Intramasseteric Schwannoma Mimicking a Parotid Tumor: Case Report and Review of Literature

Background Schwannoma is a benign neoplasm originating from Schwan cells of peripheral nerves as well as the cranial and spinal roots. It often occurred in head and neck region involving cranial nerves VII, VIII, and XII. However, schwannoma located in intramasseteric region is extremely uncommon. There have been only ten reported cases documented in English literatures to date. Here, we report a rare intramasseteric schwannoma case, who presented a painless mass in the left cheek with 2 years evolution and was treated with surgical excision through trans-parotid approach. An uneventful recovery without salivary stula or facial paralysis was recorded after the surgery. Due to its rarity and clinical signicance, we also review intramasseteric schwannoma regarding its clinical characteristics and surgical managements. In a word, schwannomas was rarely reported and might be misdiagnosed as parotid tumors. To the best of our knowledge, there have only ten cases with intramasseteric schwannoma been reported in English literatures. Here, we update an uncommon case of schwannoma located in the intramassetric muscle and describe its clinical ndings. In addition, we also performe an extensive retrospective literature review of schwannoma presenting in the region of intramassetric muscle to evaluate the clinical ndings and treatments.

Here, we report a rare intramasseteric schwannoma case, who presented a painless mass in the left cheek with 2 years evolution and was treated with surgical excision through trans-parotid approach. An uneventful recovery without salivary stula or facial paralysis was recorded after the surgery. Due to its rarity and clinical signi cance, we also review intramasseteric schwannoma regarding its clinical characteristics and surgical managements.

Conclusion
In a word, intramasseteric schwannoma is a benign lesion with favorable prognosis. MRI (magnetic resonance imaging) scanning is recommended for evaluation and differential diagnoses for this condition. Surgical excision of the tumor without parotidectomy is considered to be effective.

Background
Schwannoma is a slow growing, encapsulated tumor representing about 5% of benign soft tissue tumor (1). It was rstly described by Stout as a benign neurogenic tumor with sporadic malignant degeneration (2). Schwannoma could be originated either from cranial, peripheral or from autonomic nerve sheath cells. The signs and symptoms of this condition in different patients varied because their origin nerve and involved sites are diverse (1). The most common involved area for schwannoma is the head and neck region, which accounts for 25-45% of the whole body schwannoma (3). Schwannoma in head and neck region could arise either medially from glossopharyngeal, vagus, accessory and hypoglossal nerve, the sympathetic chain, or laterally from cutaneous or muscular branches of the cervical/brachial plexus and so on (4). It has been intensively reported with respect to the clinical, pathological and immunohistochemical characteristics of schwannoma in head and neck region in literature (3,(5)(6)(7)(8)(9). However, intramassetric schwannomas was rarely reported and might be misdiagnosed as parotid tumors. To the best of our knowledge, there have only ten cases with intramasseteric schwannoma been reported in English literatures. Here, we update an uncommon case of schwannoma located in the intramassetric muscle and describe its clinical ndings. In addition, we also performe an extensive retrospective literature review of schwannoma presenting in the region of intramassetric muscle to evaluate the clinical ndings and treatments. A 15-year-old boy presenting with a painless slow-enlarging mass in his left cheek was referred to the Department of Oral and Maxillofacial Surgery, Hospital of Stomatology, Guanghua School of Stomatology, Sun Yat-sen University, China. There was no numb feeling during the 2 years evolution. No signi cant family history was noted. An asymmetry pro le in the maxillofacial region was noted while performing facial examination. The mass was measured approximately 4.0 × 3.5 cm in diameter (Fig. 1A) on the left cheek. It was noted that the lump was palpated with poor mobility, hard elastic texture and smooth contour. The overlying skin was intact and discolored (Fig. 1A). No facial paralysis or cervical lymphadenopathy was detected. Intraorally, the buccal and oropharyngeal mucosa was normal. Computed tomography (CT) scanning showed that a well-circumscribed mass was located in parotidomasseteric region, which was hypodense to surrounding soft tissue. No involvement of bony destruction was found in the adjacent ramus of mandible (Fig. 1B). Based on these examinations, the initial impression was a tumor of parotid gland. Under general anesthesia, surgical removal of the tumor was carried out with a standard preauricular/cervical incision. However, we found that the tumor was located at the inside of musculi masseter, super cial to the ramus of mandible (Fig. 1C, D). The tumor was then completely removed and subjected to pathological examination. The parotid gland with facial nerve was preserved, repositioned and wound was closed layer by layer (Fig. 1E, F). Macroscopically, the resected specimen was a well circumscribed mass with elastic texture ( Fig. 2A). On sectioning, the mass demonstrated a white soft tissue with cystic degeneration and haemorrhage (Figs. 2B). The preliminary pathology report from frozensection specimen was schwannoma, which was con rmed by examination of the tumor in para n section. As shown in Fig. 2C, the tumor was composed of well-organized Antoni A tissue with adjacent myxoid and less organized Antoni B tissue. The Schwann cells of the Antoni A tissue form a palisaded arrangement around acellular, eosinophilic zones known as Verocay bodies (Figs. 2D). The postoperative recover was uneventful. During the follow up period, there was no salivary stula or facial paralysis occurred.

