Nasal melanoma in a 14 year old cross-breed dog

Abstract

A 14-year-old, neutered male crossbred border collie was presented for investigation of a 5 month history of sneezing and muco-haemorrhagic right-sided nasal discharge. Physical examination revealed reduced airflow through the right nares but was otherwise unremarkable. Haematology was unremarkable and biochemistry revealed only a mild elevation in amylase at 1367 U/L (<1245 U/L). A CT scan revealed a soft tissue mass effect in the dog's right nasal cavity, with significant turbinate lysis and focal destruction of the right nasomaxillary suture. No lesions compatible with metastatic disease were identified on CT of the dog's head, neck, thorax and abdomen. Rhinoscopy confirmed the presence of a nasal mass, and biopsies and cytology samples were procured. No evidence of metastatic disease was found by fine needle aspirate cytology of both mandibular lymph nodes. Cytology of the nasal mass demonstrated a population of pleomorphic polyhedral to spindloid cells among sheets of columnar ciliated epithelium. Nuclei were centrally placed and round with stippled chromatin and one or multiple prominent nucleoli; occasional macronuclei or multinucleate cells were observed. A number of cells contained green pigment, and in conjunction with the cellular pleomorphism were considered suggestive of melanoma. Histopathology described a neoplasm consisting of pleomorphic round to polygonal cells containing varying numbers of nuclei (1-9), abundant eosinophilic cytoplasm with light-brown intracellular granules, and less than 1 mitosis in 10 9400 fields, on a background of inflammation. The histogenesis of the tumour was not clear although the differentials of melanoma, olfactory neuroblastoma, neuroendocrine tumour and undifferentiated carcinoma were considered most likely. Fontana Masson stain was performed to detect melanin granules and was weakly positive. Immunohistochemistry demonstrated strong staining of the neoplastic cells for PNL-2 and Melan-A. Eight percent of the cells stained positively for cytokeratin and 20% for vimentin. These results were considered diagnostic of a melanoma. The dog received a hypofractionated radiotherapy protocol (four, weekly fractions of 8 Gray) and achieved a complete clinical response which persists at the time of writing (4 months). Malignant melanoma should be considered as a differential diagnosis for a canine nasal tumour.