Intrathoracic Lipoma in a Cat

Introduction

A lipoma is a benign mesenchymal tumor derived from mature fat cells (lipocyte/adipocyte). Lipomas tend to be nonaggressive, noninvasive, and do not metastasize. Malignant transformation to a liposarcoma is not known to occur.¹ Infiltrative lipomas are benign variants and do not metastasize.² They commonly infiltrate adjacent muscle, dissect through fascial planes, and have been shown to invade nerves, periosteum, and bone.³ Treatment consists of wide excision or surgical debulking when tumor margins are indistinct. A recurrence rate of 36% has been demonstrated after aggressive surgical resection.^4,5

Cutaneous forms of lipomas appear in all species and are particularly common subcutaneously, either singly or at multiple sites, on the trunk and proximal extremities of middle-aged to older dogs.^1,6 They account for 9% of the skin and subcutaneous tumors in dogs and approximately 5% in cats.⁷ The occurrence of nonsubcutaneous lipomas is rare, with only a handful of reports in other sites, including thoracic, abdominal, intradural, and pelvic cavity.⁸ Within body cavities, lipomas tend to have an insidious onset and may remain undetected for long periods of time.⁹ The development of clinical signs occurs upon interference with adjacent organs, as associated with compression or entrapment.¹⁰ Specific clinical signs are dependent upon the body cavity involved, the rate of tumor growth, and the specific organs affected.

Intrathoracic lipomas are rare extrapulmonary tumors and can originate within the pleura, mediastinum, or pericardium.¹¹ Cough is the predominant presenting complaint in the majority of cases.⁹ Intrathoracic lipomas remain extremely rare in dogs, with only a handful of cases reported.^12–14 The present report describes, to the authors’ knowledge, the first case of an intrathoracic lipoma in a cat. The diagnosis was supported by radiology and computed tomography (CT) and was confirmed by histopathology. The lipoma was removed surgically, and it is expected that resolution of the presenting signs will be maintained long term.

Case Report

A 10 yr old, 5.77 kg, spayed female domestic medium hair cat was referred for evaluation due to the radiographic finding of a dorso-caudal thoracic mass. Two weeks before referral, the cat presented to the referring veterinarian with a history of acute onset of arched back, regurgitation, open mouth breathing, panting, and vocalization. On referral presentation, the cat appeared comfortable and was noted to be overweight with a body condition score of 7/9. Auscultation did not indicate any abnormal lung or heart sounds. Radiographs indicated a dorso-caudal, midline, circular opacity in the thorax, with a small comma-shaped gas shadow between it and the diaphragm. The structure could not be visualized sonographically as a result of it being surrounded by inflated lung. The heart appeared normal with no obvious signs of peritoneopericardial diaphragmatic hernia or traumatic diaphragmatic hernia (DH). Esophagoscopy was performed and appeared normal, with no indication of a hiatal hernia, gastroesophageal intussuseption, esophageal diverticulum, or foreign body.

The cat was referred for thoracic CT. The cat was maintained under general anesthesia for the duration of the study. Pre- and postcontrast 3 mm images were acquired and reformatted using soft tissue and lung algorithms. Confirmation of the presence of a dorso-caudal mediastinal fat density mass was made. The average Hounsfield units of the mass, as determined on efilm^a, ranged from −100 to −130 and were considered consistent with fat tissue density. The cranial mediastinum appeared normal. Approximate length of the mass was noted to be 8.3 cm, with extension from the main pulmonary artery bifurcation to the diaphragmatic attachment (Figure 1). Maximum dimensions in the transverse plane were noted to be 5.0×3.7 cm (Figure 2). The mass was found to displace the thoracic caudal vena cava slightly and nearly encircle it at one point (Figure 2). The esophagus was compressed and shifted to the far left. Mild atelectasis was noted due to both anesthesia and compression of lung tissue by the lipoma.

View Figure

• Download as Powerpoint
• Download Figure

View Figure

• Download as Powerpoint
• Download Figure

Anesthetic premedication included hydromorphone^b (0.1 mg/kg intramuscularly [IM]), acepromazine^c (0.03 mg/kg IM), and ketamine^d (8.7 mg/kg IM). Anesthetic induction included diazepam^e (0.2 mg/kg IV) and propofol^f (3.8 mg/kg IV). Anesthesia was maintained with 1% isolfurane^g in oxygen administered through a no. 4 cuffed endotracheal tube. A right intercostal thoracotomy was performed between the fifth and sixth ribs. Upon exposure of the thoracic cavity, a routine thoracic exploratory of the right hemithorax was carried out. The mass was situated adjacent to the caudal esophagus, caudal vena cava, aorta, and caudal lung lobes. It was well encapsulated and after meticulous dissection was removed within its capsule. The mass was submitted for histopathologic examination.

