Malignant effusions part 2.....

Veterinary Radiology & Ultrasound, 2018.

Author

Obi Veterinary Education
September 14, 2025

Jennifer A. Reetz, Jantra N. Suran, Allison L. Zwingenberger, Darko Stefanovski.

Background
Pleural effusion in dogs and cats can arise from various causes, including malignancy, infection, chylous effusion, transudates, and hemorrhage. While computed tomography (CT) is used in human medicine to differentiate between malignant and benign pleural effusions, its diagnostic utility in veterinary patients remains unclear. This study aimed to evaluate CT features associated with different types of pleural effusion in dogs and cats and to determine whether specific findings could reliably predict malignancy.

Methods
This retrospective, cross-sectional study analyzed thoracic CT scans from 89 dogs and 32 cats with confirmed pleural effusion. Cases were categorized as malignant pleural disease, pyothorax, chylothorax, transudative effusion, or hemorrhagic effusion based on cytology or histopathology. CT findings, including pleural thickening, nodules, masses, effusion distribution, and lymphadenopathy, were statistically analyzed to identify predictive features for malignant effusion.

Results
Malignant pleural effusion was associated with older age (dogs: P = 0.015, cats: P = 0.005), nodular diaphragmatic pleural thickening (dogs: P = 0.021, cats: P = 0.031), costal pleural masses (dogs: P = 0.018, cats: P = 0.019), and pulmonary masses (dogs: P = 0.002, cats: P = 0.077). In dogs, the presence of any costal pleural abnormality (P = 0.002) and pulmonary masses (P = 0.004) were predictive of malignancy or pyothorax, whereas costal pleural abnormalities (P = 0.006) and sternal lymphadenopathy (P = 0.040) were inversely associated with transudative effusion. Despite these associations, many CT features overlapped among different effusion types, reducing their diagnostic specificity.

Limitations
The study was limited by its retrospective design, variable CT protocols, and the small sample size for some effusion categories, particularly in cats. Additionally, histopathological confirmation of pleural abnormalities was not available in all cases, which may have affected diagnostic accuracy.

Conclusions
While nodular diaphragmatic thickening, costal pleural masses, and pulmonary masses were strongly associated with malignant pleural effusion, many other CT features showed significant overlap between effusion types. Therefore, CT findings alone cannot definitively differentiate between malignant and non-malignant effusions, and further diagnostic testing, including cytology and histopathology, remains essential for accurate diagnosis.

Examples of pleural masses. Transverse postcontrast soft tissue algorithm (A–C). A, Mass-like proliferative thickening of the mediastinal pleura (contrast enhancing tissue denoted by white arrows) in this dog with pyogranulomatous mediastinitis/mediastinal pleuritis and pyothorax. The black arrow denotes contrast enhancing tissue in the extrapleural fat obscuring the normal internal thoracic vasculature, unlike on the right side where the vasculature is seen surrounded by fat (black arrowhead); H, heart. B, Mediastinal pleural mass/mass-like thickening (black arrowheads) and a costal pleural nodule (black arrow) in this dog with mesothelioma and bilateral pleural effusion (black asterisks). C, There is a mass along the costal pleura (white arrow) in this dog with mesothelioma and bilateral pleural effusion (black asterisks). There is smooth to irregular costal (black arrowheads) and diaphragmatic (white arrowheads) pleural thickening

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