Clinical History
A 54-year-old man presented with a painless, slowly progressive, right preauricular swelling and facial paralysis for the past year. There was no hearing loss or focal signs of inflammation.
Imaging Findings
Contrast-enhanced CT showed a homogeneous and well-defined hypoattenuating right intraparotid nodule, suggesting a benign or low-grade tumour. The lesion extended to and enlarged the stylomastoid foramen, concerning for involvement of the facial nerve (Fig. 1).
MRI confirmed a mildly enhancing well-circumscribed right intraparotid nodule, with a T2 hyperintense rim, forming a target sign. Extension into the stylomastoid foramen was confirmed (Fig. 2).
On fine needle aspiration (FNA) a definitive diagnosis could not be established, but no malignant cells were present. The lesion was surgically resected and pathology revealed an intraparotid facial nerve neurofibroma.
Discussion
Intraparotid neurogenic tumours, which include schwannomas and neurofibromas, are rare. Among these, schwannomas are more frequent, while neurofibromas are extremely uncommon, so they are usually not included in the differential diagnosis of a parotid mass [1-3].
Patients typically complain of long-standing facial swelling, with facial pain and paralysis variably present, clinically similar to other parotid tumours. Paralysis may result from neural compression within the stylomastoid canal or direct axonal involvement in the case of neurofibromas. The combination of facial paralysis with a parotid mass is usually a worrying sign, concerning for a malignant tumour. However, in our case, the protracted clinical course and enlargement of the stylomastoid foramen without bone erosion on imaging studies pointed to a benign aetiology [2,3].
Intraparotid facial nerve neurofibromas are usually associated with neurofibromatosis type 1 (NF1), a disease that results from a mutation of the NF1 gene found on chromosome 17. Solitary facial nerve neurofibromas are extremely rare and most cases affect intracranial and intratemporal portions [4,5]. In the present case, although there was no personal or family history of neurofibromatosis, no genetic study was done to exclude this entity.
Imaging findings of neurofibroma are nonspecific, so it can be challenging to distinguish from other parotid tumours, such as pleomorphic adenoma or even schwannoma. On CT neurofibromas are typically well-defined, hypodense lesions. On MRI they more frequently demonstrate isointense signal to muscle on T1 and on T2 they can have a homogeneous hyperintense signal or a peripheral hyperintense signal that surrounds a central hypointense region, resulting in the target sign. This sign, present in our case, is highly suggestive of, but not specific for, neurofibroma. Contrast enhancement is usually heterogeneous, sometimes with cystic or necrotic areas, as opposed to the homogeneous enhancing seen in the current case [1].
Preoperative diagnosis of facial nerve neurofibroma is difficult, even with FNA. Although these tumours are primarily benign, surgical excision is usually the treatment of choice because there is potential for malignant transformation [2,3].
Our patient has been in follow-up for two years and there is no clinical evidence of recurrence.
Differential Diagnosis List
Intraparotid facial nerve neurofibroma
Pleomorphic adenoma
Warthin tumour
Mucoepidermoid carcinoma
Intraparotid schwannoma
Adenoid cystic carcinoma
Final Diagnosis
Intraparotid facial nerve neurofibroma
