Bilaterally within the soft tissues located along the lateral aspect of the mandibular bodies, there are slightly lobulated, soft tissue attenuating and diffusely mildly contrast-enhancing elongated structures. These are thought to correspond to minor salivary glands, approximately 6.8cm in length and 9 mm in width.

Within the mid-aspect of the left minor salivary gland, a section of this lobulated tissue is surrounded by a thin and regular fat-attenuating and non-enhancing rim, resulting in a focal mild thickening and increased height of the gland. The portion of the gland encompassed by the fat-attenuating rim is slightly and homogeneously less attenuating (25-30 HU pre-contrast and 60-75 HU post-contrast) than the remainder of the gland (50-60 HU pre-contrast and 110-130 HU post-contrast). The portion of the gland encompassed by the fat-attenuating rim measures up to 18 mm in length, 12mm in height and 8mm in width. Multifocally, thin fat-attenuating stripes originating from the inner aspect of the rim extend between the lobulations of the encompassed part of the gland. The most medial aspect of the fat-attenuating rim focally contacts the buccal surface of the mandible, without any bony changes.

Regional lymph nodes and major salivary glands are normal.

Conclusions :

• Focal mild thickening and fatty infiltration of the minor salivary gland alongside the left buccal surface of the mandible.
• No evidence of mandibular osseous lesion.
• No regional lymphadenomegaly.

Differential diagnoses include focal sialadenitis vs salivary infarct vs sialadenoma, with secondary/concomitent focal lipomatous infiltration or a fat-containing tumor of the salivary gland.

Given the focal and unilateral distribution of these changes, sialadenosis was considered less likely. A malignant neoplasm (such as an adenocarcinoma) was not ruled out but considered less likely given the lesion’s size and margination. A sialocele, a salivary cyst or abscessation were unlikely in the absence of a fluid component.

In the absence of other tomodensitometric anormal findings, this lesion was considered the most likely cause of the oral pain.

Excisional biopsy of the gland was performed.

Histology

Microscopic description:

Skeletal muscle and loosely arranged fibrovascular tissue surrounds salivary glands that have a regular lobular organization. All glands are similar, each having acini of tall cuboidal cells with abundant, granular, lightly basophilic cytoplasm (consistent with normal mucous salivary tissue). The lumen of acini and ducts contain wispy basophilic material with occasional round cells, consistent with mucinous secretion with occasional macrophages. Low numbers of lymphocytes and plasma cells infiltrate the intralobular stroma of all glands.

The largest lobule of salivary tissue (from the area rimmed by pigmented mucosa) differs from others by being embedded within adipose tissue and having many well differentiated adipocytes throughout the inter- and intralobular fibrovascular tissues. The area of adipose tissue is well delineated. Aside from the presence of adipocytes, the involved gland has multifocal areas of proliferative tuboacinar structures, which are lined by low cuboidal epithelial cells with amphophilic cytoplasm and small dark nuclei (ie. immature/hyperplastic glands). The interstitial lymphoplasmacytic inflammation within the affected lobule is comparable to that of the others. In all other sections, adipose tissue is present only surrounding neurovascular bundles.

Histologic diagnosis

• Focal lipidosis and benign glandular hyperplasia, possible sialolipoma
• Diffuse, mild lymphoplasmacytic sialadenitis

Comments :

In this case, the inflammation is suspected to be incidental since all glands are affected similarly. There is no evidence of necrosis, ischemia or active inflammation.

A mass lesion that correlates with the size of that identified on CT consists of adipose tissue intermingled with salivary tissue. Except for benign hyperplasia, the salivary tissue is identical to the surrounding glands. The features of this lesion seem to fit the diagnosis of a sialolipoma.

The fatty gland appears to have been completely excised with narrow (minimum 2mm) margins.

Follow-up

Six months later, the patient is reported by the owners to be doing very well. Following discharge, with the exception of rare, mild left-sided hypersalivation, oral pain fully resolved and masticatory function is back to full function.

Discussion

Sialolipoma is a rare benign neoplasm, with few case reports in the dog, including one affecting a minor salivary gland. In dogs, the minor salivary glands are distributed throughout the oral cavity in the labial, lingual, and palatal mucosa, but also in the pharynx and esophagus, and collectively produce a significant amount of mucus.

In humans, fat-containing tumors of the salivary glands are uncommon and predominantly affect elderly males, with a mean age at presentation of >50 years. Their histomorphological spectrum varies greatly and lesions can be divided into true adipose tissue neoplasms (lipomas, rare lipoma variants and atypical lipomatous tumors/liposarcomas) and hybrid lipoepithelial lesions composed of epithelial derivatives admixed with a variable fatty component (including sialolipomas and lipoadenomas). Additionnally, fat-containing tumor-like salivary gland lesions can also be encountered (diffuse/interstitial lipomatosis and lipomatous lobular atrophy).

Histologically, sialolipomas (also referred to as non-oncocytic adenolipomas) are distinguished by the presence of both acini and ductal elements with myoepithelial and basal cells of non-neoplastic salivary gland tissue, entrapped by abundant mature adipocytes, encased in a thin fibrous capsule. The presence of a capsule distinguishes them from salivary lipomatosis, which is otherwise histologically similar. In this patient’s CT, sharp margination of the fat-attenuating rim is compatible with the presence of a capsule.

Lobular lipomatous atrophy (lipomatosis limited to one lobule of the gland) may resemble sialolipoma and mimic a mass. The affected lobule remains however contiguous with other lobules of gland, lacking a capsule. Besides, the multilobular pattern of sialolipoma is absent. Furthermore, lobular lipomatous atrophy is usually associated with underlying obstructive, inflammatory or other diseases of the gland. Expected histological changes of such conditions (such as thick-walled vessels and ductular hyperplasia with periductal fibrosis) were not detected on this patient’s histological sample.

References

• Sialolipoma of a minor salivary gland in a dog, Clark, Hanna and Béraud, Can Vet J, 2013; 54 :467-470

• Lipomatosis of the parotid salivary gland : 2 case reports and review of the literature on fat-containing salivary gland lesions, Serras et al., International Journal of Veterinary Science and Medicine 6 (2018) 253–257

• Fat-Containing Salivary Gland Tumors: A Review, Abbas Agaimy, Head and Neck Pathol (2013) 7:S90–S96

ACKOWLEDGEMENTS :

The author would like to thank Kate Alexander, DVM MS DACVR and Cindy Bell, DVM DACVP for their help reviewing this case.