Sialolipoma of a minor salivary gland in a dog

Abstract

A 10-year-old golden retriever dog was presented for evaluation of progressive panting, inspiratory stridor, and gagging. Oropharyngeal examination revealed a soft, nonulcerated, pedunculated mass arising from the right tonsillar fossa. The mass was completely excised and histopathology was consistent with a sialolipoma. No regrowth of the mass was evident 6 months after surgery.

Sialolipoma is a rare benign neoplasm composed of mature adipocytes encased in a fibrous capsule with entrapped non-neoplastic salivary gland tissue. Sialolipomas have only been recognized as a distinct entity in humans in the past 10 y (1). Since then, more than 35 cases of sialolipoma have been documented in humans (2). To our knowledge, sialolipomas have not been reported in a domestic animal. The purpose of this report is to describe the clinical and histologic findings of a sialolipoma in a dog.

Case description

A 10-year-old spayed female golden retriever dog was referred for suspected laryngeal paralysis. The dog was presented to its primary veterinarian with a 1-year history of progressive panting, inspiratory stridor, intermittent stertor, and gagging while eating. The stertor was more apparent while sleeping and during exercise or excitement. Hematology, serum biochemistry, serum total thyroxine, and thyroid-stimulating hormone tests performed prior to referral were within normal limits. There was no history of significant previous health problems, and the dog was not on any medications.

On physical examination, the dog was bright, alert, and panting with intermittent inspiratory stridor. A soft expiratory stertor was detected on laryngeal auscultation. Thoracic and cardiac auscultation was unremarkable. No abnormalities were detected on laryngeal and tracheal palpation. The remainder of the examination was within normal limits. Cervical radiographs revealed a 3.5 cm × 4.5 cm soft-tissue opacity in the caudal oropharynx (Figure 1). There was moderate calcification of the laryngeal cartilages. Thoracic radiographs showed no evidence of aspiration pneumonia, megaesophagus, or pulmonary masses.

Figure 1.

Lateral radiograph of the pharynx of a 10-year-old golden retriever dog diagnosed with a sialolipoma of the minor salivary glands. The soft tissue mass is seen to occupy most of the oropharynx (arrows).

The dog was premedicated with butorphanol (Torbugesic; Wyeth Animal Health, Guelph, Ontario), 0.2 mg/kg body weight (BW), IV, and induced with propofol (Diprivan 1%; AstraZeneca Canada, Mississauga, Ontario), 4 mg/kg BW, IV, for oropharyngeal examination. A 5 cm × 6 cm soft, pink, nonulcerated, pedunculated mass was found originating from the caudal margin of the right tonsillar fossa (Figure 2). The pedicle was approximately 1.5 cm long and 0.5 cm in diameter. The bulk of the mass was lying within the rima glottidis causing a 90% obstruction, and was easily displaced back into the oropharynx with a tongue depressor. The arytenoid cartilages were fully abducted during the oral examination, and laryngeal function was considered to be normal. Fine-needle aspiration of the mass yielded no cells. Due to the occlusion of the larynx caused by the mass and the risk of respiratory obstruction during anesthetic recovery, an excisional biopsy was performed. General anesthesia was maintained with isoflurane (Isoflurane USP; Pharmaceutical Partners of Canada, Richmond Hill, Ontario) in 100% oxygen via an endotracheal tube. An elliptical incision was made in the pharyngeal mucosa at the base of the pedicle, and included the right palatine tonsil. The mass was sharply dissected from the underlying submucosal tissue and removed en bloc. The entire mass was placed in 10% neutral buffered formalin and submitted for histopathologic examination. The mucosa and submucosa were closed with 4-0 poliglecaprone 25 (Monocryl; Ethicon, Somerville, New Jersey, USA) in a simple continuous pattern. The dog recovered well from surgery with no respiratory compromise, and was discharged 2 d later with tramadol (Tramadol HCl; Chiron Compounding Pharmacy, Guelph, Ontario), 3 mg/kg BW, PO, q8h, and meloxicam (Metacam; Boehringer Ingelheim, Burlington, Ontario), 0.1 mg/kg BW, PO, q24h. Following excision of the mass both the inspiratory stridor and expiratory stertor resolved.

Figure 2.

Intra-oral photograph of the oropharynx of a 10-year-old golden retriever dog demonstrating a 5 cm × 6 cm soft pedunculated mass originating from the right tonsillar fossa (arrows). The histologic diagnosis was sialolipoma.

On cut section, the mass appeared soft, white, and greasy. Microscopically, the pedunculated polypoid mass was surrounded and well-demarcated by a relatively uniform capsule of dense fibrous connective tissue (Figure 3A, B). The free outer surface of the polyp was covered by keratinized stratified squamous epithelium. The bulk of the polyp consisted of lobules of mature adipose tissue with smaller amounts of salivary tissue and ducts, which were found predominantly in the central region of the mass (Figure 3C). The salivary tissue was composed of acini, lined by well-differentiated mucous epithelial cells, which occasionally surrounded ducts that often contained small amounts of mucus. There was no apparent atrophy of the salivary tissue, metaplasia of ducts, or inflammation in any region of the mass. The palatine tissue at the base of the polyp contained some tonsillar-like lymphoid tissue and in the deeper submucosa contained some lobules of mucous-type salivary glands. The morphologic features of the mass were consistent with a diagnosis of sialolipoma.

Figure 3.

