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 neoplasms7–9. Microscopically, lipoma is composed of mature adipose tissue arranged in lobules separated by fibrous septa. However, histopathological variants of lipoma might be observed, including spindle cell lipoma, fibrolipoma, myolipoma, myxolipoma, angiolipoma, sialolipomas, chondrolipoma and osteolipoma9,10. 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 space11, 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 reported6,11−16. Considering the rarity of this case in the salivary glands, we aim to describe the first 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 findings 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.
Table 1
Osteolipoma reported in the major salivary gland regions
Authors and year | Age | Sex | Sites of lesions | Clinical presentation | Relation with salivary gland | Duration (yrs) | Tumor size (cm) | Raidographic findings | Treatment | Pathology |
Godby et al. [12].1961 | 54 | M | Sublingual area | A soft painless mass beneath the tongue | N/A | 1 | 7 × 6 | X-ray: an osseous projection was seen in the oral floor | Tumor excision | Microscopic sections showed well-formed cancellous hone surrounding striated muscle, fibrous connective tissue, and fat. |
Dutescu et al. [13]. 1973 | 40 | M | Submandibular region (L.) | A tumor mass in the left submandibular region | NA | 3 | 4 × 7 | N/A | Tumor excision | The tumor was composed of mature adipose cells, connective cells, collagen fibers and foci of osseous metaplasia similar to normal bone tissue. |
Ohno et al. [14]. 1998 | 58 | F | Submandibular region (R.) | A soft immovable mass in submandibular region | Tumor was located at extra right submandibular gland, which was displaced inferiorly | 1 | 9 × 4 | CT: low density contrast suggesting fat with bone-like tissue. MRI: Signal intensities also suggested fat and dental tissues. Bone scanning with 99 m-Tc showed accumulation in the lesion. | Tumor resection | Tumor contains fat tissue with and lamellar bone with a marrow-like appearance containing microvessels, erythrocytes and lymphocytes inside its lumen. |
Diom et al. [6]. 2011 | 21 | F | Parotid region (L.) | A sublobular, painless, hard, barely mobile mass in left cheek | The tumor was independent from the parotid located between the deep lobe of the gland externally and the parapharyngeal space internally. | 1 | 5 | NA | Tumor excision and total parotidectomy with facial nerve preservation | The tumor was made of mature non-atypical adipocytes with metaplastic ossification foci in the middle. Bone metaplasia was complete with a corticospongious tissue with adipose medulla. |
Kavusi et al. [11]. 2013 | 67 | M | Submandibular region (L.) | A firm, non-tender, and mobile mass in the left submandibular area | Tumor was located at extra gland but was involved in the anterior aspect of the left submandibular gland | 10 | 3.5 × 2.2 | CT: well-circumscribed mass with low density and a central densely calcified portion | Tumor excision | Tumor are composed of mature adipose tissue and lamellar bone |
Aboh et al. [15]. 2015 | 33 | M | Submandibular region (L.) | Painless swelling in the left submandibular region | N/A | Many years | N/A | MRI shows a submandibular mass with hyper- and hypo-intensity signals on T1-weighted image. | Tumor excision without damage submandibular gland | N/A |
Raghunath et al. [16]. 2015 | 20 | F | Sublingual area | A well circumscribed, firm and painless mass | N/A | 3 | 6 × 6 | CT scanning reveals a well-defined, hypodense lesion (− 35 to − 65 HU) measuring about 6 × 6 cm within which irregular hyperdense areas (650–850 HU) are noted to be located in the sublingual space | Tumor excision | Tumor shows a large central area of osseous tissue, characterized by branching and uniting trabeculae of osteoid. At the peripher, mature adipose tissue admixed with areas of bony trabeculae are noted. |
This case | 47 | M | Parotid region (L.) | A hard barely mobile mass in left parotid region | Tumor was located at intraparotid gland | 10 | 2.5 × 3.2 | MRI scanning shows a well-defined oval lipomatous mass with high signal intensity on T2-weighted image and suppressed signal intensity on fat-suppressed T1- and T2-weighted image. The dense, ossifying layer circumscribing the fatty core appeared as a hypointense cortical line | Tumor excision and partial parotidectomy with facial nerve preservation | Microscopic examination in paraffin embedded section reveals that the tumor exhibited abundant mature adipose tissue with bony structures |
N/A: Not available, M:Male, F:Female, L: Left, R: Right |
However, osteolipomas demonstrate various clinical manifestations, including painless soft/hard mobile mass or immovable mass. Even worsen, osteolipomas might share overlapping clinical and radiological characteristics with malignant tumors as well as benign neoplasms17. Without additional examinations, these clinical presentations are difficult 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 definition and allowing visualization of the tumor boundary from surrounding tissue18,19. As well known, MRI is the preferred method for visualizing soft tissue neoplasms, whereas CT is helpful for detecting bony structures or ossification. On CT scanning, hypodense fatty tissue with irregular hyperdense ossifying areas might be observed in osteolipoma. MRI imaging of osteolipoma shows well-defined 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 findings 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 findings supported the mass can be definitively diagnosed as tumors of adipocytic origin.
So far as now, the pathogenesis of osteolipomas is still not clarified. 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 cells20. The other widely proposed theory is the metaplasia of fibrous elements into bone tissue from the pre-existing lipoma, resulting from repetitive trauma, metabolic changes or ischemia1. Aside from the two hypotheses described above, it has also been implicated that the development of osteolipomas might result from the transformation of fibroblasts into osteoblasts induced by growth factors released from monocytes21, or perhaps the ossification due to an inadequate nutritional supply within the center of a large lipoma14.
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, superficial parotidectomy and total parotidectomy10,22,23. Unsurprisingly, superficial parotidectomy is preferred by most authors. As noted in our case, the tumor was resided at the superficial 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.