Solitary myofibroma of the mandible in an adult with magnetic resonance imaging and positron emission tomography findings: a case report

Myofibroma is a benign tumor composed of myoid spindle cells. The prevalence of myofibroma in the oral cavity is very low, with the mandible being the most common site. This report describes an adult case of myofibroma that arose on the mandible and includes magnetic resonance imaging (MRI) and positron emission tomography (PET) findings. On the MRI T1-weighted images, the tumor appeared with signal iso-intensity and was highly and heterogeneously enhanced with contrast material. On the T2-weighted images, it appeared with increased signal intensity. 18 F-fluorodeoxyglucose (FDG)-PET imaging showed abnormal strong accumulation of FDG in the left mandibular region. The tumor was removed by marginal resection of the left mandible under general anesthesia. Histopathological findings revealed that the tumor stroma contained abundant thin-walled vessels. The postoperative course was uneventful, and we found no evidence of recurrence at the postoperative 34-month follow-up.


Background
Myofibroma is a benign tumor composed of myoid spindle cells arranged around thin-walled blood vessels. Myofibromas are classified as either a solitary or multicentric type [1]. The solitary type is called myofibroma, whereas the multicentric type is known as myofibromatosis. The majority of myofibromas present in children less than 2 years of age [2]. Approximately half of myofibromas occur in the cutaneous and subcutaneous tissues of the head and neck region, followed by the trunk and the extremities. The other half occur in skeletal muscle or aponeuroses [1]. The prevalence of myofibroma in the oral cavity is very low, and the mandible is recognized as the most common site followed by the tongue and buccal mucosa [3]. Therefore, there are few reports of adult cases of oral myofibroma [4].
Some cases of oral myofibroma have been misdiagnosed as various benign and malignant tumors [5][6][7] because oral pathologists are infrequently exposed to soft tissue spindle cell neoplasms coupled with overlapping histologic patterns [5]. Although many reports dealing with oral myofibroma limited their focus to its pathological features [8,9], reports describing preoperative diagnostic imaging findings are few [10]. A preoperative image-based diagnosis using magnetic resonance imaging (MRI) and positron emission tomography (PET) would be helpful to determine whether the tumor is benign or malignant and to determine the treatment plan. In this paper, we report an adult case of myofibroma that arose on the mandible and emphasize the diagnostic value of preoperative MRI and PET findings.

Case presentation
A 52-year-old woman was referred to the Department of Oral and Maxillofacial Surgery, Tsurumi University Dental Hospital (Yokohama, Japan) on 30 October 2010 for diagnosis and treatment of a mass on the left mandible. The patient had noticed the mass for 1 month. The mass grew rapidly from the extraction socket of the lower mandibular second molar. Past medical history revealed hypertension and hyperlipidemia, which were adequately controlled with medication. The patient's family history was unremarkable. Physical examination revealed a rubbery sessile mass measuring 20 × 15 mm on the left mandible ( Figure 1). The overlying oral mucosa looked absent. There was neither neuroparalysis nor cervical lymphadenopathy.
MRI examination was performed with a 1.5 Tesla system (Intera 1.5 T; Philips Healthcare, Eindhoven, The Netherlands). On the T1-weighted axial images, the mass appeared isointense and was highly and heterogeneously enhanced with gadolinium in the left mandibular region. On the T2-weighted axial images, it appeared with increased signal intensity. On short-tau inversion recovery coronal imaging, the mass also showed high signal intensity ( Figure 2). The CT images (Brilliance CT 16; Philips Healthcare, Eindhoven, The Netherlands) showed no bony changes of the mandible ( Figure 3). The PET study was performed with a dedicated PET scanner (ECAT ACCEL; Siemens Healthcare AG, Munich, Germany). Data were acquired after administration of 181.5 MBq 18 F-fluorodeoxyglucose (FDG), using the post-injection transmission acquisition method. The image showed abnormal strong accumulation of FDG in the left mandibular region with a maximum standard uptake value of 8.5. There were no abnormal accumulations of FDG in the cervical lymph nodes nor was there any uptake in the other organs ( Figure 4A). The PET/CT fusion images showed that abnormal strong FDG accumulation corresponded to the mass on the left mandible ( Figure 4B).
In November 2010, the biopsy was performed under local anesthesia. The pathological diagnosis was myofibroblastic tumor, probably benign. However, the mass enlarged rapidly to 35 × 25 mm within 3 weeks after biopsy. According to the clinical findings, we could not completely deny the malignancy of the tumor. On 2 December 2010, it was removed by marginal resection of the left mandible under general anesthesia ( Figure 5).
The postoperative course was uneventful, and we found no evidence of recurrence at the postoperative 34month follow-up.

