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### 2.5. Molecular and Cytogenetic Analysis |
Molecular and cytogenetic findings were interpreted in the context of the WHO CNS5 framework and recent EANO recommendations for molecular diagnostic testing in CNS tumors [1,5]. These included next-generation sequencing results, IDH mutation status, TERT promoter mutation status, BRAF alterations including KIAA1549::B... |
Cases with IDH mutation but without 1p/19q codeletion were classified within the astrocytic lineage when supported by histopathological and immunohistochemical findings. Cases without IDH mutation and without 1p/19q codeletion were evaluated in the context of their histological architecture, immunophenotype, anatomical... |
CIC and FUBP1 alterations were not used as primary diagnostic criteria to establish or exclude oligodendroglioma in this cohort. According to WHO CNS5, the defining molecular criteria for oligodendroglioma are the combined presence of an IDH mutation and whole-arm 1p/19q codeletion. Therefore, tumors lacking this requi... |
Genome-wide DNA methylation profiling was not performed systematically in this cohort; therefore, methylation class results were not available for all 23 tumors and were not included in the formal analysis. |
### 2.6. Initial Impression and Diagnostic Reclassification Analysis |
To evaluate the diagnostic impact of oligodendroglioma-like morphology in routine practice, the initial microscopic impression of each case was compared with the final integrated diagnosis. This analysis was intended to document the spectrum of preliminary diagnostic considerations generated by oligodendroglioma-like m... |
Diagnostic reclassification was defined as a change from the initial microscopic impression to a different final integrated diagnostic category after incorporation of ancillary studies and clinicoradiological context. Diagnostic refinement was defined as further specification of a broad initial impression, such as low-... |
### 2.7. Assessment of Ancillary Diagnostic Contribution |
The contribution of ancillary testing was evaluated descriptively. For each diagnostic group, the immunohistochemical, cytogenetic, or molecular findings that supported final classification were recorded. The evaluated diagnostic contributions included IDH mutation and 1p/19q status for excluding true oligodendroglioma... |
This analysis was not designed as a formal diagnostic performance study and did not calculate sensitivity, specificity, or predictive values. Instead, the aim was to describe how ancillary findings contributed to diagnostic refinement in a real-world retrospective neuropathology cohort. |
### 2.8. Diagnostic Grouping |
For descriptive analysis, cases were grouped according to their final integrated diagnosis and broader diagnostic category. The main diagnostic categories were defined as glial tumors, glioneuronal tumors, neuronal tumors, ependymal tumors, and metastatic tumors. Circumscribed gliomas were included within the broader g... |
The final integrated diagnostic groups included diffuse astrocytoma, IDH-mutant; glioblastoma, IDH-wildtype, CNS WHO grade 4, molecularly defined by TERT promoter mutation; pilocytic astrocytoma, KIAA1549::BRAF fusion-positive; dysembryoplastic neuroepithelial tumor; low-grade glioneuronal tumor; polymorphous low-grade... |
### 2.9. Follow-Up Data |
Follow-up data were obtained from electronic medical records when available. The recorded follow-up parameters included duration of follow-up, postoperative residual disease, recurrence or progression, and death during follow-up. Follow-up status was categorized as no residual or recurrent disease, recurrent disease, o... |
### 2.10. Statistical Analysis |
Statistical analyses were performed using IBM SPSS Statistics version 28.0. Categorical variables were summarized as numbers and percentages. Continuous variables were expressed as median and range. Because of the retrospective design, limited cohort size, and heterogeneous diagnostic spectrum, the analysis was primari... |
### 2.11. Ethical Approval |
The study was conducted in accordance with the principles of the Declaration of Helsinki and was approved by the local ethics committee of Ankara Etlik City Hospital with approval number AEŞH-BADEK2-2026-316. Because of the retrospective nature of the study and the use of anonymized archival material and clinical data,... |
## 3. Results |
### 3.1. Cohort Characteristics and Diagnostic Distribution |
A total of 23 central nervous system tumors showing focal or predominant oligodendroglioma-like histomorphological features but not fulfilling the integrated diagnostic criteria for oligodendroglioma were included in the study. The median age at diagnosis was 35 years, ranging from 7 to 58 years. Fourteen patients were... |
The final integrated diagnoses included astrocytoma, IDH-mutant in nine cases, including CNS WHO grade 2 in four cases, grade 3 in one case, and grade 4 in four cases; dysembryoplastic neuroepithelial tumor in three; central neurocytoma in three; metastatic renal cell carcinoma in two; low-grade glioneuronal tumor in t... |
### 3.2. Oligodendroglioma-like Histomorphological Features |
All cases showed at least one histomorphological feature overlapping with oligodendroglioma. The most consistent finding was the presence of small monotonous round nuclei, observed in all 23 cases. Perinuclear halo or clearing was identified in 18 cases, chicken-wire-like capillary vasculature in 10 cases, calcificatio... |
Representative hematoxylin and eosin-stained sections are shown in Figure 1. Diffuse astrocytoma showed small round monotonous nuclei with focal perinuclear clearing. DNET and low-grade glioneuronal tumors demonstrated oligodendroglial-like cells, perinuclear halos, microcystic areas, delicate capillary networks, and a... |
