Primary diffuse intraosseous meningioma.

Primary intraosseous meningiomas are a subset of primary extradural meningiomas that arise in bones, and represent about two thirds of primary extradural meningiomas.1 Here we present a case of primary intraosseous meningioma with diffuse and symmetrical invasion of skull and scalp, which was preoperatively diagnosed as fibrous dysplasia. We described the clinical and radiological characteristics, discussed the potential pathogenesis of the diffuse lesion, and presented surgical management. A 32-year-old man was referred for neurosurgical consultation due to an 8-year history of painless, slowly growing huge scalp mass. The patient denied any past surgical history and head trauma. The patient did not present any neurological deficts. Computed tomography (CT) scans (Figure 1A and 1B) revealed diffuse thickening of the cranial vault, especially in the bilateral frontal-temporal-parietal bone. Magnetic resonance image (MRI) also showed a markedly thickened calvaria and extracranial soft-tissue mass on T1-weighted MRI (Figure 1G and 1H). Three-dimensionally reformatted CT images (Figure 1C-1F) clearly demonstrated the diffuse osteoblastic lesion. The preoperative diagnosis of fibrous dysplasia was proposed, and we planned to widely excise the diffused mass in the scalp and the lesion in the skull except the bone overlying the superior sagittal sinus, and simultaneously perform cranioplasty. The patient underwent general anesthesia before craniotomy. Unexpectedly, the thickened lesion in the scalp and skull were extensively vascular and with diffused bleeding. It was difficult for us to identify bleeding points, and consequently intraoperative hemostasis was extremely difficult to achieve. In addition, the diffused thickening of the cranium was so hard that the electric drills were damaged. Our attempt to grind the lesioned cranium by drill also failed. Thus, we had to discontinue the surgical procedure after a small piece of specimen was excised for a histological examination. The histological evaluation of a decalcified specimen revealed a meningothelial meningioma without the evidence of malignancy (Figure 1I and 1J). This patient died from severe postoperative coagulopathy that have been caused by excessive blood loss and subsequently excessive blood transfusion during surgery.Figure 1.: Axial (A) and sagittal reformatted (B) CT scans showed the osteoblastic lesion involving the outer and inner plate of the skull. Three-dimensionally reformatted CT images clearly revealed the hyperplastic lesion in the bilateral frontal-temporal-parietal bone (C-F). Axial (G) and sagittal (H) T1-weighted MRI demonstrated the hypodense lesion underlying the thickened soft tissues. Histopathological features of meningioma. Histopathological features of meningioma. I: Photograph showed the skull infiltrated by meningothelial cells (HE staining, original magnification ×10). J: Photograph demonstrating cytoarchitectural features where tumor cells had oval nuclei with vacuoles (HE staining, original magnification ×40).The origin of primary intraosseous meningioma is still controversial. Several hypotheses about its origin have been proposed. One explanation is that primary intraosseous meningiomas develop by entrapment of arachnoid cap cells in cranial sutures during the modeling process of the skull or during delivery. Another hypothesis is that arachnoid cap cells are captured in fracture sites after traumatic impact. However, only 8% of the primary intraoseouss meningiomas were associated with a cranial suture, and only a small percentage of patients with primary intraosseous meningioma had a history of head trauma followed by tumor development.1 In addition, a calvarial meningioma may arise from infiltration of the diploe by dura through haversian canals, and thus induction of corresponding osteoblastic or osteolytic reaction in the bone.2 Finally, primary intraosseous meningioma may arise from pluripotent mesenchymal cells upon unidentified stimuli. In our case, the evidence that the meningioma widely and symmetrically involves cranium and extends into the overlying soft tissues may support the hypothesis that some unknown stimuli simultaneously activate the pluripotent mesenchymal cells in the different regions of the skull which are transformed into intraosseous meningioma. Our case of primary intraosseous meningioma was preoperatively diagnosed as fibrous dysplasia. The potential reason was that we excessively relied on typical imaging features of fibrous dysplasia. Indeed, radiographic findings may help distinguish primary intraosseous meningioma from fibrous dysplasia. Primary intraosseous meningioma may invade the full thickness of the skull and demonstrate the pattern on a CT scan of inward bulging of the inner table of the skull. Those characteristics have been demonstrated in our case (Figure 1A and 1B). In contrast, fibrous dysplasia usually causes outward bulging of the outer table of the skull. In addition, the smooth surface of the invaded cranium can be seen in patients with fibrous dysplasia, whereas primary intraosseous meningioma usually has an irregular surface of the hyperostotic bone which was also seen in our case (Figure 1E and 1F). Finally, primary intraosseous meningioma tends to invade the soft-tissues overlying the lesion, but fibrous dysplasia does not, which is consistent with our case. Wide surgical resection of lesions of the scalp, skull bone, and invaded dura is the first-choice of treatment for patients with primary intraosseous meningioma, and cranial reconstruction should be simultaneously performed. Primary intraosseous meningioma is frequently supplied by terminal branches of the external carotid artery.2 In our case, it would be extremely necessary to preoperatively perform selective embolization of the tumor's feeding arteries to avoid excessive blood loss during surgery. Indeed, this selective embolization has previously been applied in two patients with primary intraosseous meningioma.2 Adjuvant therapy should be considered for patients with unresectable tumors or histologically showing malignant or atypical features. Primary diffuse intraosseous meningioma deserves special attention although its incidence is rare. Primary intraosseous meningioma arises primarily in bone and is markedly osteoblastic. The skull of those patients is expanded to form a rock-hard mass with extremely rich blood supply. Detailed features on CT and MRI of patients with primary diffuse intraosseous meningioma could help to clinch the diagnosis. In addition, a biopsy of the scalp mass is preoperatively suggested for the differential diagnosis. A selective embolization of the tumor's feeding arteries should also be performed to avoid excessive blood loss during surgery.