History

A 13-year-old 6.6-kg (14.5-lb) castrated male domestic shorthair cat was referred to the Oklahoma State University Veterinary Medical Teaching Hospital for evaluation of a large mass in the thoracic subcutis. In the year preceding referral, 3 separate attempts to excise the mass had been made. The mass recurred after each attempted removal. Histologic examination of the mass was performed 3 months prior to referral.

Clinical and Gross Findings

At the referral evaluation, a large subcutaneous mass overlying the right side of the thorax and axillary region and extending ventrally across the sternum was identified. On palpation, the mass was firm with poorly defined margins and was fixed to the adjacent unaffected tissues. Lameness of the right forelimb was evident; the limb was moderately swollen with pitting edema along its length. Thoracic auscultation revealed a grade 3/6 left apical systolic heart murmur. Because of the size and infiltrative nature of the mass, complete surgical excision was not feasible without limb amputation and extensive thoracic wall reconstruction. Given a guarded prognosis, the owners elected euthanasia, which was achieved via IV injection of 4 mL of euthanasia solution (226 mg of pentobarbital/kg [102.7 mg/lb]; 29 mg of phenytoin sodium/kg [13.2 mg/lb]).

On gross necropsy examination, the subcutis of the right axillary region was expanded by an infiltrative mass (15 × 22 × 5 cm) that compressed and deformed the underlying ribs. The overlying skin was ulcerated. On cut surface, the mass was white to yellow and multilobular; a few lobules contained necrotic debris (Figure 1). Protruding from the surface of the left renal cortex was a 0.5-cm-diameter, white, homogenous mass. The remaining cortical surface was mottled with hundreds of pinpoint, red, sunken areas that, on cut surface, corresponded to red, wedge-shaped areas extending from the cortical surface into the medulla. The right kidney was grossly normal. The heart was abnormally large and weighed 28.9 g (0.064 lb [0.44% of the cat's body weight]). The liver had a diffusely enhanced reticular pattern.

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Photographs of the cut surface of a mass in the right axillary region (A) of a cat that was referred following unsuccessful attempts at mass excision and the cut surface of the left kidney from the same cat (B). In panel A, notice that the mass is multilobular with a central area of necrosis in 1 lobule (arrow). In panel B, there is a focal, white nodule within the renal cortex.

Citation: Journal of the American Veterinary Medical Association 239, 6; 10.2460/javma.239.6.757

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Formulate differential diagnoses from the history, clinical findings, and Figure 1—then turn the page →

Histopathologic Findings

Tissues that were routinely processed and sectioned for microscopic examination included samples of the subcutaneous axillary mass, left kidney, lungs, liver, and pancreas. On histologic examination, the sub-cutis of the right axillary region was markedly expanded by a multilobular, unencapsulated mass composed of sheets of round to spindle-shaped cells that infiltrated and often obliterated the underlying skeletal muscle. Sheets of neoplastic cells were partitioned into lobules by thick, fibrovascular stroma. The neoplastic cells had abundant eosinophilic cytoplasm that often contained numerous, distinct, clear cytoplasmic vacuoles (5 to 40 μm in diameter). The nuclei were round to oval with coarsely vesiculated chromatin and a single, prominent nucleolus. There were 8 mitotic figures/10 hpf (400×), and cells were mildly pleomorphic. Large areas of coagulative necrosis were scattered throughout. Adjacent skeletal muscle fibers were separated and isolated by neoplastic cells, and a few myocytes had signs of degeneration, including flocculent, hypereosinophilic cytoplasm with loss of cross-striations and centralization of nuclei.

Within the cortex of the left kidney, there was a discrete nodule composed of neoplastic cells similar to the cells of the mass in the subcutis (Figures 2 and 3). The remaining cortical architecture was distorted by multifocal, wedge-shaped areas affected by chronic inflammation, fibrosis, and parenchymal collapse; these areas corresponded to the small, red, wedge-shaped foci seen grossly. The right kidney was not examined histologically.

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Photomicrograph of a section of the left renal cortex in the cat in Figure 1. A portion of the renal cortex is effaced by sheets of neoplastic cells; neoplastic tissue is to the left of the image, and a portion of unaffected renal cortex is to the right of the image. Notice the remnant glomerulus (arrow) and tubules (arrowhead) that are surrounded by neoplastic cells. H&E stain; bar = 200 μm.

Citation: Journal of the American Veterinary Medical Association 239, 6; 10.2460/javma.239.6.757

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Photomicrograph of the same section of the left renal cortex in Figure 2. At higher magnification, the neoplastic characteristics of the cells are apparent. Notice that there are large vacuoles within the cytoplasm of the neoplastic cells (arrowhead). An intact renal tubule (arrow) is visible. H&E stain; bar = 30 μm.

