2026 年 13 巻 p. 147-151
Intracranial schwannomas account for approximately 8% of all primary brain tumors, and the majority arise in the cerebellopontine angle. Ectopic schwannomas that originate from non-cranial nerve sites, such as the dura mater or brain parenchyma, are rare, representing less than 1% of all intracranial schwannomas. These lesions often mimic meningiomas on neuroimaging, making preoperative diagnosis challenging. A woman in her 50s was referred to our department after a brain check-up incidentally revealed a mass lesion extending both above and below the tentorium. Neurological examination was unremarkable. Magnetic resonance imaging demonstrated a well-defined, heterogeneously enhancing extra-axial mass widely attached to the tentorium, suggestive of meningioma. The lesion was completely resected via a suboccipital approach. Histopathological examination revealed spindle-shaped tumor cells with palisading nuclei, positive for S-100 protein and negative for epithelial membrane antigen, confirming the diagnosis of schwannoma. The MIB-1 (Ki-67) labeling index was below 5%, consistent with a benign lesion. Postoperative magnetic resonance imaging confirmed total resection, and the patient's postoperative course was uneventful. No recurrence was observed during a 7-month follow-up period. Tentorial ectopic schwannoma is an extremely rare entity that can closely resemble meningioma both radiologically and intraoperatively. Awareness of this entity is important when evaluating tentorial extra-axial tumors. Although the clinical course is generally favorable following complete resection, careful preoperative evaluation is essential for accurate diagnosis and appropriate management.
Intracranial schwannomas are benign tumors arising from Schwann cells that form the myelin sheath of peripheral nerves. They account for approximately 8% of all intracranial neoplasms and typically arise from cranial nerves, particularly the vestibular nerve in the cerebellopontine angle, which represents about 90% of cases.1) In contrast, ectopic intracranial schwannomas that develop independently of cranial nerves are extremely rare, comprising less than 1% of all intracranial schwannomas.2-4) These lesions can occur in atypical locations such as the brain parenchyma, ventricular system, falx cerebri, or tentorium cerebelli. Their imaging features often resemble meningiomas, particularly when arising from the dura mater, leading to diagnostic difficulty. We present a rare case of an ectopic tentorial schwannoma which expanding into both the supra- and infratentorial spaces that radiologically mimicked meningioma. We discuss its clinical, radiological, and histopathological features, along with a review of the relevant literature.
A woman in her 50s with no significant medical history underwent a brain screening ("brain dock"), which revealed an incidental mass lesion located at the tentorium, extending into both supra- and infratentorial spaces. She was referred to our neurosurgical department for further evaluation. The patient was neurologically intact, and no cranial nerve deficits were observed.
Imaging findingsMagnetic resonance imaging (MRI) demonstrated an irregularly shaped mass attached to the tentorium expanding both supratentorial and infratentorial spaces (Figure 1). On T1-weighted imaging, the lesion appeared isointense relative to gray matter; on T2-weighted imaging, it exhibited mixed intensity with focal cystic components and mild peritumoral edema. Gadolinium-enhanced T1-weighted sequences revealed heterogeneous enhancement. The tumor showed no invasion into adjacent brain tissue. The radiological impression favored atypical meningioma due to the presence of a dural attachment and heterogeneous enhancement pattern.
Preoperative MRI. (A) Axial T1-weighted image showing iso-intensity mass at the right tentorial lesion. (B) Axial T2-weighted image showing heterogenous iso-intensity mass with focal edema. (C-E) Axial/sagittal/coronal gadolinium-enhanced T1-weighted image showing well-defined heterogenous supra- and intra-tentorial extra-axial mass with dural tail sign on the tentorium.
MRI: magnetic resonance imaging
A suboccipital craniotomy was performed. Intraoperatively, the tumor was attached to the tentorium and extended above it. The lesion was firm, well-vascularized, and clearly demarcated from surrounding cerebellar and occipital cerebral tissue. At this point, the surgeons considered the tumor to be a meningioma. The tumor was completely resected together with the involved portion of the tentorium (Figure 2A; arrow).
(A) Intraoperative photograph demonstrating the tentorial attachment of the tumor, (B) Hematoxylin-eosin staining showing spindle-shaped tumor cells with palisading. Histological assessment showing positive S-100 protein (C), negative EMA (D), and low cell proliferation (Ki-67 index <5%) (E), (F) Postoperative MRI showing complete tumor resection.
EMA: epithelial membrane antigen; MRI: magnetic resonance imaging
Microscopic examination revealed spindle-shaped tumor cells with elongated nuclei arranged in interlacing fascicles and focal palisading, characteristic of Antoni type A pattern (Figure 2B). Immunohistochemical analysis demonstrated diffuse positivity for S-100 protein (Figure 2C) and negativity for epithelial membrane antigen (Figure 2D), excluding meningioma. The Ki-67 labeling index was less than 5%, consistent with a benign schwannoma (World Health Organization Grade 1) (Figure 2E).
Postoperative coursePostoperative MRI confirmed complete resection of the tumor (Figure 2F). The patient's postoperative course was uneventful, and she was discharged without neurological deficits. Follow-up MRI performed 7 months later showed no evidence of recurrence or residual enhancement.
