Xueqing Liu , Yanling Shen, Wensheng Yang*

Department of Pathology, 73rd Group Military Hospital of PLA and Affiliated Chenggong Hospital of Xiamen University, 361003, Xiamen, China

*Corresponding author. E-mail: yws_huoyun@126.com

作者 ：刘雪青，沈艳玲，杨文圣

作者单位：中国人民解放军陆军第73集团军医院（暨厦门大学附属成功医院）病理科

Abstract

Primary intraosseous schwannoma is a rare benign tumor, accounting for less than 0.2% of all primary bone tumors, which derives from Schwann cells that cover myelinated nerve fibers. A 29-year-old woman presented with a history of intermittent pain on her right ankle over three years. Radiology examination revealed a multilocular cystic and lytic lesion in the distal tibia. The patient underwent a surgical resection. Histologically, the tumor was composed of spindle-shaped cells with typical antoni A and antoni B pattern, which was similar to soft tissue schwannomas. Although primary intraosseous schwannoma has been deleted in the new World Health Organization classification of soft and bone tumors, the pathologists should be aware of the incidence of the tumor in the differential diagnosis.

Key words schwannoma, primary intraosseous schwannoma, bone tumors 

INTRODUCTION

Primary intraosseous schwannoma is a rare benign bone tumor, accounting for less than 0.2% of all primary bone tumors.¹It is an infrequent entity, especially occurring in long or short bones, such as femur,^2,3 metatarsal,⁴ calcaneus⁵ and hamate.⁶ In this article, we reported a case of a primary intraosseous schwannoma in the tibia.

CASE REPORT

A 29-year-old woman presented with a history of intermittent pain on her right ankle over three years. The pain had increased gradually in the past one year. There were no obvious initiating factors, such as trauma or inflammation. Physical examination revealed tapping pain on the right lower extremity. The overlying skin was intact and there was no evidence of fistula, induration or hyperemia. No evidence of lymphadenopathy was found. 

Plain radiograph indicated a multilocular cystic and lytic lesion in the distal tibia. The inner partial cortex was disrupted without apparent periosteal reaction. Computerized tomography scans(CT) showed that the lesion was expansive with cortical interruption and mild sclerotic borders (FIG.1).Magnetic resonance imaging(MRI) in other hospital demonstrated a well-defined lesion with hypo-intense signal on T1-weighted images and hyper-intense on T2-weighted images. Edema of periosteum and soft tissue was observed.

Based on examinations, benign or low-grade malignant tumor was suspected. The patient underwent a surgical resection. In the operation, hemorrhagic efflux was visible from the marrow cavity. Intramedullary tissue was scrapped for biopsy. Microscopically, the histological features of the tumor were consistent with schwannoma in soft tissues, which was mainly composed of spindle shaped cells. The tumor contained hypercellular regions with nuclear palisading in fibrous background (Antoni A pattern) and hypocellular regions with myxoid matrix (Antoni B pattern) (FIG. 2). In addition, bone interruption could be seen in the tumor. There was no evidence of mitotic activity and necrosis. Immunohistochemistry showed that the tumor cells were diffusely positive for S-100 and SOX-10 (FIG. 3). Ki-67 proliferation index was less than 5%.

Based on these evidences, the diagnosis of primary intraosseous schwannoma occurring in the tibia was established.

DISCUSSION

Schwannoma is a benign nerve sheath tumor deriving from Schwann cells that cover the peripheral myelinated sensory nerves. Schwannoma usually occurs in soft tissues, primary intraosseous schwannoma is exceptionally uncommon, accounting for less than 0.2% of all primary bone tumors. 

By reviewing the literature, there are three underlying hypotheses of the development of intraosseous schwannoma: (1) the tumor arises from the periosteum lesion which invades into the bone by consequent erosion. (2) the tumor originates from the nerves which enter the bone through nutrient canals. (3) the tumor occurs directly within the medullary cavity from minute non-myelinated nerves associating with vessels. In the literatures, intraosseous schwannoma is commonly found in the mandible,⁷ maxilla,⁸ sacrum⁹ and vertebrae.^10,11These bones contain nerve foramina with peripheral nerves pass through it. These schwannomas may arise from the nerves traversing intraosseous canals or from sensorimotor nerves passing through sacral foramina, which are not primary intraosseous schwannomas. Primary intraosseous schwannoma is limited to those arising from the substance of the bone, such as minute nerves accompanying vessels that nourish bone in the medullary cavity. The rarity of primary intraosseous schwannoma may be due to the low density of sensory nerves in bone. Munehisa et al. has suggested that the apoptosis of sensory nerve fibers within bone tissue may lead to the rare occurrence of intraosseous schwannoma.¹² Most intraosseous nerves are non-myelinated nerves travelling with the nutrient vessels and participate in vasomotor functions. Primary intraosseous schwannoma probably arises from schwann cells of these paravascular nerves. 

The differential diagnosis especially in biopsy include desmoplastic fibroma、osteofibrous dysplasia(OFD) or osteofibrous dysplasia-like adamantinoma(OFD-like AD). Desmoplastic fibroma is a rare locally aggressive bone tumor mainly affecting adolescents and young adults. It may involve any bone but is most frequently found in the mandible and long bones such as femur, radius and tibia. ¹³Histologically, it typically consists of low cellularity composed of fascicles of bland spindle cells set in a collagenous matrix. OFD is a benign fibro-osseous tumor of long bone and typically develops during the first and second decades of life. Although the tumor usually arises in the cortex, it can involve the medullary cavity. Histologically, it is composed of woven bone and fibrous tissue with bland spindle-shaped cells embedded within a collagenous matrix. OFD-like AD is a biphasic locally aggressive tumor of long bone, mostly occurring in the tibia.¹³ The tumor usually remains intracortical but it can also invade the medullary cavity. It is composed of small nests of epithelial cells within a bland osteofibrous component. The epithelial cells may be difficult to distinguish from endothelial cells, but they can express EMA、Keratin、P63. Both of these tumors lack typical antoni A and antoni B pattern. S-100 diffuse strong positive in schwannomas is helpful to make differentiation.    

In conclusion, the purpose of our paper is to attract attention to the fact that intraosseous schwannoma does exist as a primary bone tumor. Its rarity, especially long bone involvement, makes it easily to be ignored in the initiate differential diagnosis. 

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FIG. 1: CT showed a lesion in the distal tibia with cortical interruption. 

FIG. 2: (a) The tumor was mainly composed of spindle shaped cells. (H&E,10×). (b) Antoni A area with nuclear palisading and Antoni B area with myxoid matrix. (H&E,20×).

FIG.3: Immunohistochemistry showed that the tumor was diffusely positive for S-100 protein(a) and SOX-10 protein (b).(10×).