Osteolipoma of Parotid Gland: A Rare Presentation

Osteolipoma is an uncommon benign tumor containing mature adipose tissue with bone metaplasia foci. It is rarely observed in head and neck region as compared to the lipoma, especially in the region of parotid gland. We report the rst case of intraparotid osteolipoma with its clinical, radiographic and pathological ndings. A 47-year-old men presented with an immovable mass in his left cheek with a 10-year evolution. Magnetic resonance imaging (MRI) scans showed a 3.2-cm fat-containing mass involving in the left parotid. The patient was taken to the operating room for excision of the mass and the nal pathologic diagnosis was osteolipoma. Due to its rarity and clinical signicance, we also reviewed the osteolipoma presenting in major salivary gland regions.


Abstract Background
Osteolipoma is an uncommon benign tumor containing mature adipose tissue with bone metaplasia foci. It is rarely observed in head and neck region as compared to the lipoma, especially in the region of parotid gland.

Case presentation:
We report the rst case of intraparotid osteolipoma with its clinical, radiographic and pathological ndings. A 47-year-old men presented with an immovable mass in his left cheek with a 10-year evolution. Magnetic resonance imaging (MRI) scans showed a 3.2-cm fat-containing mass involving in the left parotid. The patient was taken to the operating room for excision of the mass and the nal pathologic diagnosis was osteolipoma. Due to its rarity and clinical signi cance, we also reviewed the osteolipoma presenting in major salivary gland regions.

Conclusion
Intraparotid osteolipoma is an extremely rare neoplasm originating from adipogenic tissue, which should be taken into consideration in the differential diagnosis of parotid tumor.

Background
Osteolipoma is a benign mesenchymal neoplasm originating from fatty tissue with bone formation, which is a rarer histological variant among lipomas and accounts for less than 1% of cases reported in the literature 1,2 . It has been documented in the upper and lower extremities, spine, subcutaneous soft tissue and head and neck region [1][2][3][4][5] . To the best of our knowledge, only one case of osteolipoma in the parotid region was documented in the English language literature, which was reported by Diom et al. 6 in 2011. They described a 21-year-old female patient with osteolipoma located between the deep lobe of the parotid gland externally and the parapharyngeal space internally, indicating the tumor was independent from the parotid gland and should be classi ed as periparotid osteolipoma. Here, we describe the rst case of intraparotid osteolipoma located at the super cial lobe of parotid gland. Moreover, we also reviewed the osteolipoma presenting in major salivary gland regions due to its rarity and clinical signi cance.

Case Presentation
A 47-year-old male patient was admitted to the department of Oral and Maxillofacial Surgery at Hospital of Stomatology, Guanghua School of Stomatolgy, Sun Yat-sen University. He presented a slowgrowing and painless mass in the left cheek with a 10-year evolution. The patient had a history of trauma in that region in 10 years ago. The medical history and systemic review were noncontributory. Physical examination showed a 2.5 × 3.2 cm 2 , painless, hard, barely mobile mass with a smooth surface in the left parotid region (Fig. 1A). There was no intraoral expression and peripheral facial palsy. After the physical examination, the initial impression was diagnosed as the tumor of parotid gland and MRI scanning was requested. MRI ndings revealed that a 2.5 × 2.0 × 3.2 cm 3 , well-de ned oval lipomatous mass located in the super cial lobe of left parotid with high signal intensity on T2-weighted image and suppressed signal intensity on fat-suppressed T1 and T2-weighted image ( Fig. 1B-E). A hypointense line was also observed on the MRI images, indicating that the fatty mass was circumscribed with an ossifying layer (Fig. 1E). However, the internal boundary of tumor with parotid gland is not clear (Fig. 1F). Based on these clinical and radiographic ndings, the differential diagnosis including pleomorphic adenoma with ossi cation, osteolipoma, chrondrolipoma, sialolipoma and other salivary gland tumors was considered. Under general anesthesia, surgical removal of tumor was done with a standard preauricular/cervical incision.
After exposure the parotid gland, the facial nerve was identi ed and preserved. Then, a yellowish tumor was dissected and resected completely. In agreement with the MRI ndings, we found that the tumor was attached to the adjacent parotid gland at the internal boundary during the operation. After removal of the tumor, the surgical specimen was then sent to the Department of Oral Pathology, Hospital of Stomatology, Guanghua School of Stomatology, Sun Yat-sen University. Macroscopically, the resected specimen was a well circumscribed and yellowish mass with hard consistency ( Fig. 2A). On sectioning, the mass demonstrated a yellow soft tissue with various intermixed ossifying structures and a brous capsule (Figs. 2B). Microscopic examination in para n embedded section revealed the tumor comprising predominantly of mature adipose tissue partly encapsulated by osseous bone shell and admixed with areas of bony trabeculae, along with intervening brous connective tissue, few in ammatory cells and blood vessel ( Fig. 2C-F). The nal established diagnosis was the 'osteolipoma'.