After surgical excision of the mass, the thoracic cavity was lavaged and closed in a routine fashion. A Jackson-Pratt closed suction drain was placed and subsequently removed 24 hr later.^h The animal recovered without incident in the intensive care unit. Immediate postoperative management included cefazolinⁱ (22 mg/kg IV q 8 hr), fentanyl^j (2 μg/kg/hr IV constant-rate infusion), and meloxicam^k (0.2 mg/kg subcutaneously once). Routine postoperative instructions were given with regard to incision care and exercise restriction, and the cat was discharged 1 day after surgery. The administration of amoxicillin/clavulanic acid^l (10.8 mg/kg per os q 12 hr) and buprenorphine^m (0.01 mg/kg per os q 8–12 hr) were continued at home. The cat continued to recover well at home and returned for a recheck and staple removal 2 wk later. Personal communication with the owner 21 mo after discharge indicated that the cat continued to do well, was eating and drinking normally, and had not redeveloped any signs of respiratory difficulty. Repeat thoracic radiographs demonstrated an unremarkable lung field, with no evidence of recurrence.

Gross examination of the thoracic mass revealed lobulated pale yellow adipose and membranous tissue. Induration of the mass was grossly evident. Cut surfaces were pale and homogenous. Histologic examination revealed mature adipocytes with eccentric nuclei, among which were interspersed capillaries with sporadic lymphocytic perivascular accumulations and occasional larger vessels encircled by connective tissue. A diagnosis of intrathoracic lipoma was made.

Discussion

The clinical presentation of dyspnea and arched back are consistent with reports of common clinical signs in dogs diagnosed with intrathoracic lipomas.¹⁰ In this case, the additional clinical sign of regurgitation was most likely due to poor esophageal transit past the lipoma. All presenting signs resolved upon surgical excision and recovery.

The adipose tissue within a lipoma has the same physical density as that of fat located elsewhere in the body. As such, it is important to be able to distinguish an intrathoracic lipoma from mediastinal or pericardial fat associated with obesity or herniated omentum that has entered the thoracic cavity via a traumatic DH.^10,14 In this case, the mass remained well circumscribed and encapsulated, which aided in its differentiation. Invasion into adjacent musculature and fascia, as occurs with infiltrative lipomas, was not seen in this case.

Histologic examination classified the excised mass as a simple lipoma. Lipomas are the most common of the fatty tumors in cats. Atypical and mixed lipomas such as angiolipomas and thymolipomas are extremely rare.² They have characteristic histopathologic appearances that were not present in this case. A recent report described a rare variant of a simple lipoma as an angiolipoma, which consisted of solitary subcutaneous nodules composed of thin-walled blood vessels randomly distributed throughout lobules of well-differentiated adipose tissue.⁶ Differentiation of an angiolipoma with sparse vascularity from a simple lipoma presents a diagnostic challenge, but in this case, the small vessels were few in number and none formed large blood-filled channels.⁶ Within small proportions of the excised mass, small mature lymphocytes were noted to surround the vasculature. In both dogs and cats, a thymolipoma has been described, in which adipose tissue contained cords and nests of thymic tissue.^15,16 Although perivascular mature lymphocytes were noted, there was no indication of thymic arrangement in this mass.

Conclusion

An intrathoracic lipoma was diagnosed in a 10 yr old female cat. Although this was the first reported case in a cat, there have been several reports described in dogs. Along with more common occurrences such as peritoneopericardial diaphragmatic hernia and traumatic DH, an intrathoracic lipoma should be considered as a differential for space occupying lesions within the thoracic cavity. Radiation therapy can be used to decrease blood supply and diminish growth of unresectable lipomas. In this case, CT demonstrated that the mass was fully encapsulated, thus making surgical excision a viable treatment option. Surgical excision of intrathoracic lipomas resulted in complete resolution of clinical signs, providing for an excellent prognosis. Recurrence, which is not uncommon with subcutaneous lipomas, remains a possibility.