Abstract

Background: Parosteal lipomas are rare benign soft-tissue tumors that develop in close association with the periosteum. Skull involvement is exceptionally uncommon, with only a few cases reported in the literature.

Case Description: A 14-year-old female presented with a slowly enlarging, painless lump above her right ear, first noticed at the age of five. Physical examination revealed a firm, well-defined, immobile mass measuring 2.5 × 1.5 cm in the supra-auricular region. A non-contrast computed tomography scan demonstrated a hypodense mass with thickening of the underlying squamous and mastoid bones, along with mastoid air cell enlargement. Surgical excision was performed for cosmetic reasons and histopathological diagnosis. Intraoperatively, the mass was found to be adherent to the periosteum and underlying bone. Histopathological examination confirmed the diagnosis of parosteal lipoma, showing mature adipose tissue, fibrous tissue, and focal lymphocytic aggregates without signs of malignancy. The patient recovered well postoperatively, with no recurrence or complications over 6 months of follow-up.

Conclusion: Parosteal lipomas of the skull are exceedingly rare and require clinical suspicion for diagnosis. Imaging aids in preoperative planning, while histopathological assessment confirms the diagnosis. Complete surgical excision is curative, with a favorable prognosis. This case adds to the limited literature on skull-based parosteal lipomas.

Keywords: Histopathology, Lipoma, Skull neoplasm, Soft-tissue neoplasm, Surgical excision

INTRODUCTION

Parosteal lipomas are rare benign soft-tissue tumors that develop in close association with the periosteum, accounting for only 0.3% of all lipomas, with skull involvement being exceptionally uncommon.[ 2 ] These tumors typically present as slow-growing, painless masses, often discovered incidentally or due to cosmetic concerns. Although their exact etiology remains unclear, some reports suggest a potential link to localized trauma or chronic mechanical irritation. Definitive diagnosis requires histopathological examination to distinguish parosteal lipomas from other fat-containing tumors such as osteolipomas and liposarcomas.[ 1 ] In this article, we present a rare case of parosteal lipoma located in the supra-auricular region of a pediatric patient. This report is structured as a descriptive case report accompanied by a focused literature review to contextualize our findings within the existing body of knowledge.

CASE PRESENTATION

History

A 14-year-old female patient presented to the outpatient clinic with a complaint of a lump above her right ear, which she had first noticed at the age of 5. The lump had gradually enlarged and remained stable over the past 3 years. Other complaints such as headaches, seizures, vomiting, hearing impairment, or ear discharge were denied. There was no history of ear infections or similar complaints among family members.

Examination and radiological findings

Physical examination revealed a firm, well-defined, immobile mass measuring approximately 2.5 × 1.5 cm above the right ear [ Figure 1 ]. The overlying skin appeared normal, with no redness or tenderness on palpation. A non-contrast head CT scan showed a hypodense mass in the right supra-auricular region, with thickening of the underlying mastoid and squamous bones, as well as enlargement of the mastoid air cells [ Figures 2a - d ]. Based on these findings, the differential diagnoses are parosteal lipoma, osteolipoma, and calcified lipoma.

Figure 1:

Physical examination reveals a mass located above the right ear.

Figure 2:

Non-contrast head computed tomography scan shows a hypodense mass in the right temporal and mastoid regions, accompanied by thickening of the underlying bone and enlargement of the mastoid air cells. (a) Axial view, brain window. (b) Coronal view, brain window. (c) Axial view, bone window. (d) Coronal view, bone window.

Operative procedure

The patient was positioned in the supine position with the head tilted to the left. The incision site was marked according to the mass location [ Figure 3a ]. Layer-by-layer dissection was performed until the temporalis muscle was exposed. On muscle dissection, a reddish-yellow mass with minimal vascularity was observed, covered by the periosteal layer [ Figure 3b ]. The mass was adherent to the underlying bone, making separation difficult. Monopolar cautery was used to facilitate dissection. After complete resection of the mass, the underlying squamous and mastoid bones appeared thickened, with an irregular, rough surface [ Figure 3c ]. Bone rongeurs were used to thin the affected bone, extending into the mastoid air cells, and the surrounding walls were smoothed [ Figure 3d ]. The mastoid air cells were sealed using bone wax, and the temporalis muscle was repositioned over the site. The incision was closed in layers. There was no intraoperative complication and the patient was able to extubated immediately post-operation.