Discussion And Conclusions
A review of the English-language literature reveals only 10 reported cases of schwannomas located at the intramasseteric region (10)(11)(12)(13)(14)(15)(16)(17)(18)(19). As shown in Table 1, a female-to-male ratio of 1:1 was observed and there is no signi cant gender predilection. The patient age was ranged from 12 to 61 years. The average size of tumor was 1.45 cm, with a range of 2 to 4 cm. All lesions were described as a painless and slow-growing mass. However, the mass showed various characteristics on palpation, including soft or hard elastic consistency, well or poor mobility. Notably, all ne-needle aspiration (FNA) biopsies were Inconclusive but the nal diagnosis can only achieved based on pathological examination. All of the ten reported tumors were surgically removed, among which seven cases were recorded with follow-up information. There were no recurrences observed within the follow-up period. Some of the lesions (6/10) were comprised with both Antoni A and B components. Two patients were diagnosed as Antoni A type schwannoma and only one was classi ed as Antoni B type schwannoma. The histological pattern of the case reported by Schreiber et al (13) was not available. Schwannoma, also known as neurilemomas or neurilemmomas, was originated from schwann cells. It was reported that schwannoma account for as rare as 5% of all benign soft tissue tumors, and the malignant schwannoma represent only 5% of all soft tissue sarcomas (1). Head and neck region is the most common site affected by the extracranial schwannoma (20). However, intramasseteric schwannomas are extremely uncommon. Currently, there were only 10 cases with intramasseteric schwannoma reported in English literatures, indicating that intramasseteric region is an extremely rare site for schwannoma (Table 1) (10)(11)(12)(13)(14). The epidemiological characteristics of schwannoma in head and neck region were in agreement with those in other regions of the body, which was prone to affect patients in their second and third decade and an equal gender distribution was observed. [21][22][23][24]. Similar results were also observed in these case series of intra-masseteric schwannoma.
Clinically, intramasseteric schwannomas might be misdiagnosed as the tumor originating from parotid therefore it should be differentiated from parotid tumors. The differential diagnosis of these two kinds of tumors is even more tricky when the intra-masseteric schwannoma become soft due to their mucinous degeneration, bleeding and cystic changes [20]. It's still a great challenge to diagnose this palpable mass preoperatively for a junior head and neck surgeon (1). Medical history, physical examination, ne needle aspiration (FNA), ultrasound scanning, computed tomography (CT), magnetic resonance imaging (MRI) and positron emission tomography (PET) are used as diagnostic modalities. However de nitive diagnosis and identi cation of the affected nerve are often di cult before surgery. (21,22). The most common presentation of schwannoma is an asymptomatic slow-growing mass, which is similar with benign tumor of parotid gland. Generally, CT, MRI and FNA might provide helpful information in diagnosis of parotid tumors. As shown in Table 1, all cases received CT or MRI scanning before operation. Both of this two imaging examination can provide useful information for differential diagnosis and development of surgical plan. On CT scanning, schwannoma typically demonstrates a heterogeneous low density with only limited soft tissue de nition. MRI imaging, in which schwannoma demonstrates isointense T1 signal relative to skeletal muscle and increased heterogeneous T2 signals, is preferred for visualizing soft tissue neoplasms.. The ultrasound scanning is another optional for preoperative imaging since vessels and blood ow within the tumor could be detected [16]. However, These radiographic ndings provide good but often limited information in the diagnosis of schwannomas (23). As evident in Table 1, FNA biopsies were performed in 4 cases and all inconclusive, indicating that FNA has a low accuracy in the diagnosis of schwannoma. Therefore, the conclusive diagnosis is only achieved based on microscopic ndings of the harvested tissues, characterizing with a speci c cellular Antoni A areas and less organized hypocellular Antoni B areas [16][17][18][19][20].
It was suggested that both surgical excision and observational approach are applicable in management of schwannomas in head and neck region due to the noninvasive nature (24,25). Intracapsular enucleation and resection are both recommended to manage schwannoma in head and neck region. During the operation, resection of tumor was performed if the lesion was originated from the terminal branch of trigeminal nerve and lack of postoperative neurological de cit. However, intracapsular enucleation was highly recommended when the tumor involved the important nerve, such as facial nerve or trunk of trigeminal nerve. For intramasseteric schwannoma, it is di cult to identify the original nerve of the tumor since the tumor always obliterate a small nerve in masseter muscle. All reported cases were treated with tumor excision through different approach, including extraoral or intraoral approach, which was mainly depended on the location and involvement of tumor (15,17,19). An extraoral incision such as a preauricular incision or a submandibular incision is often unavoidable for the intramasseteric schwannoma. Intraoral approach accompanied with endoscope is feasible for tumor removal when the tumor size is limited (15). Similar with the surgical approach for parotid tumor, facelift incision was also used to excise the intramasseteric schwannoma (19). Super cial or total parotidectomy is not required for management of intramasseteric schwannomas. In the present case, the tumor was excised with transparotid approach. The parotid gland and facial nerve were preserved. The prognosis of schwannoma is usually outstanding as reported in literatures [21]. No recurrence was seen in any cases till date in the present study.
Taken together, intramasseteric schwannoma is a benign lesion with favorable prognosis. MRI scanning is recommended for evaluation and differential diagnoses of schwannoma preoperatively, but FNA biopsy is always inconclusive and not necessary. For treatment of these lesions, surgical excision of the tumor is considered to be effective, which neighboring structures and major nerve should be protected and preserved. Super cial or total parotidectomy is not required for managing intramasseteric schwannomas.