Intracerebaral Schwannoma: Case Report and Review of the Literature

Najeeb Thomas, MD; Joe Nadell, MD; Richard Coulon, MD and Deepak Awasthi, MD

Department of Neurosurgery; LSUHSC; New Orleans, LA

Abstract:
Intracranial schwannomas account for approximately 8% of primary brain tumors; however, intraparenchymal schwannomas of the CNS are rare. Since 1965 only 36 cases of intraparenchymal schwannomas have been reported. Of the 36 reported cases only four have been malignant. Two cases of intra cerebral schwannomas are reported in this series, one primary and one metastatic. We will review the demographics of the known cases, discuss the pathologic basis for interpretation of schwannoma and review the theories of how these tumors arise in the brain parenchyma.

Introduction
:
Peripheral nerve sheath tumors represent a spectrum of pathology commonly encountered in surgical practice. Indeed, intracranial schwannomas represent about 8% of primary intracranial malignancies.18 However, intraparenchymal schwannomas represent a rare subset of these tumors.

There have been thirty six reported cases of primary intraparenchymal schwannomas in the literature since first reported in 1965.22 Four of these have been considered malignant.4,22 There have been three reported cases of schwannomas metastasises from a distant site to the brain parenchyma.4,14,15 This entity was first recognized in 1991.4

The current series illustrates a primary intracerebral and a metastatic intracerebral schwannma. The incidence, radiographic findings, pathogenesis and outcome as well as the literature will be reviewed..

Case Reports:
Case #1 -A 13 year old right handed male was admitted to the hospital with a five month history of intermittent seizure activity. Further questioning revealed an almost 2 year history of headaches. His past medical history was non-contributory and he elicited no signs of Von Recklingshausen's disease. Physical examination was normal. MRI revealed a right sided parietal lobe mass (Figure 1). The patient underwent a right sided parietal craniotomy, and with the assistance of a frameless image guided system the tumor was completely resected.

Figure 1: Left: Pre-operative contrast enhanced T1-weighted sagittal MRI scan shows a homogenously enhancing lobulated mass in the right parietal lobe. Right: Postoperative contrast enhanced T1-weighted sagittal MRI scan showing complete excision of the right parietal mass


Postoperatively, the patient has done well and has remained tumor free 9 months postop. He continues to take dilantin for seizure prophalyxis.

Case #2-A 27 year old right handed male was admitted with right upper extremity monoplegia and facial paresis. His history was significant for a right lower extremity amputation for a malignant schwannoma. Nine days prior to craniotomy he also had a left lung lesion resected which was confirmed to be malignant schwannoma. MRI of the head revealed a solitary lesion in the left parietal lobe (Figure 2). The patient underwent a left parietal craniotomy with complete resection of the lesion.

Figure 2: Left: Pre-operative contrast enhanced T1-weighted coronal MRI scan showing an inhomogenously enhancing mass with irregular borders in the left parietal lobe. This mass has a malignant appearance. Right: same lesion in an axial T1-weighted image with contrast.


Postoperatively, the patient underwent whole brain radiation and chemotherapy. He had multiple recurrences 5 months postop and was sent home on hospice care.

Pathologic Findings:
Case# 1- A 3.0x3.5x2.2 cm fragment of lobulated, tan, multinodular mass with hemorrhagic foci seen on cut surfaces. The neoplastic cell react to antibodies to S100 and to a lesser extent Collagen IV which outlines the presence of a basal membrane. Reticulin stain highlights the presence of dense pericellular network of reticulin. Secondary opinion at The Mayo Clinic concurred with diagnosis of schwannoma.
Case#2- Pathologic specimen was examined against primary specimen from leg and metastatic lesion from lung and determined to be the same tumor ( malignant schwannoma).

Discussion:
Intraparenchymal schwannomas are a rare clinical entity. Worldwide there have been reported 36 cases of intraparenchymal schwannoma.10,11,12,22 Consideration of the demographics suggest that these tumors have a male preponderance of almost 2:1 and mean age of presentation of just under thirty years of age.22 The most common signs and symptoms of presentation include seizure, headache, and focal neurologic deficit.1,6,7,19,20,23 Additionally, this tumor has mimicked subarachnoid hemorrhage and torcular mass.2,9 It is important to recognize the radiologic characteristics of these tumors as well as potential adjuvant treatment options for the malignant variety.

Metastatic schwannomas represent an even more rare clinical entity. There have been three reported cases of malignant schwannoma metastasizing to the brain parenchyma,4,14,15 the first in1991. There has been a fourth case of metastasis to the dura mater.8 These tumors most commonly locally recur or metastasize frequently to the liver, lung and lymph nodes.4 Brain metastasis are considered rare even in the animal model. Injection of malignant schwannoma cells in rats failed to yield cerebral metastasis despite widespread disease in other organ systems.13

There are no pathgnomonic findings of intracerebral schwannomas on either CT or MRI scan.3 However, there are some features seen that are distinctive from the more common vestibular schwannoma. First, there is a propensity for cyst formation; this was confirmed in 19 cases. Second, twelve cases have been reported periventricular or just superficial to the ventricle. Third calcification was seen in six of the cases, and this is rarely seen in the vestibular schwanomma5,25. MR imaging characteristics have been variable on both T1 and T2 images.

Primary malignant schwannomas represent more rare subset of intraparenchymal schwannomas. There have been four reported cases of this variety, three of which subsequently died secondary to tumor recurrence.22 The fourth case at last report, 17 months post-op, has remained tumor free.
Several theories exist regarding the histiogenesis of intraparenchymal nerve sheath tumors. Some suggest that schwann cells from the telia choriodea provide the basis for these tumors.3,16 Another suggestion has been the failure of neural crest cells to migrate appropriately leaving rests of cells within the brain parenchyma.3 Another hypothesis has been that these cells arise from hamartomous regions within the brain.16,17 Others suggest that the schwann cells arise from multipotential cells in the CNS. Finally, these lesions have been attributed to disordered embryogenesis or some even hypothesize they may be related to MS plaques or old infarcts as schwann have been found in both of these conditions.

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