Could we tell the type of pleural effusion?

Can Malignant and Inflammatory Pleural Effusions in Dogs Be Distinguished Using Computed Tomography?

Thom C. Watton, Ana Lara-Garcia, Christopher R. Lamb.

Background
Pleural effusion in dogs can result from various pathological conditions, including malignant neoplasia and inflammatory diseases such as pyothorax or chylothorax. While cytological analysis of pleural fluid is commonly used for diagnosis, distinguishing between malignant and inflammatory effusions can be challenging. In humans, computed tomography (CT) is a key imaging tool for differentiating pleural malignancies from pleuritis, but there is limited information on its diagnostic utility in dogs. This study aimed to identify CT features that could help differentiate pleural malignant neoplasia from pleuritis in dogs.

Methods
A retrospective case-control study analyzed medical records and CT scans of 52 dogs diagnosed with either malignant pleural effusion (20 dogs: mesothelioma, metastatic carcinoma, lymphoma) or inflammatory pleural effusion (32 dogs: pyothorax, chylothorax). CT images were assessed for pleural thickening, thoracic wall invasion, mediastinal lymphadenopathy, fluid distribution, and pannus formation. Statistical analyses compared CT findings between the two groups to determine distinguishing features.

Results
Dogs with malignant pleural effusion were significantly older than those with pleuritis (median 8.5 vs. 4.9 years, P = 0.001). CT findings associated with malignancy included pleural thickening (85% vs. 44%, P = 0.03), isolated parietal pleural thickening (65% vs. 13%, P = 0.01), and greater median pleural thickness (3 mm vs. 0 mm, P = 0.01). Thoracic wall invasion was observed exclusively in malignant cases (15%, P = 0.05). There were no significant differences in pleural fluid volume, fluid attenuation, pannus formation, mediastinal adenopathy, or pulmonary nodules.

Limitations
The study was limited by a small sample size, particularly for dogs with malignant pleural effusion, reducing statistical power. Additionally, there was overlap in CT findings between malignant and inflammatory effusions, limiting the ability to make definitive diagnoses based on imaging alone. Pathological confirmation was not available for all cases.

Conclusions
Marked parietal pleural thickening and thoracic wall invasion on CT support a diagnosis of pleural malignant neoplasia, whereas visceral pleural thickening is more suggestive of pleuritis. Although CT alone cannot definitively differentiate malignant from inflammatory pleural effusions, it can provide valuable diagnostic insights to guide further testing and clinical decision-making.

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