Intraosseous metaplastic meningioma: A case report
A 65-year-old male with a history of headaches for a few months was referred to neurosurgery department due to a cranial mass, which was revealed by magnetic resonance imaging (MRI) performed during a previous headache workup. Although previous cranial MRIs had also been taken 4 years prior due to other reasons, there was allegedly no significant change in the lesion's size or appearance between the 2 examinations. The patient's medical history included hypertension, diabetes mellitus, and bronchial asthma, all of which were under medical control. He had no history of malignancy. In addition to mild headache, the patient had no significant neurological abnormalities. There was no visible skull bulge, and a laboratory examination revealed no abnormal readings.

Computed tomography (CT) showed a 19-mm-long, well-defined, left parietal bone mass near the squamosal suture. The main part of the mass was located in a diploic space, and the trabeculae were sparse but well preserved. There was also a mild bony bulge, although there were no tears in the inner or outer plates. MRI performed at our hospital showed an isointense mass relative to the muscles, which included internal hyperintense dots on T1-weighted images (T1WI). Meanwhile, the mass had heterogeneous intensity on T2-weighted images also showing restricted diffusion. Due to a history of asthma, contrast-enhanced MRI was not performed. The hyperintense dots on T1WI were thought to be bone marrow fat, and thus we thought that the mass did not destroy the pre-existing structures, such as the bone trabeculae and bone marrow fat. This finding led to the differential diagnosis of osseous hemangioma (venous malformation), hyperplastic bone marrow (red marrow), IOM, Paget disease of bone, and metastatic bone tumor (particularly, intertrabecular type). The mass was suspected to be benign, because there was no increase in size for 4 years and there was no history of malignancy. However, the mass was still resected as per the patient's request.

A histopathological examination was then performed after decalcification using Plank-Rychlo fluid, revealing tumor growth in the intertrabecular space and whorl patterns of spindle-shaped cells. Moreover, the tumor was marked with ossification and lipomatous tissue inclusions, which were different from the pre-existing bone trabeculae and bone marrow fat. Immunohistochemistry showed that the tumor cells expressed epithelial membrane antigen (EMA) and progesterone receptor, but they were negative for MIB-1 (Ki-67). Given these findings, the final diagnosis was intraosseous metaplastic meningioma (World Health Organization [WHO] Grade I).