Neoplastic pleural effusion in a dog

Samples of pleural fluid were submitted from a 10-year-old Golden Retriever for cytological evaluation.

Fig 1. This is a low power view of a direct smear of the pleural effusion, revealing a highly cellular sample.
Fig 2. On closer inspection, the majority of nucleated cells consist of pleomorphic, neoplastic mast cells mixed with moderate numbers of eosinophils.

Final Diagnosis

Mast cell neoplasia


The dog had a history of multiple cutaneous mast cell tumours and in the absence of other primary lesions, the neoplastic effusion is consistent with metastatic disease. Similar to carcinomatosis, regardless of the site of primary neoplasia, the neoplastic cells can exude via the lymphatics into the thoracic, pericardial and abdominal cavities, resulting in a neoplastic exudate.