Photomicrographs of a sialolipoma in a dog. A — The sialolipoma on the right is well-demarcated from the adjacent palatine tissue on the left by a capsule of fibrous connective tissue (arrow). Hematoxylin and eosin (H&E) stain, original magnification ×12.5. B — Higher magnification showing the relatively thick fibrous capsule (arrow). H&E stain, original magnification ×100. C — Clusters of mucous acinar glands (arrows) adjacent to ducts that contain some mucus (*) and are surrounded by well-differentiated adipose tissue. H&E stain, original magnification ×100.

Six months after surgery, oropharyngeal examination under heavy sedation with dexmedetomidine (Dexdomitor; Pfizer Animal Health, Kirkland, Quebec), 7 μg/kg BW, IV, and butorphanol (Torbugesic; Wyeth Animal Health), 0.1 mg/kg BW, IV, revealed no visible regrowth at the site of tumor removal. Clinically the dog was doing well, with no inspiratory stridor or excessive panting.

Discussion

Histologically, sialolipomas are well-circumscribed masses of mature adipocytes with islands of salivary gland parenchyma surrounded by a thin fibrous capsule (1). The adipose tissue may account for 40% to 90% of the tumor volume (1,3). The salivary gland elements comprise acini and ducts, which may be normal or may have evidence of acinar atrophy, ductal dilatation, and ductal oncocytic metaplasia (1,3–5). The epithelial islands may be distributed throughout the tissue or located at the periphery. Immunohistological results have shown that the acini-duct complexes express normal cellular phenotypes and have no proliferative activity (1). Some tumors have lymphoplasmacytic infiltration and discrete areas of fibrosis (6). The mass in this case report consisted of mature adipose tissue with scattered islands of normal acini and ducts surrounded by a capsule of fibrous tissue, which is consistent with the classification of sialolipoma.

Some researchers have proposed that the histogenesis of sialolipomas may be related to a type of salivary gland dysfunction, similar to sialoadenosis, which results in an altered salivary gland structure (7). A more common histogenic theory is that it is a simple lipoma that entraps normal salivary gland tissue (1).

Salivary glands in dogs are divided into major and minor salivary glands. The major salivary glands include the parotid, mandibular, sublingual, and zygomatic (8). The minor salivary glands are distributed throughout the lips, cheeks, tongue, hard palate, soft palate, pharynx and esophagus, and collectively produce a significant amount of mucus (8).

In humans, sialolipomas affect both the major and minor salivary glands in equal proportions. They occur most commonly in middle-aged humans, but have also been reported in newborns (9). Sialolipomas are typically slow-growing, soft, and painless (2). Growth in children is reported to be more rapid than in adults (9). Depending on the size and location, they may interfere with mastication, swallowing, and respiration. In the present case, the sialolipoma was in an older dog and presented as a soft and painless mass. It is unknown how long the mass had been present in the dog, but the progression of clinical signs over the course of 1 y suggests that it was slow-growing.

Sialolipomas of the major salivary glands affect males more frequently than females (5). Approximately 90% of the reported cases in humans affected the parotid salivary gland; the balance affected the submandibular gland (3,5,6). Sialolipomas of the major salivary glands have not been reported in animals, but could be considered as a differential diagnosis for sialomegaly.

In humans, sialolipomas of the minor salivary glands affect men and women equally (5). Most cases involve the hard and soft palates (37.5%); sialolipomas have also been reported on the lip, tongue, floor of the mouth, retromolar pad, buccal sulcus, and buccal mucosa (2,6,7,10,11). On cut section they usually resemble a lipoma, as did the sialolipoma in this report (1,2). There have been no reports of malignant transformation or recurrence after marginal excision in humans (1,4,5). Likewise, no recurrence of the reported sialolipoma was seen in this case 6 mo after surgical excision.

Imaging with computed tomography typically reveals a well-demarcated mass with a low-density signal (6). Magnetic resonance imaging may show a hyperintense mass on the T1-weighted images and isointensity on the T2-weighted images, consistent with subcutaneous fat (6). Other authors have found a heterogenous signal on the T1- and T2-weighted images (1,12). If the sialolipoma in this case had been located in a less obvious and accessible location, advanced imaging may have increased the index of suspicion for a well-circumscribed benign lipomatous mass. In future cases, advanced imaging and/or incisional biopsy may prove beneficial in the preoperative planning for sialolipomas.

Histologically, sialolipomas must be differentiated from lipomatous infiltration, adenolipomas, lipomatous adenomas, true lipomas, interstitial lipomatosis, lipomatous atrophy, fibrolipomas, and spindle cell lipomas (5,13). Sialolipomas are distinguished by the presence of both acini and ductal elements, abundant adipocytes, and a thin fibrous capsule (1). Lipomatous infiltration of the major salivary glands has been\reported to affect the parotid and mandibular salivary glands of dogs (13,14). Histologically, these glands have extensive replacement of normal salivary tissue by well-differentiated adipocytes, but unlike sialolipomas there is no distinct fibrous capsule (1,13). Other differential diagnoses for a mass in this location include salivary cyst, palatine tonsil cyst, branchial cyst, tonsillar polyp, sialocele, abscess, foreign body, and malignant neoplasia (15–18).

Sialolipoma is a rare benign tumor of adipose tissue with incorporated salivary elements surrounded by a fibrous capsule. The findings in the present case indicate that sialolipoma should be considered as a differential diagnosis for oral masses and sialomegaly in dogs. Complete surgical excision of the reported sialolipoma resulted in no evidence of regrowth after 6 mo. Complete surgical excision in humans is curative. CVJ