Discussion
There are few reports of adult patients with myofibroma of the mandible [4]. Clinical differences between adults and pediatric patients with myofibroma have suggested that pediatric patients are likely to demonstrate bone involvement [7,11] and that lesions in adults almost invariably presented as the solitary form, with exceptional cases of the multicentric form [6]. However, no specific differences in histologic features between adult and pediatric patients have been reported [6,11]. The clinical features of our solitary case without bone destruction are in accordance with those of adult oral myofibroma.
In the present case, the preoperative diagnosis based on the biopsy specimen findings was a myofibroblastic tumor, probably benign. However, the tumor grew rapidly in a few weeks. In the oral and maxillofacial region, approximately one-fourth of cases of myofibroma exhibit rapid enlargement, and some lesions show accelerated growth after incisional biopsy [11]. Spindle cell tumors are difficult to diagnose with a small piece of biopsy specimen alone [5,7]. Therefore, the final pathological diagnosis must rely on a specimen from the entire surgically removed lesion. Consequently, we performed marginal  resection of the mandible with consideration of the possibility that the tumor was malignant.
To the best of our knowledge, PET findings of myofibroma have been reported in only one case in the literature. In that case, strong accumulation of FDG in a myofibroma of the mandible was documented [10]. The FDG-PET findings in the present case are in agreement with those of the previously reported case [10]. FDG accumulation could be caused partially by blood supply. For glucose to be taken up and used by tumor cells, an adequate blood supply is needed; therefore, FDG-PET is assessed by not only the glucose metabolism, but also the blood supply [12]. In our case, the strong FDG accumulation was assumed to be caused by activated glucose metabolism due to the high proliferative activity of tumor cells and rich blood supply to the tumor. High proliferative activity of tumor cells was proven macroscopically from the rapid growth of the tumor, and microscopically from the mitoses sometimes seen in the HE-stained sections. The rich blood supply to the tumor was evident from many blood vessels confirmed on the immunohistochemically stained sections, which showed CD34 positivity in vascular endothelial cells.
In the present case, gadolinium-enhanced T1-weighted MRI showed strong enhancement in the tumor. Strong enhancement on T1-weighted images by contrast material was also documented in some reported cases of myofibroma [13][14][15]. In the head and neck region, on the other hand, some tumors other than myofibroma (for example, hemangiomas, solitary fibrous tumors, carotid body tumors, and malignant tumors) also demonstrate strong contrast enhancement on T1-weighted images. However,  hemangioma [16] and solitary fibrous tumor [17] could be discriminated through FDG-PET findings of weak accumulation of FDG within the lesions. Moreover, carotid body tumors can be clinically ruled out by the specific site of the tumor [18]. However, malignant tumors are difficult to differentiate from myofibroma on the basis of MRI and PET findings. To avoid unnecessarily aggressive surgery, the clinician should be aware that myofibroma could show false positive results in a PET study aiming for detection of malignant tumors. Although a preoperative specific diagnosis of myofibroma could not be established in our case, clinical findings in combination with MRI and PET could narrow down the differential diagnoses. Complete local excision is performed for the treatment of solitary myofibroma [6]. Recurrence is rare, and such cases are usually controlled with re-excision [1,3]. In the present case, the tumor was completely removed by marginal resection of the left mandible and we found no evidence of recurrence at the postoperative 34-month follow-up.

Conclusions
In this report, we described an adult case of myofibroma that arose on the mandible and discussed the diagnostic value of preoperative MRI and PET findings.

Consent
Written informed consent was obtained from the patient for publication of this case report and any accompanying images. A copy of the written consent is available for review by the Editor-in-Chief of this journal.