History
A 13-year-old 33.2-kg (73-lb) spayed female Labrador Retriever was admitted for a recheck examination 22 months after undergoing an incomplete resection of an intermediate grade peripheral nerve sheath tumor, which was located over the lateral aspect of the right forelimb. Local tumor control had been achieved by means of adjuvant radiation therapy consisting of 18 fractions of 3 Gy each (54 Gy total). On physical examination, the dog appeared anxious and had inspiratory stridor. Measurements of vital signs were within reference limits, and multiple subcutaneous lipomas were observed. At the previous excision site, no evidence of tumor recurrence was found. Serum biochemical analyses and a CBC were performed; the only abnormality detected was a high serum alkaline phosphatase activity (1,294 IU/L; reference range, 20 to 142 IU/L). Thoracic radiography was performed (Figure 1).
Left lateral (A) and ventrodorsal (B) radiographic views of the thorax of a 13-year-old dog evaluated during an examination for metastasis approximately 22 months after incomplete resection of a low-grade peripheral nerve sheath tumor of the right forelimb.
Citation: Journal of the American Veterinary Medical Association 233, 11; 10.2460/javma.233.11.1691
Determine whether additional imaging studies are required, or make your diagnosis from Figure 1—then turn the page →
Diagnostic Imaging Findings and Interpretation
A 6 × 6 × 9-cm soft tissue opaque mass is located in the ventral aspect of the cranial mediastinum immediately cranial to the heart (Figure 2). The cardiac silhouette and pulmonary vasculature appear to be within reference limits. Review of thoracic radiographs obtained approximately 6 months earlier confirms that the cranial mediastinal mass is a new finding.
Same radiographic views as in Figure 1. Notice the soft tissue opaque mass in the cranial mediastinum (white arrows).
Citation: Journal of the American Veterinary Medical Association 233, 11; 10.2460/javma.233.11.1691
To better evaluate the cranial mediastinum, ultrasonography of the ventral aspects of the second through fourth left intercostal spaces was performed. A linear probe (3 to 9 MHz) was used with the dog positioned in right lateral recumbency. An ovoid, hyperechoic mass with a central hypoechoic area in the third intercostal space is evident (Figure 3). The hypoechoic center creates a distal acoustic enhancement artifact that is indicative of fluid.
Representative ultrasonographic image of the cranial mediastinum. Notice the cavitated, hyperechoic mass (white arrows) with a hypoechoic center. The distal enhancement artifact indicates a fluid center (wide arrow).
Citation: Journal of the American Veterinary Medical Association 233, 11; 10.2460/javma.233.11.1691
Differential diagnoses included a neoplastic process such as thymoma, lymphoma, or possibly metastatic spread of the previously treated peripheral nerve sheath tumor. Alternatively, ectopic thyroid tissue, cyst, or abscess was considered.
Comments
Cytologic examination of an ultrasound-guided, fine-needle aspirate specimen revealed an eosinophilic, proteinaceous background consisting of abundant RBCs, leukocytes, and a few foamy macrophages. The owners elected for surgical exploration without additional diagnostic imaging. A lateral thoracotomy was performed over the left fourth intercostal space from ventral to the vertebral bodies extending toward the sternum. A 9 × 9-cm soft, fluctuant cranial mediastinal mass was identified and digitally excised from the thoracic cavity. The dog recovered from surgery and was discharged from the hospital the following day. Histologic evaluation revealed sheets of large adipocytes surrounded by a layer of collagen and a fibrous connective tissue capsule with areas of fat necrosis and inflammation. These findings were consistent with a diagnosis of a necrotic lipoma.
Intrathoracic lipomas have been reported in the veterinary literature and have included mediastinal, pleural, pericardial, and intrapericardial sites.1–4 Although lipomas can often be suspected on the basis of their radiographic appearance (fat density), infarcted or necrotic lipomas can be misleading in that they may appear as soft tissue or mixed density masses, even with an advanced imaging technique such as computed tomography (CT).5,6 A CT scan can use Hounsfield units to define tissue density and is often recommended preoperatively to evaluate vascular invasion and to aid in preoperative planning. In the dog of this report, a preoperative CT scan may have also defined this lesion as partly consisting of fat. The dog had a necrotic mediastinal lipoma that was discovered as an incidental finding during a routine radiographic examination for metastasis. Although intrathoracic lipomas are rarely reported, they should be included as a differential diagnosis for dogs with cranial mediastinal masses.
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