Abstract

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 3mm vs. 0mm, 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.

Highlights

  • Pleural fluid accumulation may occur as a result of several different pathologic mechanisms that determine the nature of the fluid.[1]

  • Pleural thickening was the sign most frequently observed in dogs with malignant pleural effusion whereas enlarged mediastinal lymph nodes was the sign most frequently observed in dogs with pleuritis

  • Likelihood ratios for pleural malignant neoplasia for categorical Computed tomography (CT) signs of significance or borderline significance were: pleural thickening 1.9; parietal pleural thickening only 5.2; and thoracic wall invasion 11.0

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Summary

Introduction

Pleural fluid accumulation may occur as a result of several different pathologic mechanisms that determine the nature of the fluid.[1]. Mesothelioma, a primary malignant neoplasm of the pleura[5,6,7], pericardium[8,9,10] or peritoneum[10,11,12], can be difficult to diagnose on routine cytologic preparations because malignant mesothelial cells may appear similar to the reactive mesothelial cells seen with inflammatory pleural conditions.[1,6,13,14] Carcinoma metastasis to the pleura can be difficult to diagnose cytologically and may require immunocytochemical analysis of the effusion or histopathology.[15] Metastasis to the pleura could occur with any type of carcinoma, but in the dog this condition is mainly associated with primary epithelial lung tumors, mammary carcinoma, prostatic carcinoma and transitional cell carcinoma of the urinary bladder.[16] Dogs with pleural carcinoma (or sarcoma) usually have a primary tumor elsewhere, the detection of which aids diagnosis of the pleural effusion

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