Abstract
Ectopic schwannomas account for less than 1% of surgically treated schwannomas of the central and peripheral nervous system. Intracerebral schwannomas, without any apparent connection to cranial nerves, are exceedingly rare. Due to rare intracerebral occurrence and lack of pathognomonic imaging features, intraparenchymal schwannoma may be overlooked in the initial differential diagnosis of an intra-axial tumor with heterogeneous ring enhancement such as high-grade glioma, metastasis or lymphoma. Brain metastasis from papillary thyroid carcinoma is very rare with a frequency of 0.1 to 5 %. In this report, we presented a 21-year-old woman with papillary thyroid carcinoma and recent history of seizure who had heterogeneous ring enhancing left frontal lobe mass. The patient underwent uneventful gross total resection of the tumor and histopathology was compatible with benign intraparenchymal schwannoma.
Keywords: frontal, intraparenchymal schwannoma, metastasis, papillary thyroid carcinoma, seizure

Introduction
Intraparenchymal occurrence of schwannoma is uncommon and first reported in 1966 by Gibson et al (1). Since then, greater than sixty five cases of intracerebral schwannomas have been described (2, 11). These lesions are generally solitary, and in very few cases, an associated neurofibromatosis was found (9,12). Although the neuroradiological characteristics of intraparenchymal schwannoma are thought to be of high frequency calcifications, cystic formations and peritumoral edema, these findings are not the rule (8, 11). Prospective imaging diagnosis is challenging due to lack of pathognomonic features (2). The treatment of choice is complete surgical excision which is associated with cure and good long-term outcome (9). In this report, we review the histopathology and clinical features of intraparenchymal schwannoma involving the left frontal lobe of a 21-year-old woman with history of papillary thyroid carcinoma. Intraparenchymal schwannoma was overlooked in the initial differential diagnosis of this intra-axial tumor in a patient with papillary thyroid carcinoma.
Case Report
 A 21-year-old woman with history of papillary thyroid carcinoma presented to the neurosurgery clinic with a recent history of seizure. She had suffered from a secondarily generalized tonic-clonic seizures that lasted for 1-2 minutes with a short postictal period. There was no significant history of neurofibromatosis and no other known genetic disorder in patient and her family.  On admission, the results of the physical examination were unremarkable. Neurological examination revealed no significant findings. Computerized tomography (CT) scans revealed a 2.6 × 2.4 cm heterogeneous left frontal lobe mass, with a hyperdense peripheral rim and surrounding vasogenic edema (figure 1A).
Magnetic resonance imaging (MRI) of the brain demonstrated an intra-axial left frontal lobe mass which was hypointense with isointense rim on T1 (figure 1B) and mixed isointense and hyperintense on T2 with extensive surrounding vasogenic edema (figure 1c). The mass revealed a heterogeneous ring enhancement with central area of non-enhancement (figure 1D) and restricted diffusion is noted within the wall of the mass.  There was no evidence of hemorrhage or calcification on susceptibility-weighted MR images or on computed tomography (CT) imaging. The initial differential diagnosis was considered high-grade glioma, metastasis or lymphoma.
A decision for microsurgical resection was made. The patient underwent a left frontal craniotomy with nearonavigation guidance. Gross total resection was achieved (figure 1E). Intraoperative Squash preparation and frozen section was consistent with a slow growing process.
The specimen consisted of three pieces of pink to light tan tissue ranging in size from 0.5 × 0.2 × 0.1 cm to 0.2 × 0.1 × 0.1 cm. The excised tissue reveals a spindle cell neoplasm which is well-circumscribed and surrounded by a thin rim of gliotic cerebral parenchyma. The spindle cells contain scant cytoplasm and elongated nuclei where at times, palisaded pattern is suggested in different foci. The center of the neoplasm consists of hyalinized and hypocellular tissue. Abundant hyalinized blood vessels are also present.  The trichrome stained section confirms presence of collagen in the neoplasm, and the neurofilament antibody reveals that the cerebral parenchyma is at the periphery of the neoplasm and no axons are identified within the neoplasm. The gliotic cerebral periphery is strongly reactive with glial fibrillary acidic protein (GFAP) antibody and there is mild cytoplasmic reactivity of the spindle cells with the same antibody. These features are diagnostic of a schwannoma arising intraparenchymally. There is no anaplastic or unusual features, and this schwannoma is benign.
The patient tolerated the surgery well, the postoperative course was uneventful and she was discharged home without any complication. The patient is doing well six years after the surgery with no recurrence (figure 1E) or neurologic deficit.
Discussion
Intracerebral schwannomas, without any apparent connection to cranial nerves, are exceedingly rare (3, 11,12). Ectopic schwannomas account for less than 1% of surgically treated schwannomas of the central and peripheral nervous system (12, 13). In the earlier research reports, the majority of tumors were located in the temporal and frontal regions, which are more common sites for producing schwannoma-associated seizures (12). In these locations, schwannomas can resemble other more common intra-axial cystic neoplasms including a high- grade glial tumor (4). In our case, tumor was in the left frontal lobe and at first place was misdiagnosed as metastatic tumor or high grade glioma.
Males are found to be more affected than females (9, 10, 12) or no gender bias (11). They can present with headache and seizures (10, 11), and these generally occur either in the first two decades with a slow indolent course or in elderly with a rapid course (9). Seizure has been regarded as the most common symptom (12, 14). First presentation in our report was seizure.
 The origin of intraparenchymal schwannoma is much debated in the literature (2).  Because Schwann cells are not indigenous to cerebral parenchyma, a lot of speculation has been attached to their origin (9). There are two theories for their occurrence in the central nervous system. One is that they have a developmental origin related to native central nervous system (CNS) cells such as the mesenchymal pial cells or neural crest cells. Another theory attributes the presence of schwann cells in the brain to the perivascular nerve plexuses and the large arteries in the subarachnoid spaces (2, 5, 6). 
MRI characteristics are variable, with T1 isointensity and hypointensity and mixed T2 isointensity and hyperintensity. They show intense but often inhomogeneous contrast enhancement depending on calcification, cystic and/or necrotic component (11). Due to rare intracerebral occurrence and lack of pathognomonic imaging features (9, 12), intraparenchymal schwannoma may be overlooked in the initial differential diagnosis of an intra-axial tumor. Further, their radiological findings are difficult to differentiate from glioma (9). Several characteristic findings have been described, although significant variability exists. These lesions are often superficial or periventricular in location and may be enhanced homogenously or heterogeneously (2, 7, 8). The presence of cyst has been the most consistent finding with these tumors (9). In our reported case, the lesion had heterogeneous ring enhancement with extensive peritumoral edema without cystic component and calcifications. Magnetic resonance imaging was ineffective for the definitive diagnosis and was instead afforded by histology (11). Care must be taken to include this entity in the differential diagnosis of solid brain lesion. Because most of these are benign, complete excision is the treatment of choice (9).
Brain metastasis from papillary thyroid carcinoma is very rare with a frequency of 0.1 to 5 % (15). Although the incidence of metastasis from papillary thyroid carcinoma is very low, it should be considered in differential diagnosis of cerebral intra-axial mass with heterogeneous ring enhancement in MRI. The histopathology will reveal the distinct diagnosis.
Conclusion
Due to rare intracerebral occurrence and lack of pathognomonic imaging features, intraparenchymal schwannoma may be overlooked in the initial differential diagnosis of an intra-axial tumor with heterogeneous ring enhancement. The histopathology will reveal the distinct diagnosis.

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