Necrotic lung tumor mimicking primary pulmonary abscess on computed tomographic imaging study in a dyspneic dog

A 14-year-old spayed female Shih-Tzu dog was evaluated for a 1-month history of progressive coughing. Results of the complete blood count and serum biochemistry panel indicated inflammation. Thoracic radiography showed pleural effusion, and cytology revealed septic pyothorax. After antibiotics medication, the patient improved clinical signs but relapsed pleural effusion with a suspicious lung lesion. Computed tomography (CT) of the thorax showed a cavitary lesion with ring-enhanced irregular walls and poorly defined margins in the left cranial lung lobe. The main differential diagnosis considered was lung abscess, but the histopathological diagnosis was necrotic lung adenocarcinoma. This case suggests that both a necrotic lung tumor and a lung abscess are differential diagnoses for ring-enhancing lesions on contrast-enhanced CT images. Especially in the case of septic pyothorax that does not improve or relapse with medical treatment, the underlying cause of a primary lung neoplasm should be suspected.