Intraosseous Schwannoma of the Frontal Bone: A Case Report and Review of the Literature

Introduction

Intraosseous schwannomas are rare, owing to the low density of sensory nerve fibres within bone.¹ Most common in the mandible or sacrum, they comprise ~0.2% of primary bone tumours.² Rarer still is their presence within the skull, with nineteen reported cases: three of which occurred in the frontal bone.^1,3,4 Here, we present the fourth case – a 46-year-old male with an intraosseous schwannoma of the frontal bone – and review the limited body of literature on this rare entity.

Case Presentation

A 46-year-old male presented to the Neurosurgery clinic with a 3-month-history of severe left-sided headache. He had an unremarkable medical history with a normal examination. Contrast-enhanced head computed tomography (CT) revealed a destructive, lucent soft tissue lesion of the left frontal bone. Brain magnetic resonance imaging (MRI) with and without intravenous contrast highlighted a heterogeneously enhancing, 24x20x30mm (AP x trans x SI) osseous lesion with destruction of both inner and outer table and thickening of the underlying dura mater concerning for invasion. Nuclear medicine scanning (SPECT/SPECT-CT) of the whole body confirmed a solitary, osteoblastically active lesion (Figure 1). Metastatic disease and myeloma were considered likely differential diagnoses. Laboratory tests – including those deranged in myeloma (creatinine, calcium, alkaline phosphatase, haemoglobin) – were unremarkable.

Figure 1 Pre-operative imaging. (A and B) Axial and coronal computed tomography showing an intraosseous lytic lesion in the left frontal bone with destruction of the outer cortex. (C) Nuclear medicine scan (SPECT/CT) demonstrating mild radiotracer uptake indicative of osteoblastic activity, concerning for malignancy. (D–F) T1, T2, and T1 post-contrast magnetic resonance imaging demonstrating T1 iso-hypointensity, T2 iso-hyperintensity and heterogenous contrast enhancement. Dural thickening is observed deep to the lesion, concerning for invasion.

The patient proceeded to undergo a craniectomy and resection of the lesion with duroplasty and titanium mesh cranioplasty (Figure 2). Intra-operatively, tan coloured tumour was seen invading both skull and adjacent dura mater. Mild inflammatory reaction of the arachnoid mater was noted, but no brain invasion was identified.

Figure 2 Post-operative imaging. (A and B) Axial and coronal computerised tomography showing the craniectomy site and surgical cranioplasty and expected post-surgical small volume pneumocephalus.

Histopathological analysis (Figure 3) of the specimen revealed moderately cellular, spindle cell tumour in patternless sheets and intersecting fascicles, with occasional Verocay bodies. Immunohistochemistry revealed cells strongly and diffusely positive for S100 protein and SOX10. Markers suggestive of melanoma (Melan-A, HMB45, BRAF mutation, GNAQ mutation) were negative. These findings led to the diagnosis of an intraosseous schwannoma.

Figure 3 Histopathology of the tumour. (A) Low powered haematoxylin and eosin stain highlighting bone involvement. (B) Highly cellular (Antoni A) and myxoid hypocellular (Antoni B) regions typical for schwannoma. (C and D) High powered H&E stain showing spindle cells. (E) Strong, diffuse positivity for S100 and (F) SOX10 suggestive of schwannoma.

The postoperative course was uneventful, and the patient discharged 3-days post-operatively. At 2-month -follow-up, he reported complete resolution of his headache, with no neurological deficit. Magnetic resonance imaging of the whole spine had been arranged to assess for spinal lesions suggestive of neurofibromatosis type II (NF2).

Discussion and Literature Review

With an estimated incidence of 2 in 100,000, schwannomas are a relatively common, benign nerve sheath tumour.⁵ Uncommon in an intraosseous location – and rarer still in the cranial vault – this patient represents the fourth known case of an intraosseous frontal bone schwannoma.^1,3,4 With a reported age range of 2 to 65 years, intraosseous schwannoma affects both males and females equally.² Three mechanisms for their development within bone have been postulated: tumour originating in the centre of bone; erosion of bone by extraosseous tumour; or tumour arising within the nutrient canal of bone, forming a dumbbell-shaped configuration, and enlarging this canal.⁶

Radiological features of intraosseous schwannoma are often non-specific: CT commonly shows expansile and lytic bone lesions without cortical erosion, whilst MRI reveals a hypo-iso intensity on T1-weighted images, and iso-hyper intensity on T2-weighted images with moderate contrast enhancement.² Histologically, intraosseous schwannoma appear as spindle cells arranged in palisading patterns with Verocay bodies (Antoni A pattern), and a small number of tumour cells dispersed in oedematous stroma (Antoni B pattern).^4,7,8 Immunohistochemistry in the form of S100 uptake – expressed in neural crest tumours – helps differentiate schwannoma from sarcoma, which is histologically similar.^2,8

Table 1 provides a summary of the limited literature on intraosseous schwannoma of the skull. Except for petrous apex lesions (in which 78% of cases presented with hearing/auditory disturbance)^7,9–14 and one case of a clival lesion causing cranial nerve IX/X/XI disturbance,¹⁵ symptoms appear nonspecific for lesions elsewhere with the most reported being a painless mass (55%),^3,4,16–18 followed by headache (18%),¹ ocular symptoms (9%),¹⁹ paraesthesia (9%),¹ or asymptomatic (9%).²⁰

Table 1 Published Literature on Intraosseous Schwannoma of the Skull

Optimal treatment of intraosseous schwannoma has not been established, though surgical resection appears the mainstay of therapy, being utilised in 89% of existing cases,^{1,3,4,7,9–11,13,15–20} and all cases of frontal bone lesions. When combined with complete tumour excision in 70% of these cases, there was no documented recurrence within the mean follow-up of 45 months. Incomplete excision was associated with tumour recurrence in one of five cases (mean follow-up of 33 months).⁷ Stereotactic radiosurgery (SRS) was used in one case – with stable tumour size 12 months post – but longer-term data remains unavailable.¹² Rozman et al⁷ used adjuvant SRS for recurrent schwannoma following incomplete resection with favourable radiological and clinical outcomes at 6 months. One recent paper describes an endoscopic endonasal approach to a clival schwannoma.¹⁵

The patient in this case was asked to organise whole spine magnetic resonance imaging to assess for spinal lesions suggestive of NF2. This is an important learning point, given approximately 10% of schwannomas are associated with multisystem disorders such as neurofibromatosis, schwannomatosis, multiple meningiomas and Carney complex.^2,21 One patient in the literature to date had schwannoma elsewhere,⁷ though none had been formally diagnosed with such disorders.

Intraosseous schwannoma is not a commonly considered differential diagnosis, with just two cases considering this possibility pre-operatively.^14,15 Interestingly, these cases are the most recently published, and may reflect increasing awareness of this pathology as more cases emerge. Given its extreme rarity; however, its absence from most differential diagnoses is forgivable.

Conclusion

Intraosseous schwannoma of the skull remains a rare entity. This report describes the fourth known case of frontal bone intraosseous schwannoma. The tumour was totally resected with excellent clinical outcome. Evidence suggests symptoms are often non-specific for non-petrous lesions, and gross total resection an effective therapy. Intraosseous schwannoma should remain a differential diagnosis for patients with undifferentiated skull lesions.