The study focused on cats with pleural effusion and a cytologic or histopathologic diagnosis of pulmonary carcinoma, or metastatic epithelial cancer considered consistent with a pulmonary origin based on imaging and findings elsewhere. Fine-needle aspiration of a pulmonary mass confirmed carcinoma or malignant epithelial neoplasia in 12 of the 22 cats. For the remaining 10, the diagnosis was presumptive: malignant epithelial cells were demonstrated in pleural fluid, pleural or thoracic-wall lesions, or another metastatic site while imaging supported a primary pulmonary neoplasm. This distinction matters because an occult primary tumor outside the lung could not be completely excluded in every presumptive case, and the selected cohort likely represents cats with clinically apparent or advanced thoracic disease rather than all cats with lung cancer.

Ultrasonography was performed with linear probes reaching 18 MHz or microconvex probes reaching 9 MHz. Archived images from three ultrasound systems were reviewed in consensus by two board-certified veterinary radiologists and a veterinarian with 15 years of diagnostic imaging experience. The investigators recorded the location and distribution of pleural abnormalities, maximal thickness, shape, echogenicity relative to neighboring intercostal muscle, internal homogeneity, and whether invasion of the chest-wall musculature was suspected. Pleural effusion was probably central to this assessment: fluid can separate lung from the thoracic wall and create an acoustic window through which parietal and visceral pleural surfaces can be examined.

Based on the stored images, pleural thickening appeared diffuse in 12 cats, focal in six, and multifocal in four. The representative or measured abnormality was most often located in the cranial pleural region, ahead of the cardiac silhouette, appearing in 18 cats; it was in the middle region in three and the caudal region in one. These figures should not be interpreted as a complete map of pleural disease. The clinical protocol began with the cranial pleura and moved to the middle and caudal regions only when no cranial thickening was found, so the investigators could not determine the total number of lesions or their true anatomical distribution.

The maximal visible thickness of the pleural abnormalities averaged 5.6 millimeters, with a median of 4.9 millimeters and a range from 1.2 to 16.2 millimeters. Shape was smooth in nine cats, irregular in eight, and nodular in five. Most lesions, 15 of 22, were hypoechoic relative to adjacent intercostal muscle, although two were isoechoic and five had mixed echogenicity. Internal echoes were homogeneous in 14 cats and heterogeneous in eight. These overlapping appearances mean that a thickened pleura should not be considered malignant on ultrasound alone. Pleural thickening can also arise from inflammatory or reactive processes, and the study did not include cats with non-neoplastic pleural effusion for comparison.

One potentially important feature was an irregular or poorly defined interface between the thickened pleura and adjacent intercostal muscle, with apparent extension into the muscle or loss of the normal fascial plane. Such suspected intercostal muscle invasion was recorded in eight cats. All eight lesions were irregular or nodular, whereas none of the smooth lesions showed this feature. The median maximum thickness among these lesions was 7.1 millimeters, compared with 4.9 millimeters for the group as a whole. The appearance may help flag advanced local involvement, but the investigators did not have histological confirmation of muscle invasion in every cat, so it should be treated as an imaging suspicion rather than a validated sign of true invasion.

Pleural fluid cytology produced neoplastic cells in 15 of the 20 cats whose effusion was examined, but malignant cells were not detected in five. Within this selected group, therefore, a negative fluid cytology result did not exclude pulmonary carcinoma. The findings also identify a possible alternative route to diagnosis. Six cats underwent ultrasound-guided sampling of thickened pleura or an associated thoracic-wall lesion, and cytologic or histopathologic results supported carcinoma in all six. Five samples were interpreted as epithelial malignant neoplasia. In one cat, an initial fine-needle aspiration was suspicious for a non-epithelial malignant neoplasm, while subsequent biopsy of a thoracic-wall mass confirmed squamous cell carcinoma.

The sampled targets included four diffuse and two multifocal lesions; five were irregular and one was smooth. No focal or nodular lesion was sampled. These results support the feasibility of lesion-directed sampling when a safe acoustic window exists, especially if a primary pulmonary mass is central, surrounded by aerated lung, or otherwise difficult to reach directly. They do not establish that pleural sampling is safer, more accurate, or superior to aspirating the lung mass. Only six cats underwent the procedure, selection depended on clinical judgment and accessibility, and records inconsistently documented needle gauge, number of passes, and aspiration technique. The procedure’s diagnostic performance, safety profile, and comparative value therefore require prospective testing.

The study’s central clinical message is practical but cautious. In a cat with suspected pulmonary carcinoma and pleural effusion, the pleural surface deserves deliberate inspection rather than evaluation only for the presence of fluid. Ultrasound may show thickening that is smooth, irregular, or nodular; hypoechoic, isoechoic, or mixed; and variable in thickness and distribution. Such findings are suspicious in the presence of known or suspected carcinoma, particularly when malignant cells are present in the effusion, but imaging alone cannot distinguish neoplastic infiltration from reactive disease. Pleural fluid cytology remains an important test, yet the study shows that its negative result need not settle the question. Prospective studies using standardized whole-thorax scanning, comparison with computed tomography, cytologic or histopathologic reference standards, and controls with non-neoplastic effusion are needed to determine the true prevalence, diagnostic accuracy, staging value, and prognostic significance of these ultrasound findings in cats with pulmonary carcinoma.

Subject of Research: Ultrasonographic pleural changes and diagnostic sampling in cats with pulmonary carcinoma

Article Title: Ultrasonographic Features of Pleural Thickening in Cats With Pulmonary Carcinoma and Pleural Effusion: A Retrospective Descriptive Study

Article References: Toshima, A., Fulkerson, C. V., & Murakami, M. (2026). Ultrasonographic Features of Pleural Thickening in Cats With Pulmonary Carcinoma and Pleural Effusion: A Retrospective Descriptive Study. Veterinary Medicine and Science, 12(6), Article e71290. https://doi.org/10.1002/vms3.71290

Image Credits: AI Generated

DOI: 10.1002/vms3.71290

Keywords: feline pulmonary carcinoma, thoracic ultrasonography, pleural thickening, pleural effusion, fine-needle aspiration, veterinary oncology, thoracic imaging, intercostal invasion, cancer staging, Ultrasonographic, Features, Pleural