A Rare Case of Periosteal Osteoblastoma Located in the Frontal Cranial Bone

Although osteoblastomas account for approximately 3.5% of all benign primary bone tumors and less than 1% of all bone neoplasms,1 osteoblastomas located on the surface of cortical bone, or periosteal osteoblastomas, are extremely rare. The periosteal location of an osteoblastoma was first described by Lichtenstein and Sawyer2 in 1964. Since then, several periosteal osteoblastomas have been reported in the literature,3 but few have been completely documented, and only 2 have been located in cranial bone.4 Although osteoblastomas may affect virtually any bone, the most common site is the vertebra.5 Osteoblastomas are infrequent in the skull and, when periosteal in origin, may be radiologically indistinguishable from a meningioma. The rare periosteal nature of this bony lesion should be kept in consideration as part of the differential diagnosis of the far more common dural-based meningioma.

REPORT OF A CASE

A 23-year-old white woman presented with a 3-month history of headaches and a 1-week history of double vision. On physical examination, she exhibited a left abducent nerve palsy associated with papilledema. Magnetic resonance imaging with contrast was performed (Figure 1), revealing an 8-cm right frontal extraaxial mass that was relatively isointense to brain on Tl-weighted sequences and relatively isointense to gray matter on T2-weighted sequences. It was described as being well circumscribed and vascular with underlying bony hyperostosis with a durai base along the right frontal convexity consistent with a meningioma. Further review of the radiologie studies demonstrated no evidence of bony expansion by the tumor. Rather, very focal bone involvement was present. There was significant mass effect and a 1-cm midline shift from right to left, with associated flattening of the right frontal horn. No hydrocephalus was noted, and no underlying parenchymal edema or infarction was noted. Subsequent to magnetic resonance imaging and 1 day prior to surgery, diagnostic cerebral angiography and preoperative transcatheter, transarterial particle embolization of intracranial and extracranial branches supplying the tumor were performed without complications. These were followed by bicoronal craniotomy with resection of mass.

PATHOLOGIC FINDINGS

Gross inspection revealed a tan to dark-brown coarse hard mass measuring 7.5 × 7.0 × 2.5 cm (Figure 2). The surface of the mass was porous, with a firm to spongy texture. Cut surface of the mass showed a central core that was tan-yellow and firm to slightly spongy, surrounded by a dark-brown rim of firm to gritty tissue with scattered hemorrhagic regions. Additional fragments of dura were submitted separately and grossly showed small foci of adherent tan-brown tumor.

Microscopically, the mass was composed of anastomosing osteoid and bony trabeculae in a loose fibrovascular stroma (Figure 3). The trabeculae showed rimming by a single layer of osteoblasts with large vesicular nuclei. No overt anaplasia of the osteoblasts was present, and no prominent mitotic activity was noted (Figure 4). No large sheets of osteoblasts or entrapped fragments of native lamellar bone were seen. The intertrabecular stroma was composed of capillaries and loosely arranged spindle cells without atypia. Osteoclastic giant cells were scattered throughout the stroma (Figure 5). There were areas of necrosis, with embolization material noted in some blood vessels. Small foci of hemorrhage were noted, particularly around areas of necrosis; however, no aneurysmal bone cyst features were present. No periosteum was seen. The fragments of dura showed adherent tumor with reactive fibroblastic proliferation at the interface with the tumor.

COMMENT

Osteoblastoma is a rare tumor that primarily affects young adults, with 80% presenting in the first 3 decades of life. There is a male predominance, with a male-female ratio of 2:1. Unlike most other bone neoplasms, osteoblastomas are most commonly found in the vertebral column, followed by the long bones, in particular the femur and tibia, and they are much less common in other bones.1 Periosteal osteoblastomas are extremely rare. Of the fewer than 30 cases reported in the literature, most appear to be located in long bones. The presenting symptoms include sharp pain and discomfort with palpation, but many patients are asymptomatic.3

The radiologic and pathologic features of a periosteal osteoblastoma have been described as being essentially identical to those of its intramedullary counterpart,6 with the exception of its intraperiosteal location. This distinction is made radiologically, because an osteoblastoma, because of its intramedullary origin, results in overall expansion of the involved bone, whereas a periosteal osteoblastoma shows little, if any, involvement of the overlying bone. In the brain, the periosteal location of an osteoblastoma may indeed render it radiologically indistinguishable from a dural-based meningioma, as in the present case.

The radiologie findings of osteoblastomas, regardless of origin, are otherwise often nonspecific. In a review by Lucas et al,7 only 8 of 116 lesions had a calcified central nidus with a lucent halo suggestive of the diagnosis.

Histologically, differentiation toward bone, cartilage, and adipose tissue can be seen in otherwise typical meningiomas. These metaplastic components, however, are usually superimposed on classic meningiomas in the meningotheliomatous-transitional-fibrous spectrum.8 These features were absent in the present case. In contrast, the characteristic features of a periosteal osteoblastoma were clearly demonstrable, including the macroscopic finding of a round to lobulated, red-brown, firm to gritty mass, and the microscopic findings of the typical histologie appearance of an intramedullary osteoblastoma, as described earlier. Other features that suggest a benign nature include sharp circumscription of the lesion with no permeation of surrounding bone (which would otherwise suggest an osteosarcoma or injury lesion), absence of entrapped native bone by the tumorous process,6 and absence of sheets of osteoblasts without bone production.1 Epithelioid osteoblasts, trabecular or sheetlike osteoid, increased mitotic activity, and osteoclastic resorption have been suggested by some authors to indicate more aggressive clinical behavior,9 but the relationship between these histologic features and aggressive behavior has been disputed.10 These findings were also absent in the present case.

The response of benign osteoblastomas to conservative surgical treatment is generally good. Repeat surgery is occasionally indicated when a tumor is incompletely removed. True malignant transformation has rarely been reported, although in such cases the original benign diagnosis is often questioned.1

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