Ⅰ. INTRODUCTION

Myofibroma is a rare benign soft tissue tumor that consists of myofibroblasts. It can occur as a solitary or as multiple form¹⁾. There is a high incidence of the solitary myofibroma than the multiple form. Myofibroma occurs often in the head and neck soft tissue, skin, or bone. And it has male predilection in children at the first decade of life. However, it is extremely rare in adult jaws²⁾. To our knowledge, only 6 cases of solitary myofibroma in adult jaws have been reported^1-6). In this article, we added a case of solitary myofibroma in the mandible that occurred in a 41-year-old Korean woman.

Ⅱ. CASE REPORT

A 41-year old woman visited a complaint of swelling and a radiolucent lesion in the left posterior mandible. She became aware of the lesion 3 years ago when she had been implanted on the left maxillary 2^nd molar area at the local dental clinics. Although this lesion gradually had increased to the present size, but she did not feel any discomfort without the swelling. Her premedical and family history was unremarkable. The panoramic radiographic image demonstrated a well-defined radiolucency in the left mandibular 2^nd molar periapical area (Fig. 1 a). This tooth was asymptomatic and positive on electronic pulp test. Computed tomography demonstrated irregular cortical bone resorption with perforation of lingual cortical bone (Fig. 1b, c). The presumptive clinical diagnosis was a periapical cyst, odontogenic keratocyst or unicystic ameloblastoma.

Under general anesthesia, surgical excision of the lesion in the left mandible was performed with preservation of inferior alveolar nerve that was easily detached the lesion. No remarkable postoperative complications were reported. The patient was free of local recurrence at the 3-year follow-up.

Postoperative histopathology revealed as a circumscribed nodule composed of fascicles of uniformly distributed spindle shaped cells. The tumor was of moderate cellularity and contains branching endothelial-lined vascular channels (Fig. 2). Immunohistochemically, the cells were decorated antibodies to smooth muscle actin (SMA) and vimentin, but negative to desmin and S-100 (Fig. 3). Additionally, Ki-67 and c-kit positive cells were rarely identified (Fig. 4). The pathological diagnosis was an intraosseous solitary myofibroma.

Ⅲ. DISCUSSION

Myofibroma is an uncommon tumor occurring in soft tissues of head and neck, but rare in the jaws. It is commonly diagnosed in children at the first decade of life with male predominance^1,2). However, solitary myofibroma of the adult jaws is extremely rare with only six cases having been reported^1-6). An analysis of these cases including our case is presented in Table 1. The age of the patients with solitary myofibroma ranged from 20 to 52 years and affecting predominantly female than male patient. All cases were occurred in the mandible. One case was occured in the anterior mandible, and the others in the posterior mandible. Long-term follow up was not reported in these cases.

Myofibroma is generally treated by curettage and surgical excision, and recurrence rates for the lesions are very low. Incomplete removal is due to the difficult surgical access and anatomic hindrances, and this is related to recurrences of the lesions²⁾.

Radiologic feature of intraosseous myofibroma is usually unilocular radiolucent lesions. So unilocular lesion should be differentiated from periapical lesion, unicystic ameloblastoma, ameloblastic fibroma or odontogenic keratocyst⁷⁾.

Microscopically, myofibroma is characterized by biphasic growth of pericyte-like vascular cell or smooth muscle-like spindle cell proliferations^2,4,8). Histopathological differential diagnosis includes leiomyoma, neurofibroma, hamangiopericytoma, nodular fasciitis, and fibrosarcoma.

Immunohistochemistry should be done to confirm the diagnosis. In this case, the tumor cells are immunoreactive for vimentin and SMA, but negative for desmin and S-100 protein. These antibodies help to differentiate myofibroma from leiomyoma and neurofibroma⁹⁾. Because neurofibroma would show positivity for S-100 & vimentin, and leiomyoma was positive for SMA & desmin. Additionally CD34 and CD99 are negative for myofibroma. Therefore, they help to separate myofibroma from hemangiopericytoma.

In this case, c-kit and Ki-67 positive cells are rarely present. For diagnosis of myofibroma in gastrointestinal tract, a possibility of gastrointestinal stromal tumor should be considered in the differential diagnosis. In such case, the use of c-kit would be helpful in diagnosis, because c-kit has never been described for myofibrma¹⁰⁾. Ki-67 is a cellular marker for proliferation. Its expression in myofiborma has been known for some time²⁾. To our knowledge, Ki-67 has not been characterized in this tumors yet. It would be interesting to investigate, with a greater number of cases, the use of Ki-67 as a viable adjunct in the diagnosis of myofibroma.

In conclusion, solitary myofibroma of the adult mandible is uncommon lesion, but it should be differentiated with an unilocular radiolucent lesion in periapical lesion. And owing to similarities of microscopic characteristics, immunohistochemical profilings against vimentin, SMA, desmin, S-100 and CD34 are needed to avoid possibility of misdiagnosis.