An unusual presentation of intraosseous malignant peripheral nerve sheath tumour of mandible

Abstract

Malignant peripheral nerve sheath tumour (MPNST) is a rare mesenchymal tumour which usually presents high grade malignancy. We report an atypical case of intraosseous malignant peripheral nerve sheath tumour on mandible in a 36-year-old male. Patient presented with an incidentally discovered radiolucency on the left anterior mandible and did not complain of any symptoms. Panoramic radiograph and computed tomography showed enlargement of inferior alveolar nerve canal, thinning of the cortex and calcified foci within an expansile mass. Magnetic resonance images revealed heterogeneous hyperintense signal intensity with a well-defined margin on T₂ weighted images, heterogeneous enhancement on contrast-enhanced T₁ weighted images and intermediate signal intensity and inferior alveolar nerve canal enlargement on T₁ weighted images. In spite of benign radiologic characteristics as mentioned above, histopathologic examination of biopsied specimen otherwise revealed a low-grade malignancy. Wide excision of mandible was performed and there has been no loco-regional recurrence or distant metastasis over 1 year following surgery. This case indicates that, even when imaging modalities clearly demonstrate benign nature of intraosseous neurogenic tumour, care must be taken to establish proper treatment plan for assumed malignancy with a definitive histopathological analysis.

Introduction

Malignant peripheral nerve sheath tumour (MPNST) has referred to any malignant mesenchymal tumour that arise from the peripheral nerve sheaths. MPNST exhibits various degrees of differentiation of nerve sheath elements including schwann cells, fibroblasts and perineural cells.¹ Although 50% of MPNST have been found in patients with neurofibromatosis Type 1 (NF1), they can also occur sporadically or be induced by radiation.² In the head and neck region, MPNST composes 2–6% of head and neck sarcoma, the 5 year overall survival rate having been reported as 20–51%.^2–4

Intraosseous variants of the head and neck MPNST in jaw bone are exceedingly rare in the literature. In the previous reports, intraosseous MPNST (IOMPNST) have demonstrated an irregular, osteolytic radiolucency and root resorption of involved tooth in radiographs.^3,5–7 We report an unusual presentation of IOMPNST within the mandible showing a unilocular, well-defined radiolucency and migration of the inferior alveolar nerve canal (IANC), which is likely to be misdiagnosed as a benign entity.

Case report

A 36-year-old male with non-specific medical history was presented with complaint of pain on tooth #19. The diagnosis of tooth #19 was previously treated tooth with symptomatic apical periodontitis, but a radiolucency on anterior mandible region was discovered in panoramic view. The patient was referred to department of oral and maxillofacial surgery for further evaluation of the incidental finding. There were no facial swelling or pain, only a slight bony bulging being identified on the buccal vestibule region. Involved teeth demonstrated normal response to electronic pulp test without mobility and percussion pain.

Panoramic radiograph revealed unilocular radiolucent lesion with well-defined corticated margin from lateral incisor to second premolar of the left mandible. The lesion was suspected to have widened the mental foramen, yet the patient did not complain of any neurosensory disturbance (Figure 1). Based on the radiographic signs, simple bone cyst or odontogenic keratocyst was considered in a preliminary diagnosis. From practical point of view, cone beam CT with field of view 90 × 50 (mm²) would be beneficial option for the intraosseous lesion. Nevertheless, on the basis of a tertiary dental hospital setting, CT scan was also available and performed for the internal feature of the lesion. CT scan (HiSpeed Advantage; GE Medical System, Milwaukee, WI) was performed with the following parameters: 120 kVp, 200 mAs, 3 mm slice thickness and 0.33 mm pixel spacing. Axial view in soft tissue window revealed a low attenuated, well-demarcated expansile mass (26 × 20 × 13 mm). A foci of calcification was noted at the center of the lesion (Figure 2a,b). In bone window, thinning of the buccolingual cortex on the anterior mandibular body (Figure 2c) was shown, but not cortical perforation. These findings suggested a central origin of tumour, excluding secondary bony involvement by extraosseous origin.⁸ The enlargement (or widening) of IANC was also shown (Figure 2d). Radiologic findings suggested a benign lesion derived from central origin, suggestive of odontogenic tumour (solid/multicystic type of ameloblastoma; calcifying epithelial odontogenic tumour), neurogenic tumour (intraosseous neurofibroma; intraosseous schwannoma) or vascular malformation.

Figure 1.

Pre-operative panoramic radiograph shows a well-defined unilocular radiolucency in left anterior mandible (white arrows).

Figure 2.

Computed tomographic images. Axial (a) and coronal view (b) in soft tissue window shows low attenuated intraosseous lesion containing a calcified foci (black arrows) on left anterior mandible. In bone window, axial view (c) and coronal view (d) demonstrates ill-defined anterior loop and enlargement (white dotted arrows) of inferior alveolar nerve canal.

The patient underwent an incisional biopsy under local anesthesia. The histopathological examination reported a malignant mesenchymal tumour from neural origin. Microscopic examination revealed bundles of irregular spindle cells with low levels of mitosis as well as moderate cellular density, atypias, and hyperchromatic nuclei varying in shape throughout the specimen. Immunohistochemical analysis with S-100 protein confirmed the diagnosis of neural origin.

MR images (3T system; Intera Achieva or Achieva TX; Philips Medical Systems, Best, the Netherlands) revealed the detailed contents and extent of the mass. T₂ weighted images [fast spin echo; repetition time (TR), 6528 ms; echo time (TE), 65 ms] revealed heterogeneous hyperintense signal intensity, especially on the rim area (Figure 3a). Gadolinium-based contrast enhanced T₁ weighted images (spin echo; TR, 572 ms; TE, 10 ms) showed similar heterogeneous and rim enhancement (Figure 3b). T₁ weighted images showed intermediate signal intensity and IANC enlargement compared to the contralateral side (Figure 3c).

