Sialolipoma, an uncommon variant of head and neck lipoma, is composed of proliferative adipocytes with entrapped normal salivary gland islands . Almost any site within the oral and maxillofacial region may be involved with the parotid gland being the most frequently reported location [1-3, 6, 7, 9, 14, 21]. To our knowledge, 34 cases of sialolipoma have previously been reported in the English literature and eighteen of them were found in minor salivary glands [seven on the palate [1, 8, 10, 16, 17, 19], three in buccal mucosa [4, 10, 20], three on floor of the mouth [5, 11, 20], two on the tongue [4, 20], two in lower lip [12,the present case], and one on retromolar pad .
Clinically, sialolipomas usually present as a solitary painless palpable mass with an average size of 2.74 cm in diameter. Females are affected slightly more than males (with ratio 1.1:2). Patient’s ages range from 6 weeks to 84 years, with average of 47.6 years. The duration of the lesion range from two months to ten years, with average of three years. In the present report, the lesion is in the lower lip and the diameter is 0.6 cm. Because lower lip is a preferable site of mucocele, it is probable that superficially located sialolipoma might be misdiagnosed clinically as mucocele. The other most common preoperative diagnoses are fibroma and salivary gland tumor. There is no distinguishable radiographic sign for sialolipoma in either computed tomography scan (CT) or magnetic resonance imaging (MRI) compared to a typical fatty lesion in the head and neck region .
Histological findings of haematoxylin and eosin staining in previous studies include a well circumscribed mass surrounded by a delicate fibrous tissue. The tumors are composed of mature adipose elements mixed with salivary gland tissues. The glandular components, consisting of acinar cells and ductal components, may be scattered through out the tumor or located in the periphery of the tumor [1, 5]. The 80 % of sialolipomas in major salivary gland are composed of adipose tissue while in minor salivary gland the glandular elements are clustered and evenly distributed around fat tissue [10,11]. No mitosis is seen in adipocytes or acinar and ductal cells . The glandular components may be showed ductal dilation, oncocytic changes and squamous ductal metaplasia [1, 5-10]. In some cases areas of fibrosis are seen while myxoid changes are reported only in one case [9, 15]. Additionally, lymphocyte infiltration and enlarged congested vessels are reported [1, 10, 15].
The pathogenesis of the sialolipoma is not completely understood. However, immunohistological and ultrastructural studies confirmed that the glandular elements of the lesion could arise from entrapment of minor salivary gland during lipomatous proliferation rather than representing neoplastic process [1, 4, 5].
The morphologic differential diagnosis includes a variety of entities. Adenolipoma has histologic characteristic similar to sialolipoma; but it is composed of adipocytes and duct elements without acinar cells. Adenolipoma also differs from sialolipoma by the lack of organoid arrangement of the ductal type tissue [1, 5, 8]. Lipomatosis which typically occurs in older patients can be excluded by the microscopic lack of the fibrous capsule in addition to the absence of any medical condition associated with lipomatosis, for instance diabetes mellitus, malnutrition, chronic alcoholism and liver cirrhosis [1, 7, 9]. The distinction from pleomorphic adenoma is made by the presence of extensive fatty elements within the normal salivary gland tissue and lack of ducts and strands of dark-staining myoepithelial cells in sialolipoma [1, 5, 10].
Sialolipoma in the minor salivary glands is treated by complete surgical excision. However, most of tumors in parotid glands are treated with superficial parotidectomy. A complete parotidectomy with preservation of the facial nerve has been reported in two cases [7, 21] (Table 1). Malignant transformation of sialolipoma has not been reported yet in the literature . The follow up period ranged from 2 months to 10 years and there is no evidence of recurrent sialolipoma.
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The authors confirm that this article content has no conflicts of interest.