Sialolipoma of the parotid gland: A rare entity. Case report

Introduction and importance Lipomatous neoplasms of the parotid gland represent an exceptionally rare and often underdiagnosed category of tumors, accounting for an incidence ranging from 0.6 % to 4.4 % of all neoplasms detected within the parotid gland. Sialolipoma is defined as an uncommon variant of lipoma, characterized by a well-defined proliferation of mature adipocytes with secondary entrapment of salivary gland elements, including serous acini, ducts, and myoepithelial cells. Case presentation The current case pertains to a 17-year-old female who presented with a one-year history of enlargement in the left preauricular region. Clinical discussion The case we present poses a complex diagnostic challenge due to two distinct characteristics. The diagnostic challenge lies in its remarkably low incidence and the propensity for confusion with pleomorphic adenoma, which is the most common tumor of the parotid gland. It is a benign disease entity characterized by the absence of dysplasia, in marked contrast to pleomorphic adenoma. Conclusions The infrequency in the manifestation of these tumor types, coupled with their prolonged asymptomatic course, can pose a diagnostic challenge. Enhancing our knowledge to comprehensively delineate these entities is imperative to effectively address the diagnostic complexities from both clinical and histopathological standpoints.


Introduction
Salivary gland tumors make up a small proportion, less than 3 %, of all neoplastic lesions affecting the craniocervical region.The parotid gland is the primary site of incidence for these tumors, whether they are benign or malignant.Although most of these tumors originate from epithelial tissue, about 2-5 % arise from mesenchymal cells [1].Based on the reviewed literature, very few cases have been documented globally.Most of these cases (51.4 %) have specifically originated in the parotid gland [2].The reported symptoms in these cases mimic those of a pleomorphic adenoma, with a significantly lower incidence of complications related to facial venous entrapment (14 %) [3][4][5].
Sialolipoma represents a particular histological variant of lipoma, first documented by Nagao et al. in 2001 [6].It appears in the 2005 World Health Organization (WHO) classification and has been incorporated as a separate entity in the most recent 2017 WHO classification of head and neck tumors.This tumor type typically manifests as a welldefined mass with a fibrous envelope, composed of neoplastic mature adipose tissue and non-neoplastic salivary gland components [7].
From a clinical perspective, the sialolipoma typically manifests as a precisely delineated localized swelling.Its observed maximum diameter can extend up to 3 cm, displaying a soft consistency and a yellowish appearance.In certain instances, when associated with traumatic factors, its surface may adopt a yellow-white tint.Submersion in formalin results in the characteristic floating of the surgical specimen, indicating the presence of adipose content within its structure.Initially, it is often perceived as a glandular anomaly akin to a pleomorphic adenoma.Accurate diagnosis is only achievable through histopathological examination.When involving the parotid gland, it may incite facial paralysis.Surgical excision stands as the primary therapeutic approach, with no reported instances of recurrence documented to date [8].
Lipoadenoma shares significant histological similarities with sialolipoma.The main distinction is that lipoadenoma consists of adipose tissue and ductal components, whereas sialolipoma, in addition to these elements, contains glandular acini.We present a case report of a 17year-old female with a diagnosis of parotid sialolipoma.The work has been reported in line with the SCARE criteria [9].

Clinical case presentation
The case involves a 17-year-old female patient who presented for consultation due to the identification of a mass in the left preauricular region persisting for 1 year.On palpation, a soft, mobile, and painless tumor measuring 2 cm in diameter was noted.This enlargement was associated with paresthesias in the left cheek, the sole reported symptom.The patient had a history of percutaneous drainage of a similar lesion in the same region at 8 years of age, which showed a favorable outcome.No other pertinent medical history was identified.Considering the clinical presentation, comprehensive laboratory tests revealed no pathological findings, prompting an excisional biopsy of the tumor.A 3 cm longitudinal incision was made over the tumor, which was subsequently dissected and excised completely.The resulting wound was closed using nylon material in a simple interrupted suture technique.A sample was sent for pathological analysis.
The macroscopic examination of the specimen exhibited dimensions of 4 × 3 × 0.3 cm and displayed preserved, soft, greyish-yellow, granular tissue for detailed microscopic analysis (Image 1a and b).Microscopic evaluation revealed an unaltered epidermis in underlying connection with fragments of a benign mesenchymal neoplasm of adipose origin with lobulated contours.This neoplasm demonstrated a proliferation of adipocytes with minimal variations in shape and size, resembling mature adipocytes, and lacked septa.Furthermore, transitions with serous glandular acini of the parotid gland were identified, and the lobules were demarcated by adipose tissue with similar characteristics (Image 2a and b).Ultimately, the diagnosis of sialolipoma in the left parotid gland was confirmed.The patient was discharged on the same day.A one-week follow-up revealed substantial improvement without any complications.Concerning the initial clinical manifestation of paresthesias, the patient persisted in experiencing paresthesias in the left cheek for the two weeks subsequent to the surgical procedure, coupled with a mild loss of movement in the left labial commissure.At the twoweek mark post-surgery, the patient became asymptomatic.Throughout a one-year follow-up period, no indications of recurrence were noted.

Discussion
The case we present poses a complex diagnostic challenge due to two distinct characteristics.Firstly, the higher incidence of sialolipomas has been reported in the male population [3] Secondly, the occurrence of paresthesia in the left cheek as the sole symptom, without any accompanying medical history or pathological indications, added further complexity to the diagnosis.Furthermore, the age factor contributed to the decreased suspicion, as reported by Kidambi et al., where 90 % of documented sialolipoma cases were observed in adult individuals over 18 years of age.Notably, three cases of sialolipoma were documented in patients under the age of 5. Additionally, only two cases of congenital sialolipoma were identified [10].These instances comprised one case in a patient at birth and the other in a 6-week-old male infant, with both cases showing sialolipoma in the parotid gland [10,11].
The etiology of sialolipoma remains not completely elucidated.Some researchers suggest that the pathogenesis of sialolipoma might be associated with salivary gland malfunction, leading to an altered configuration of the salivary glands.This microscopic alteration is characterized by the substitution of normal salivary gland tissue with mature adipose tissue, accompanied by atrophic salivary glandular elements, chronic changes in ductal epithelial cells such as oncocytic metaplasia, fibrosis, and lymphocytic infiltration [12].Microscopically, sialolipomas are typically recognized as well-demarcated, often encapsulated masses comprising benign neoplastic adipose tissue interspersed with foci of acini and usually atrophic, non-neoplastic salivary gland ducts within the lipomatous proliferation.Adipose tissue accounts for 90 % of sialolipomas in the parotid gland and 50 % of cases affecting the  palate [6].In the presented case, the microscopic examination details the presence of fragments of benign neoplasia characterized by adipocyte proliferation.
Finally, the history of percutaneous drainage of a similar lesion in the same region, with a favorable evolution, is another noteworthy aspect, as cases of recurrence in this type of tumor have been described in extremely rare cases [13].

Image 1 .
a and b: Excisional biopsy of nodule in left parotid region.

Image 2 .
a and b: Fragments of benign mesenchymal neoplasm of adipose lineage with lobulated contour.The proliferation of adipocytes stands out.