Figure 3.

Magnetic resonance images. (a) Axial T₂ weighted images revealed an inhomogeneous tumour with high-signal intensity in the left anterior mandible. The lesion demonstrated a relatively well-defined smooth margin. (b) Contrast-enhanced T₁ weighted images demonstrated a lesion with inhomogeneous signal intensity and rim enhancing. (c) Axial T₁ weighted image showed lesion with inhomogeneous intermediate signal intensity. Enlargement of inferior alveolar nerve canal was observed (white arrows) compared to the contralateral side (white empty arrows).

After identifying the absence of radiologic evidence of regional and distant metastasis using PET-CT system (Discovery LS; GE Healthcare, Milwaukee, WI), a treatment plan including surgery alone was established. The patient underwent radical excision and immediate reconstruction, involving segmental mandibulectomy and reconstruction with fibular free tissue transfer. The surgical margin was free from tumour and the final histopathological examination confirmed the primary tumour as a low-grade MPNST (Figure 4a,b). Post-operative evolution was favourable and no adjuvant chemoradiation therapy was adopted. On close surveillance, MRI have presented no evidence of recurrence after postoperative 12 months (Supplementary Material 1).

Figure 4.

Histopathological findings. (a) Irregular spindle cells with moderate cellular density, atypias, hyperchromatic nuclei and low level mitosis. (b) Intense immunohistochemical staining for S-100 and positive reaction was found in most cells. (original magnification x 100)

Discussion

Differential diagnosis of a well circumscribed, corticated and radiolucent lesion at anterior mandible comprises several categories of pathology, such as chronic inflammatory processes, odontogenic tumours and other neoplastic conditions. With normal response to pulp vitality test of adjacent teeth, chronic inflammatory processes from odontogenic origin were excluded. Then, odontogenic tumours including ameloblastoma and calcifying epithelial odontogenic tumour (CEOT) would be probable based on the location. But, the lesion of present case demonstrated a single calcification rather than multiple snow flake calcification that is typical of CEOT. Furthermore, current lesion did not show any aggressive behaviour such as root resorption or cortical perforation, which are relatively common findings of ameloblastoma and CEOT.

The lesion did not appear to be associated with odontogenic tumours, accordingly, non-odontogenic pathologic conditions were taken into the differential diagnosis. The continuity with IANC is favourable for vascular malformation or neurogenic tumour. Vascular malformation has been characterized by heterogeneously hyperintense signal in T₂ weighted MR images, isointense and/or flow voids in T₁-MR images and irregular bone destruction in CT images.^9,10 However, the lesion of present case did not show irregularity on surrounding cortical bone in CT images, but rather demonstrated the enlargement of IANC in T₁ weighted MR images. The above findings suggested the possibility of benign neurogenic tumour.^8,11,12

In the present case, IOMPNST demonstrated somewhat unusual findings in comparison to previous reports. IOMPNST usually exhibits aggressive features which have been described as an ill-defined radiolucency with rapid growth, root resorption and cortical bone destruction.^5,7However, panoramic radiograph revealed a well-circumscribed, unilocular lesion and patient did not present any symptoms such as pain, swelling or sensory discomfort. CT and MR images also did not provide any malignant characteristics of this lesion. Only histopathological examination revealed, although there has been some disagreement on histopathological criteria of low grade MPNST,^1,13 nuclear atypia, moderate cellularity as well as the mitotic activity that is known as a key feature distinguishing MPNST from benign counterpart. In addition, a radiopaque spot within the lesion was an uncommon finding in intraosseous neurogenic tumour.³ Perkins et al found that calcified focus had been detected in only 5% (4/83) of neurogenic tumour cases.¹¹

IOMPNSTs of the mandible are likely to affect younger (≤40 years) females in previous reports.^5,7,14 Our patient was a male of late thirties diagnosed as a sporadic variant of IOMPNST in the absence of NF1 related features. Approximately, 38–60% of entire MPNSTs are observed in patients with NF1, while the other cases arise as a sporadic lesion.^1,7 In the head and neck region, previous studies have demonstrated that MPNSTs develop sporadically rather than on the basis of NF1.^5,14 According to Ma et al., 69.8% (30/43) of MPNST had been diagnosed as a sporadic MPNST.¹⁴ The age and sex predilection of sporadic MPNST is distinct from the NF1 associated MPNST. NF1-associated MPNST patients are generally younger, with a mean age of 28 years, whereas the median age at diagnosis among sporadic MPNST patients is 41 years.¹ Regarding the sex predilection, Bowers et al mentioned a male predilection in trigeminal nerve of MPNST.¹⁵ On the other hand, incidence of sporadic MPNST has been reported to be almost equal between the male and female.¹

IOMPNSTs of mandible to date have been reported as exceedingly rare entities that exhibit high-grade malignancy and aggressive characteristics. We described a low-grade IOMPNST mimicking a benign lesion in the mandible which could only be diagnosed as an unspecified neurogenic tumour based on the imaging characteristics. Both clinically and radiographically benign manifestation of this case could have led to a conservative treatment inappropriate for a malignant tumour. When an intraosseous neurogenic tumour is suspected from the radiologic findings, apparent benign pathology notwithstanding, an incisional biopsy is mandatory to discard malignancy before the treatment planning.