Gliomas that arise primarily in leptomeninges have rarely been reported in either cerebral hemispheres or the spinal cord [1-3]. Most reported cases have presented diffuse leptomeningeal infiltration of the tumor on their imaging studies. However, the reports of focal solitary glioma in leptomeninges are extremely rare as there have been only 16 cases reported in the literature [4]. Their differential diagnosis is very challenging because the diffuse form may mimic the clinical manifestation and radiographic appearance of chronic meningitis, while the solitary form may bear a strong histological resemblance to other central nervous system (CNS) tumors. We herein report a case of focal solitary leptomeningeal glioma which mimicked an extra-axial tumor.

A primary leptomeningeal glioma was described by Cooper and Kernohan [5] as a "rare neoplastic condition in which glial tumor cells extend diffusely throughout the leptomeninges without forming any intra-axial lesions". As a primary leptomeningeal glioma is extremely rare, there is no consensus on the diagnostic criteria and treatment. Two anatomical and clinical forms have been described: a focal mass-forming lesion and a diffuse gliomatosis. The diffuse form constitutes the majority of this condition, in which glial tumor cells are widely disseminated outside the CNS parenchyma without mass-forming. Keith et al. [6] defined a diffuse form of leptomeningeal glioma as a primary leptomeningeal gliomatosis and reviewed 50 previously reported cases. Their diagnosis is very challenging since it only depended on imaging studies. Histopathological examination is very important because there are many conditions which should be differentiated; viral and fungal infections as well as tuberculosis and sarcoidosis. The prognosis of a diffuse primary leptomeningeal glioma is dismal and its diagnosis is often made on autopsy. On the other hand, a solitary and focal form of primary leptomeningeal glioma has been rarely reported as summarized in Table 1. They often mimic an extra-axial CNS tumor such as a meningioma [7]. Some authors defined this condition as a meningeal glioma [3]. Its prognosis is known to be better than that of a diffuse form of primary leptomeningeal glioma [8]. In our case, the tumor was apparently located outside brain parenchyma and was completely separate without continuation. The tumor did not mimic a meningioma on preoperative MRI as its enhancement on T1-weighted image was very subtle. In addition, it showed neither dural tail nor calcification. Preoperative imaging findings were not suggestive of any specific conditions such as a metastatic tumor, a glioma, as well as a meningioma.

The pathogenesis of primary leptomeningeal glioma has not been well elucidated. However, there have been several reports that heterotopic glial tissue is frequently founded in the leptomeninges [5]. Some authors proposed a possibility that this heterotopic glial cell nest is the origin of primary leptomeningeal gliomas, having been separated from normal brain parenchyma during embryogenesis and having subsequently undergone neoplastic transformation [9]. The histopathology of our case demonstrated typical findings of WHO grade 3 of glioma, and the results of immunohistochemical staining were also consistent with a glial cell tumor. The tumor mass consisted predominantly of oligodendroglioma components. It was reported that leptomeningeal glial cell nests consist predominantly of astrocytes and rarely contain oligodendrocytes and even ependymal cells [1]. It is interesting that the histopathological diagnosis of our case was an anaplastic oligoastrocytoma as most of the reported cases were diagnosed as an astrocytoma of various grades of malignancy [10].

In conclusion, although primary leptomeningeal glioma is very rare, the possibility should be taken into consideration in the differential diagnosis of unusual gliomas. It is very difficult to diagnose this rare condition based on imaging studies alone. However, when imaging findings are neither typical nor specific for a certain tumors, presumption of a rare condition is important and pathological confirmation is mandatory.