Figure 3:

Surgical procedure. (a) Marking of the incision site above the right ear. (b) Intraoperative view of the tumor mass adherent to the underlying bone. (c) Post-excision appearance, showing thickened, irregular, and rough bone surface. (d) Exposure of the mastoid air cells after bone surface smoothing and resection using a bone rongeur.

Histopathological finding

Histopathological examination revealed mature adipose tissue and fibrous tissue, along with focal aggregates of lymphocytes [ Figures 4a and b ]. In addition, bone tissue and hematopoietic cells were observed [ Figure 4c ]. No signs of malignancy were detected, such as the presence of lipoblasts, cellular atypia, or increased mitotic activity.

Figure 4:

Histopathological findings. (a) The mass was predominantly composed of mature adipose tissue (Hematoxylin and eosin [H&E] stain; 100× magnification). (b) Mature adipose tissue (black arrow) surrounded by fibrous tissue (red arrow) (H&E stain; 200× magnification). (c) Interface between adipose tissue (black arrow) and bone tissue (red arrow) (H&E stain; 100× magnification).

Postoperative management

The patient was hospitalized for 2 days, during which she remained in good condition without any hearing disturbances or a sensation of fullness in the right ear. Follow-up over 6 months showed no tumor recurrence or any hearing-related complaints in the right ear.

DISCUSSION

Parosteal lipomas are rare benign soft-tissue tumors that develop in close association with the periosteum, typically involving the long bones of the extremities. They account for only 0.3% of all lipomas, with skull involvement being exceedingly rare.[ 2 ] To date, only a handful of cases have been reported in the literature [ Table 1 ], including our case of a skull parosteal lipoma, in the supra-auricular region.[ 1 , 7 ]

Table 1:

Reported cases of parosteal lipoma of the skull.

Parosteal lipomas often present as slow-growing, painless masses. Symptoms occur primarily due to pressure on adjacent structures. In our case, the patient had a longstanding, painless supra-auricular mass without neurological or auditory symptoms. This aligns with previously reported cases where patients primarily sought medical attention for cosmetic concerns.[ 5 ] Rarely, nerve compression can cause sensory or motor deficits.[ 7 ] In addition, some cases have reported tenderness especially with underlying bone changes.[ 1 ] Trauma related or post-inflammatory lesions should be considered to be a differential diagnosis, especially if there is history of blunt trauma, although the possible relationship with parosteal lipoma remains speculative.[ 2 ]

Radiographic imaging plays a critical role in diagnosing parosteal lipomas. Computed tomography (CT) is often the initial imaging modality, revealing a well-defined hypodense mass with possible underlying bony changes such as cortical thickening and hyperostosis.[ 3 ] In our case, CT demonstrated a hypodense mass over the right supra-auricular region with mastoid and squamous bone thickening and mastoid air cell enlargement. These findings are consistent with prior cases showing cortical irregularities or osseous projections.[ 1 , 2 ] Magnetic resonance imaging (MRI) is considered the gold standard, offering superior soft-tissue contrast to differentiate lipomas from other fat-containing tumors.[ 4 , 7 ] MRI typically shows a well-circumscribed, high-intensity lesion on T1- and T2-weighted images, with signal suppression on fat-saturation sequences.[ 3 ] A fibrous capsule and lack of significant contrast enhancement support the diagnosis of a benign lesion. However, MRI was not available in our case, limiting assessment of soft tissue and adjacent structure involvement.

Histopathological examination is necessary for definitive diagnosis. Parosteal lipomas consist predominantly of mature adipose tissue with a fibrous capsule and may show varying degrees of osseous or cartilaginous metaplasia.[ 5 ] Reactive hyperostosis in the adjacent bone, as in our case, may suggest a chronic interaction between the tumor and the underlying bone, possibly due to mechanical stimulation or localized metabolic changes.[ 3 , 7 ] The histopathological assessment plays a critical role in differentiating these from liposarcoma, with benign findings such as the absence of lipoblasts, cellular atypia, or increased mitotic activity.[ 2 ]

Histopathological examination of parosteal lipomas typically reveals mature adipose tissue with a fibrous capsule, and occasionally bone and hematopoietic components, as observed in our case. The presence of lymphocytic aggregates suggests a localized inflammatory response, which has been documented in previous cases.[ 6 ] Some reports described varying degrees of osseous metaplasia, with some cases exhibiting trabeculae of lamellar bone interspersed within the adipose tissue.[ 6 ] While reactive hyperostosis has been noted in skull-based cases, hematopoietic components are rare and may indicate chronic tumor-bone interactions.[ 2 ] These findings reinforce the importance of thorough pathological assessment to distinguish parosteal lipomas from other fat-containing tumors such as osteolipomas and liposarcomas.