Discussion
Lipomas are common benign mesenchymal neoplasm of soft tissue in various areas of body, but account for only 0.2-0.8% of major salivary gland tumors and 22.5% of all benign mesenchymal salivary neoplasms [7][8][9] . Microscopically, lipoma is composed of mature adipose tissue arranged in lobules separated by brous septa. However, histopathological variants of lipoma might be observed, including spindle cell lipoma, brolipoma, myolipoma, myxolipoma, angiolipoma, sialolipomas, chondrolipoma and osteolipoma 9,10 . Among them, osteolipoma is an extraordinary lipoma variant characterized by mature bone formation within the neoplastic fatty tissue. In head and neck region, more than half of the osteolipoma were reported in oral cavity and naso/parapharyngeal space 11 , but extremely rare in parotid gland. Although oncology of the salivary gland and parotid region is tremendously rich and varied, only seven cases of osteolipoma located at the major salivary gland regions have been reported 6,11−16 . Considering the rarity of this case in the salivary glands, we aim to describe the rst case of intraparotid osteolipoma and summarize the osteolipoma presenting in the region of major salivary glands, which would be of particular interest to head and neck surgeons and pathologists.
As shown in Table 1, osteolipoma can be observed in all three major salivary gland regions, regardless of the proximity to bone. Two cases were occurred in the sublingual area, four in the submandibular space and two in the parotid region. Among these cases, the submandibular osteolipomas, sublingual osteolipomas and one periparotid osteolipoma were all independent from salivary glands, indicating that the tumor might originate from fat tissue in the submandibular, sublingual and parapharyngeal space. Interestingly, the tumor was resided at the intraparotid gland in our case, which may closely mimic pleomorphic adenoma with bone formation and easily misdiagnosis as pleomorphic adenoma in clinics. In addition, the age, sex, clinical presentation, tumor size, duration of the lesion, radiographic ndings and managements of these cases are also described in Table 1. In this series of cases, all cases were reported in adult patients with a relative long history of slow progression and seemed to more common in men, which is similar with salivary gland lipomas 9,10 .  17 . Without additional examinations, these clinical presentations are di cult to narrow differential diagnosis and most often mistaken for pleomorphic adenoma, Warthin tumor or malignant tumors. It has been implicating that MRI and CT examination can be used to further characterize osteolipoma, and provide the best tissue de nition and allowing visualization of the tumor boundary from surrounding tissue 18,19 . As well known, MRI is the preferred method for visualizing soft tissue neoplasms, whereas CT is helpful for detecting bony structures or ossi cation. On CT scanning, hypodense fatty tissue with irregular hyperdense ossifying areas might be observed in osteolipoma. MRI imaging of osteolipoma shows well-de ned mass with high signal intensity on T1-weight imaging but suppressed signal intensity on fat-suppressed T2-weight images. Therefore, osteolipoma might be diagnosed based on the radiographic ndings though combination MRI and CT examination. In our case of osteolipoma, the tumor demonstrates high signal intensity on T2-weighted image and suppressed signal intensity on fat-suppressed T1-and T2-weighted image. A hypointense line was also observed on MRI imaging, indicating that the fatty core mass was circumscribed with an osseous layer. These ndings supported the mass can be de nitively diagnosed as tumors of adipocytic origin.
So far as now, the pathogenesis of osteolipomas is still not clari ed. Some researchers suggested that osteolipomas might be derived from the bidirectional differentiation of multipotent mesenchymal stem cells, which claimed that a lipoma with ossifying tissue is the result of lineage differentiation into both bone and adipose tissue form mesenchymal stem cells 20 . The other widely proposed theory is the metaplasia of brous elements into bone tissue from the preexisting lipoma, resulting from repetitive trauma, metabolic changes or ischemia 1 . Aside from the two hypotheses described above, it has also been implicated that the development of osteolipomas might result from the transformation of broblasts into osteoblasts induced by growth factors released from monocytes 21 , or perhaps the ossi cation due to an inadequate nutritional supply within the center of a large lipoma 14 .
The treatment of osteolipoma is similar with lipoma and tumor excision is recommended. The recurrence is rare after complete excision. As evident in Table 1, the simple tumor excision was used to manage the osteolipoma from salivary gland regions except for the parotid region. All the submandibular and sublingual glands were not damaged and still kept intact in these cases. However, it remains under controversy for the management of parotid gland osteolipoma, as well as lipoma. Several methods have been proposed for managing the parotid lipoma, including enucleation or excision with a small border of parotid gland for encapsulated intra or paraparotid lipomas, super cial parotidectomy and total parotidectomy 10,22,23 . Unsurprisingly, super cial parotidectomy is preferred by most authors. As noted in our case, the tumor was resided at the super cial lobe of parotid and closely attached to facial nerve. MRI examination suggested that the internal boundary of ostelipoma was involved in parotid gland. To preserve the function of parotid gland and remove the tumor completely, we performed partial parotidectomy and excise the tumor but preserve most of parotid gland and facial nerve. In the case reported by Diom et al., total parotidectomy with facial nerve preservation was employed to treat the ostolipoima due to the mass located between the deep lobe of the gland externally and the parapharyngeal space internally. Taken together, we recommended that tumor excision should be used to manage parotid osteoliopma. To remove tumor completely and preserve the function of facial nerve and healthy parotid gland, the tumor location and its relationship with parotid must be taken into considerations during surgery.