Citation: Journal of the American Veterinary Medical Association 239, 6; 10.2460/javma.239.6.757

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The enhanced reticular pattern of the liver observed grossly was a result of centrilobular congestion and subsequent multifocal dissociation of centrilobular hepatocellular cords. Although gross lesions in the lungs and pancreas were not detected at necropsy, histologic examination findings included bronchiolar smooth muscle hyperplasia and multifocal replacement of the pancreatic islets with acellular, eosinophilic material consistent with amyloid. In addition, portions of the exocrine pancreas were fibrotic with multifocal hyperplastic nodules.

Morphologic Diagnosis

Liposarcoma in the subcutis of the right axillary region and focal metastatic liposarcoma in the left kidney.

Comments

In the cat of this report, the large subcutaneous mass was most suggestive of either neoplasia or inflammation. Subcutaneous neoplasms including fibrosarcoma, lipoma, liposarcoma, and lymphoma in cats have been reported.¹ Inflammatory masses within the subcutis may be a result of infection with Mycobacterium spp, Actinomyces spp, Nocardia spp, or Staphylococcus spp or development of xanthogranuloma or sterile nodular panniculitis.¹ Considering that the mass recurred following attempted excision without evidence of systemic disease, neoplasia was considered the most likely differential diagnosis. Because the mass infiltrated surrounding tissues, a malignant neoplasm (eg, fibrosarcoma, liposarcoma, or lymphoma) was suspected, rather than a benign neoplasm (eg, lipoma). Fibrosarcoma, liposarcoma, and lymphoma can have a similar appearance and may be difficult to distinguish grossly. As for the subcutaneous mass, differential diagnoses for the renal mass included neoplasia (eg, lymphoma, adenoma, or adenocarcinoma) or inflammation (eg, feline infectious peritonitis or Histoplasma capsulatum infection).¹

Histologic examination revealed that the subcutaneous mass was a liposarcoma that had metastasized to the left kidney. Histologic subtypes of liposarcoma include well-differentiated, myxoid, and pleomorphic liposarcomas,² but subtype has not been shown to affect prognosis in dogs.^3,4 The morphological characteristics of the neoplasm in the cat of this report were most consistent with a well-differentiated liposarcoma because most of the neoplastic cells contained large lipid vacuoles, as do normal adipocytes. Oil red O staining is often used to identify lipid within cytoplasmic vacuoles. In this case, oil red O stain was not used because the fixed tissue samples were discarded prematurely, and the process of paraffin-embedding removes lipids from tissues.⁵

Liposarcomas are rare, malignant neoplasms of adipocyte origin that can develop in all domestic species.⁴ As in the cat of this report, primary tumors most commonly develop in the subcutis.² Metastatic disease is considered rare.⁴ In cats, metastatic or multifocal disease in the liver, spleen, kidneys, adrenal glands, round ligament of the urinary bladder, and mediastinum has been reported.^6–9 Although liposarcomas typically develop spontaneously in many species, development of this type of neoplasm may be associated with vaccination in cats.⁷ In addition, there are reports^8,9 that infection with FeLV may be associated with development of liposarcoma. There are reports of 2 dogs that developed liposarcoma in association with subcutaneous foreign bodies (ie, a piece of glass¹⁰ and a microchip¹¹).

In general, excision is the treatment of choice for liposarcoma,^3,4 although results of case series focusing on treatment of subcutaneous liposarcoma in cats have not been published, to our knowledge. However, studies of treatment of feline vaccine-associated fibrosarcoma have been performed.^12,13 Although the biological behavior of vaccine-associated sarcomas is reportedly more aggressive than that of spontaneous sarcomas,¹⁴ the principles of surgical treatment are the same. Excision of the mass with wide surgical margins and reconstruction is appropriate, and the likelihood of success is enhanced when the tumor is sufficiently small to obtain adequate margins without compromising the functioning of the adjacent normal anatomic structures.¹³

The cat of this report underwent 3 attempts at tumor resection prior to referral, and each time the mass recurred. Recurrence of liposarcoma following excision attempts is common. In 1 study,³ 12 of 43 (28%) dogs with liposarcoma had local tumor recurrence after surgical resection. Results of that study also indicated that, in dogs, excision of liposarcomas with wide surgical margins significantly improved survival time, compared with survival times following tumor excision with narrow surgical margins or incisional biopsy. In the cat of this report, tumor excision with wide surgical margins was not possible at the time of referral because of the location and extent of the mass. In a similar case of a cat with thoracic liposarcoma, successful treatment was achieved via radical excision of the mass, including removal of ribs and mesh reconstruction of the thoracic body wall.¹⁵ Such treatment may have been an option for the cat of this report if the procedure could have been performed earlier in the disease process.

This case serves as a reminder that treatment of neoplasms is best approached with early histologic diagnosis followed by resection and reconstruction using proper oncological surgical techniques. With such treatment protocols, local recurrence rates can be decreased and functional outcomes improved. Furthermore, because of the potential for metastasis of liposarcomas in cats, it is advisable to evaluate feline patients for metastatic disease.