHC) examinations. Material and methods: Serial tissue samples were processed routinely and stained with hematoxylin & eosin. IHC stainings were performed with the following antibodies: anti-pancytokeratin, anti-vimentin, anti-NSE, anti-synaptophysin, anti-chromogranin A, anti-CD3, anti-CD79, anti-S-100 and anti-GFAP. Results: The mass was submucosal, not encapsulated, multilobular, ulcerated, with invasive growth and was composed by packed nests of polygonal to round cells embedded in a fine fibrovascular stroma. The cells showed indistinct cell borders, moderate amount of clear cytoplasm, round nucleus with coarsely stippled chromatin and two to three evident nucleoli. The mitoses were 5-6 for high power field. Anysocytosis and anisokaryosis were moderate. The neoplastic cells were strongly positive for NSE and vimentin, variably positive for chromogranin A, and negative for synaptophysin, pankeratin, CD3, CD79, S-100 and GFAP. Discussion: Neuroendocrine tumours occur in tissues that contain cells derived from the embryonic neuronal crest, neuroectoderm and endoderm. Thus they may develop at many sites in the body. The immunohistochemical results confirm the present case as a neuroendocrine tumour. The exact primary origin of the tumour cannot be estabilished, since both oral or nasal neuroendocrine tumour have the same features and the horse described was affected at both sites. Differential diagnosis included neuroendocrine tumours (derived from APUD cells), neuroendocrine carcinoma (from Merkel-like cells) and olfactory neuroblastoma (from olfactory membrane). According to the WHO classification of tumours in domestic animals, olfactory neuroblastomas are characterized by the presence of typical rosettes and NSE and S-100 positivity; neuroendocrine carcinomas are positive to cytokeratin and Chromogranin A. In the present case the pankeratin negativity excluded the diagnosis of neuroendocrine carcinoma, and the absence of rosettes and S-100 negativity excluded that of olfactory neuroblastoma. The morphology and the positivity of the neoplastic cells to vimentin, NSE and mildly to chromogranin A suggested the origin of this tumour from the dispersed neuroendocrine system (APUD).