Conclusion
Osteolipoma is an extremely rare lesion in salivary gland region, which should be considered along with common salivary gland tumors in the differential diagnosis. CT and MRI can accurately characterize osteolipoma and should be utilized to de ne the size, location, and extent of the neoplasms prior to surgery. De nitive diagnosis can only be achieved with histopathologic examination. Tumor excision is recommended for managing the salivary osteolipoma.
However, partial, super cial or total parotidectomy might be required for treating parotid osteolipomas dependent on the tumor location and its relationship with parotid gland.

Declarations
Ethics approval and consent to participate Not applicable.

Consent for publication
The patient provided informed consent to publish this all presentations of case reports.

Availability of data and materials
The data and material supporting the conclusions of this article are included within the article.

Figure 1
Clinical and MRI imaging revealed an oval mass located at the left parotid gland. A barely mobile mass with a smooth surface was noted in the left cheek (A).
A well-de ned oval mass was observed in the super cial lobe of left parotid gland with suppressed signal intensity on fat-suppressed T1-(B and C) and fatsuppressed T2-weighted image (D). The mass showed a high signal intensity on T2-weighted image, which was circumscribed with a hypointense line (E). The internal boundary of tumor with parotid gland is indistinct (F).

Figure 2
Osteolipoma was diagnosed based on the pathological examination of resected specimen. The resected specimen was a well circumscribed and yellowish mass with hard consistency in general (A). On sectioning, the mass showed a yellow soft tissue with bony structures (B). Microscopic examination revealed the tumor was comprised by mature adipose tissue admixed with areas of bony trabeculae, along with intervening brous connective tissue, few in ammatory cells and blood vessel (C: 0.3×, D-F: 10×).