The differential diagnosis for parosteal lipomas includes osteolipoma, ossifying lipoma, calcifying synovial sarcoma, and tumoral calcinosis.[ 1 ] Osteolipomas containing mature adipose tissue with osseous metaplasia and may mimic parosteal lipomas radiologically.[ 6 ] Calcifying synovial sarcoma shows irregular calcifications and more aggressive features, usually with pain and rapid growth. Tumoral calcinosis presents as a firm radiopaque mass with calcium deposits.[ 7 ] Dermoid and epidermoid cysts should also be considered, particularly in cases where MRI is unavailable as they may present with fat-containing components and calcifications but typically lack bone involvement.[ 3 ]

The standard treatment for parosteal lipomas is complete surgical excision, particularly when symptomatic or for cosmetic reasons.[ 5 ] Surgical removal is often curative, and recurrence rates are low.[ 7 ] The key surgical challenge lies in cases where the tumor is firmly attached to the underlying bone, requiring careful dissection to avoid residual tissue or excessive bone removal.[ 1 ] Bone smoothing or minor reconstructive procedures may be necessary for contour restoration.[ 2 ] There have been no documented cases of malignant transformation in parosteal lipomas, supporting a favorable prognosis.[ 7 ]

CONCLUSION

This case illustrates a rare occurrence of parosteal lipoma in the supra-auricular region of the skull, which is extremely uncommon in clinical practice. CT imaging played a key role in detecting both soft tissue and underlying osseous changes, especially in settings where MRI is unavailable. Histopathological analysis confirmed the diagnosis, revealing mature adipose tissue, fibrous components, lymphocytic aggregates, and hematopoietic cells – suggesting chronic interaction with the adjacent bone. Complete surgical excision achieved excellent cosmetic and clinical outcomes without recurrence. These findings expand the current understanding of the radiological and pathological spectrum of skull-based parosteal lipomas and reinforce the value of tailored surgical approaches in their management.

Ethical approval:

The Institutional Review Board approval is not required.

Declaration of patient consent:

The authors certify that they have obtained all appropriate patient consent.

Financial support and sponsorship:

Nil.

Conflicts of interest:

There are no conflicts of interest.

Use of artificial intelligence (AI)-assisted technology for manuscript preparation:

The authors confirm that there was no use of artificial intelligence (AI)-assisted technology for assisting in the writing or editing of the manuscript and no images were manipulated using AI.

Disclaimer

The views and opinions expressed in this article are those of the authors and do not necessarily reflect the official policy or position of the Journal or its management. The information contained in this article should not be considered to be medical advice; patients should consult their own physicians for advice as to their specific medical needs.

References

1. Cil Y, Kocer NE, Yapici AK. Frontal parosteal lipoma. J Craniofac Surg. 2008. 19: 1122-5

2. Morishita T, Toriyama K, Yagi S, Takanari K, Fujii M, Nishida Y. Frontal parosteal lipoma with thickening of diploic space. JPRAS Open. 2015. 5: 19-23

3. Murakami M, Hirai M, Sakakibara T, Yamaki T, Kusuzaki K. Skull parosteal lipoma with reactive hyperostosis: A case report. Neurol Med Chir (Tokyo). 2014. 54: 314-6

4. Murphey MD, Johnson DL, Bhatia PS, Neff JR, Rosenthal HG, Walker CW. Parosteal lipoma: MR imaging characteristics. AJR Am J Roentgenol. 1994. 162: 105-10

5. Nur M, Bin D, Aziz A, Sim R. Skull parosteal lipoma-Hard lump on the head of bony origin?. Asian J Case Rep Surg. 2018. 1: 32-6

6. Potter J, Richards C, Collin J. Parosteal lipoma of the mandible: A case report and review of the literature. J Oral Maxillofac Pathol. 2022. 26: 129-30

7. Varshney S, Arora S, Swarnkar M, Gaur G, Prasad A, Taneja A. Unusual presentation of paraosteal lipoma in skull. Indian J Otolaryngol Head Neck Surg. 2023. 75: 1241-4