Can malignant and inflammatory pleural effusions in dogs be distinguished using computed tomography?

Computed tomography (CT) is the primary imaging modality used to investigate human patients with suspected malignant or inflammatory pleural effusion, but there is a lack of information about the clinical use of this test in dogs. To identify CT signs that could be used to distinguish pleural malignant neoplasia from pleuritis, a retrospective case-control study was done based on dogs that had pleural effusion, pre- and postcontrast thoracic CT images, and cytological or histopathological diagnosis of malignant or inflammatory pleural effusion. There were 20 dogs with malignant pleural effusion (13 mesothelioma, 6 carcinoma; 1 lymphoma), and 32 dogs with pleuritis (18 pyothorax; 14 chylothorax). Compared to dogs with pleuritis, dogs with malignant pleural effusions were significantly older (median 8.5 years vs. 4.9 years, P = 0.001), more frequently had CT signs of pleural thickening (75% vs.44%, P = 0.04), tended to have thickening of the parietal pleura only (65% vs. 13%, P = 0.01) and had more marked pleural thickening (median 3 mm vs. 0 mm, P = 0.01). Computed tomography signs of thoracic wall invasion were observed only in dogs with malignant pleural effusions (P = 0.05). There were no significant differences in pleural fluid volume, distribution or attenuation, degree of pleural contrast accumulation, amount of pannus, or prevalence of mediastinal adenopathy. Although there was considerable overlap in findings in dogs with malignant pleural effusion and pleuritis, marked thickening affecting the parietal pleural alone and signs of thoracic wall invasion on CT support diagnosis of pleural malignant neoplasia, and may help prioritize further diagnostic testing.