History
A 13-year-old spayed female Labrador Retriever was evaluated by the University of Georgia Oncology Service for treatment of an incompletely excised malignant melanoma of the caudal aspect of the oral cavity that was first observed during dental prophylaxis. On physical examination, the dog was panting and eupneic with muffled heart sounds and decreased lung sounds bilaterally on auscultation of the ventral aspect of the thorax. Results of a CBC and serum biochemical analyses were within reference ranges, and urinalysis was unremarkable. No cytologic abnormalities were identified on evaluation of an ipsilateral mandibular lymph node aspirate. Abdominal ultrasonography revealed no remarkable abnormalities. Thoracic radiographs were obtained (Figure 1).
Left lateral (A) and ventrodorsal (B) radiographic views of the thorax of a 13-year-old spayed female Labrador Retriever evaluated because of an incompletely excised malignant melanoma in the caudal aspect of the oral cavity observed during dental prophylaxis.
Citation: Journal of the American Veterinary Medical Association 247, 12; 10.2460/javma.247.12.1365
Determine whether additional imaging studies are required, or make your diagnosis from Figure 1—then turn the page →
Radiographic Findings and Interpretation
In the ventral aspect of the pleural space, a large volume of heterogeneous fat and soft tissue material is present, causing severe retraction and dorsal displacement of the lung margins from the ventral aspect of the thoracic body wall. The cardiac silhouette is dorsally displaced with loss of cardiosternal contact. The pleural fissure between the right cranial and right middle lung lobes is mildly wide with rounding of the adjacent lung lobe margins. On the left lateral thoracic radiograph, in the ventral aspect of the pleural space at the level of the sixth to seventh intercostal spaces, a small round soft tissue structure is present. Best seen on the ventrodorsal view, the cranial mediastinum is wide, causing lateral and caudal displacement of the cranial lung lobes (Figure 2).
Same radiographic images as in Figure 1. Notice the heterogeneous soft tissue and fat opacity in the ventral aspect of the thorax (white arrows), retraction of the lung lobes, and mediastinal widening (black arrows).
Citation: Journal of the American Veterinary Medical Association 247, 12; 10.2460/javma.247.12.1365
Differential diagnoses for the wide cranial mediastinum included cranial mediastinal lymphadenopathy, thymic mass, or abundant fat accumulation. The differential diagnoses for the wide pleural fissure included pleural effusion or a thickened pleura secondary to age or fibrosis from prior disease. Differential diagnoses for the nodule in the intercostal space included granuloma, hematoma, or nodular fat necrosis. The radiographic signs were not typical for pleural effusion, and differential diagnoses for lobar retraction are limited; therefore, further imaging was needed.
To better characterize the pleural and mediastinal abnormalities, thoracic ultrasonography was performed. Inhomogeneous parenchymal tissue extended from the thoracic inlet to the level of the diaphragm, with only scant mildly echogenic pleural effusion present. Minimal blood flow was identified in the parenchymal mass by color flow Doppler ultrasonography. Differential diagnoses included adipose tissue, tumor (thymoma or lymphoma), or aberrant tissue (thymic or thyroid). Fine-needle aspiration was performed. Results of cytologic evaluation of the tissue were inconclusive as a result of low cellularity.
Thoracic CT was performed (Figure 3). On CT images, the ventral aspect of the thorax was filled with a large volume of fat-attenuating tissue, causing dorsal displacement of the cranial lung lobes. Identified within this tissue were 3 soft tissue nodules, measuring from 1 to 1.4 cm in diameter and containing stippled mineral foci. On the basis of the CT scan, a large intrathoracic lipoma was considered the most likely diagnosis, and the mineral density foci within the fat were suspected to be granulomas or nodular fat necrosis (ie, Bates bodies), although neoplastic nodules could not be completely ruled out.
Transverse CT image (soft tissue lung algorithm; window level, −151 Hounsfield units [HUs]; window width, 1,749 HUs; slice thickness, 2 mm) obtained at the level of the sixth thoracic vertebra of the same dog as in Figure 1. Notice the fat-attenuating material that displaces the lungs dorsally (white arrows). Additionally, notice a mineral density focus (black arrow) within the fat, most consistent with a granuloma or nodular fat necrosis (Bates body).
Citation: Journal of the American Veterinary Medical Association 247, 12; 10.2460/javma.247.12.1365
Treatment and Outcome
Multiple fine-needle aspirates of the tissue mass and a nodule were obtained following the CT scan; results of cytologic examination were inconclusive because of low cellularity, but samples appeared as greasy fluid on the slide. On the basis of the results of the imaging and examination of fine-needle aspirates, a presumptive diagnosis of intrathoracic lipoma was made. The patient was treated with 4 weekly 8-Gy doses of radiation therapy to the site of the incompletely excised melanoma and mandibular lymph nodes. The dog received a vaccine for the treatment of malignant oral melanoma,a according to the recommended protocol.1
Five months after diagnosis, a large soft tissue mass in the left cranial lung lobe and 2 smaller nodules in the left caudal lung lobe were noted on routine recheck thoracic radiographs. Nine months after diagnosis, the patient was euthanized because of dyspnea. Necropsy identified extensive intrathoracic fat deposition or lipoma, mild to moderate pleural effusion, multifocal pulmonary melanoma metastasis, and a single large pulmonary mass consistent with melanoma on the basis of positive immunohistochemical staining for melan-A.2
Comments
Although lipomas are generally associated with a characteristic radiolucency on radiographs, in the thoracic cavity they can appear radiopaque and mimic the appearance of pleural effusion.3 Several radiographic findings in the dog of the present report were inconsistent with any 1 specific disease process. The finding of severe mediastinal widening in conjunction with caudal displacement, retraction, and rounding of the cranial lung lobes was most consistent with a soft tissue mass or severe pleural effusion. However, the lack of border effacement with the heart and diaphragm was consistent with a fat opaque structure and not fluid or soft tissue. Thoracic ultrasonography and CT were superior to thoracic radiography in defining the nature of the pleural and mediastinal opacities. Ultimately, histopathologic findings were required to confirm the diagnosis.
Intracavity lipomas are rare.4–9 Growth of these lipomas can result in clinical signs associated with distension and compression of organs and other intracavitary structures. Surgical resection has resulted in resolution of clinical signs and long-term tumor control.4–9 Because the dog of the present report had no clinical signs of a space-occupying mass and oral melanoma had been diagnosed, the owner decided to treat the dog for malignant melanoma and monitor the intrathoracic lipoma. The lipoma did not progress for more than 9 months; the dog was euthanized because of dyspnea attributable to diffuse metastasis of the oral melanoma and concurrent pleural effusion. It is possible that the large lipoma limited the pulmonary reserve capacity, and the clinical signs secondary to the neoplastic changes were exacerbated.
ONCEPT, Merial Ltd, Duluth, Ga.
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