Ectopic tentorial schwannomas are exceedingly rare, with 19 cases, including ours, reported in the literature (Table 1).5) Although the definitive conclusion is difficult to draw, several hypotheses have been proposed regarding the pathogenesis of ectopic schwannomas:
1. Origin from meningeal branches of cranial nerves coursing along the dura;
2. Ectopic Schwann cells along perivascular nerve plexuses;
3. Schwann cell migration into the leptomeninges during embryogenesis.
Reported cases of ectopic tentorial schwannoma
| Study | PMID | Age/sex | Clinical features | Tumor location/features | Surgical approach | Surgical result | Outcome |
|---|---|---|---|---|---|---|---|
| CPA: cerebellopontine angle; ELSCI: extreme lateral supracerebellar infratentorial; F: female; M: male; N/A: not available; PMID: PubMed Identifier; TS: transverse sinus | |||||||
| Flickinger 1988 | 3366967 | 22/M | N/A | Supra- & infra-tentorial | N/A | N/A | N/A |
| Jabbour 2002 | 11919450 | 9/F | Headache | Infratentorial CPA cistern | Retrosigmoid | Total excision | No new deficit |
| Oikawa 2002 | 12015855 | 41/F | Headache | Infratentorial CPA cistern | Retrosigmoid | Total excision | No new deficit |
| Ozawa 2003 | 12814930 | 29/M | Headache | Infratentorial | Transpetrosal | Total excision | No new deficit |
| Du 2003 | 12646734 | 17/F | Headache | Anteromedial edge of tentorium | Orbitozygomatic trans-sylvian | Total excision | No new deficit |
| Anton 2006 | 16200344 | 23/M | Dizziness, dysphagia | Supra- & infratentorial | Transpetrosal retrolabyrinthine | Total excision | No new deficit |
| Chung 2007 | 17532220 | 49/F | Headache, right facial palsy | Infratentorial CPA cistern | Retrosigmoid | Total excision | No new deficit |
| Calisaneller 2008 | 18814126 | 60/F | Headache | Supra- & infratentorial | Occipital and suboccipital | Total excision | No new deficit |
| Hayashi 2008 | 19061145 | 20/M | Headache | Infratentorial | Subtemporal and ELSCI | Total excision | No new deficit |
| D’Urso 2011 | 22204567 | 42/M | Right hemiparesis | Anteromedial edge of tentorium | Retrosigmoid | Total excision | No new deficit |
| Nagata 2011 | 21613767 | 58/F | Headache | Infratentorial | Retrosigmoid | Subtotal resection | No new deficit |
| Nitta 2011 | 21441745 | 64/F | Transient global amnesia | Supra- & infratentorial, CPA cistern | Subtemporal | Subtotal resection | No new deficit |
| Kumar 2017 | 28642183 | 21/M | Headache, dizziness | Right tentorium cystic supra/infratentorial | Occipital and suboccipital | Total excision | No new deficit |
| Ribeiro 2017 | 28626414 | 75/M | Visual loss, headache | Supratentorial | Occipital | Total excision | No new deficit |
| Xinrui 2017 | 27876660 | 37/M | Memory disturbance | Infratentorial | Occipital transtentorial | Total excision | No new deficit |
| Tsutsui 2020 | 32569764 | 48/M | Gait abnormality | Infratentorial CPA cistern | Retrosigmoid | Total excision | No new deficit |
| Yan 2023 | 36684627 | 39/F | Blurred vision | Supra- & infratentorial near TS | Posterior median approach | Total excision | No new deficit |
| Ahamed 2025 | 40705297 | 45/M | Headache | Supra- & infratentorial | Occipital and suboccipital | Near-total resection | Recurrence ×3 (Malignant transformation) |
| Present study | 50/sF | Incidental | Tentorium supra/infratentorial mimicking meningioma | Suboccipital approach | Total excision | No new deficit | |
Radiologically, ectopic schwannomas often mimic meningiomas because both are extra-axial, dural-based, and well-enhancing masses. However, certain features may aid differentiation. Schwannomas tend to demonstrate cystic changes, intratumoral heterogeneity, and less intense dural tails than meningiomas. In our case, the tumor exhibited heterogeneous enhancement and mild mass effect, features that overlapped with atypical meningioma. High-grade meningiomas can also present with such heterogeneity, which contributed to preoperative misdiagnosis. Advanced imaging techniques, such as diffusion tensor imaging and MR spectroscopy, may improve diagnostic accuracy, but definitive distinction remains difficult without histopathological evaluation.
Management and prognosisComplete surgical excision is the treatment of choice for tentorial ectopic schwannomas, as these tumors are usually benign and well-circumscribed. In our review of the literature, no tentorial schwannoma required adjuvant therapy or showed recurrence after complete resection, although malignant transformation has been reported in one case.5) Thus, complete excision is highly recommended. Our patient remained recurrence-free at 7 months postoperatively, consistent with the favorable outcomes reported previously.
Literature reviewReported cases of tentorial schwannomas frequently present with headache as the initial symptom. The ages of the patients ranged from 9 to 75 years. Schwann cell migration into the leptomeninges during embryogenesis is highly suspected for pediatric patients, whereas the tentorial branch of a cranial nerve is considered to be the pathogenesis in elderly patients. Radiologically, most lesions mimic meningiomas, which exhibit a dural tail sign. Ribeiro da Cunha et al.4) reported a similar tentorial schwannoma that was initially diagnosed as meningioma on MRI but was confirmed as schwannoma pathologically. These findings underscore the necessity of considering ectopic schwannoma in the differential diagnosis of tentorial dural-based lesions. Prognosis after complete resection is promising, therefore, total excision is strongly recommended.
ConclusionWe report a rare case of an ectopic tentorial schwannoma that mimicked meningioma on imaging studies. Complete surgical resection yields an excellent prognosis, and awareness of this rare entity is essential for neurosurgeons and radiologists when evaluating dural-based tentorial lesions.
All authors have no conflict of interest.
Author Miki Fujimura is one of the Editorial Board members of the Journal. This author was not involved in the peer-review or decision-making process for this paper.
All authors agreed to submit the manuscript, read and approved the final draft and take full responsibility of its content, including the accuracy of the data