One pilocytic astrocytoma case showed focal oligodendroglioma-like morphology. Preoperative imaging described a subcortical mass lesion containing cystic and hemorrhagic components. Histologically, the oligodendroglioma-like component was focal and consisted of monotonous round nuclei, perinuclear halos, and calcificat... |
### 3.3. Initial Microscopic Impressions and Diagnostic Reclassification |
The initial microscopic impressions varied considerably across the cohort, reflecting the diagnostic uncertainty generated by oligodendroglioma-like morphology. The most common initial impressions were low-grade glial tumor in six cases (26.1%) and high-grade glial tumor in five cases (21.7%). Other initial impressions... |
Comparison of initial microscopic impressions with final integrated diagnoses demonstrated that oligodendroglioma-like morphology did not lead to a single uniform diagnostic consideration, but rather generated a broad range of preliminary interpretations. Cases initially considered as low-grade glial tumors were finall... |
### 3.4. Immunohistochemical Findings |
Immunohistochemistry contributed substantially to the final classification of the cases. In the diffuse astrocytoma group, IDH1 R132H positivity, ATRX loss, p53 nuclear positivity, and GFAP/Olig2 expression supported astrocytic lineage and helped distinguish these tumors from oligodendroglioma in the absence of 1p/19q ... |
Representative immunohistochemical findings are illustrated in Figure 2. IDH1 R132H positivity, ATRX loss, and p53 nuclear expression are shown in a diffuse astrocytoma. Diffuse CD34 positivity is demonstrated in PLNTY. Synaptophysin positivity in a low-grade glioneuronal tumor and NeuN-positive neuronal cells in DNET ... |
### 3.5. Molecular and Cytogenetic Findings |
Molecular and cytogenetic evaluation was available according to the original diagnostic work-up of each case. IDH mutation with absence of 1p/19q codeletion was identified in nine diffuse astrocytoma cases. The molecularly defined glioblastoma case showed TERT promoter mutation in the absence of IDH mutation and 1p/19q... |
No case in the cohort fulfilled the integrated molecular criteria for oligodendroglioma, IDH-mutant and 1p/19q-codeleted. Thus, molecular/cytogenetic assessment, particularly IDH status and 1p/19q evaluation, was essential in excluding true oligodendroglioma in tumors with overlapping morphology. In the pilocytic astro... |
### 3.6. Diagnostic Contribution of Ancillary Testing |
The contribution of ancillary testing was evaluated descriptively according to its role in diagnostic refinement or reclassification. IDH mutation analysis and 1p/19q status were central to excluding true oligodendroglioma and supporting astrocytic lineage in IDH-mutant diffuse gliomas. ATRX and p53 patterns further su... |
The diagnostic contribution of ancillary tests in the context of oligodendroglioma-like morphology is summarized in Table 3. |
### 3.7. Follow-Up Findings |
The median follow-up duration was 18 months, ranging from 6 to 48 months. Fourteen patients had no residual or recurrent disease during follow-up. Recurrent disease was documented in six cases, and three patients died during follow-up. The pilocytic astrocytoma case showed no postoperative residual tumor, and follow-up... |
### 3.8. Practical Diagnostic Clues and Proposed Diagnostic Reappraisal Workflow |
The practical differential diagnostic clues for oligodendroglioma-like central nervous system tumors are summarized in Table 4. In routine practice, diffuse astrocytoma should be considered when oligodendroglioma-like morphology is accompanied by IDH mutation, ATRX loss, p53 nuclear accumulation, and absence of 1p/19q ... |
Based on the observed diagnostic pitfalls and the reclassification patterns in this cohort, we propose a practical diagnostic reappraisal workflow for tumors showing oligodendroglioma-like morphology (Figure 3). Initial recognition of small monotonous round nuclei, perinuclear clearing, delicate branching capillaries, ... |
## 4. Discussion |
In this single-center retrospective diagnostic reappraisal study, we analyzed 23 non-oligodendroglial central nervous system tumors that showed focal or predominant oligodendroglioma-like morphology but did not fulfill the integrated diagnostic criteria for oligodendroglioma. The cohort included a broad spectrum of fin... |
The diagnostic relevance of this observation lies in the contemporary definition of oligodendroglioma. Historically, oligodendroglioma was recognized by uniform round nuclei, perinuclear clearing, delicate branching capillaries, and calcification. However, in the molecular era, these features are insufficient for diagn... |
The most frequent final diagnostic category in our series was diffuse astrocytoma, IDH-mutant. Recent updates on IDH-mutant glioma diagnosis further emphasize the integrated interpretation of IDH status, 1p/19q codeletion, ATRX expression, p53 expression, and TERT promoter status in distinguishing astrocytoma from olig... |
The inclusion of one glioblastoma, IDH-wildtype, CNS WHO grade 4, molecularly defined by TERT promoter mutation further illustrates the impact of molecular testing in this morphological setting. cIMPACT-NOW update 3 established that IDH-wildtype diffuse astrocytic gliomas harboring molecular features such as TERT promo... |
The pilocytic astrocytoma case expands the diagnostic spectrum by showing that focal oligodendroglioma-like morphology may also occur in circumscribed MAPK pathway–altered gliomas. In this case, the oligodendroglioma-like component consisted of monotonous round nuclei, perinuclear halos, and calcification, whereas the ... |
Low-grade glioneuronal and epilepsy-associated tumors represented another important diagnostic group. DNET has long been recognized as a cortically based, epilepsy-associated tumor that may contain oligodendroglial-like cells, floating neurons, and microcystic architecture [7,8]. In our series, DNET and low-grade glion... |
PLNTY is another diagnostically relevant mimic in this setting. Since its initial description, PLNTY has been characterized by oligodendroglioma-like components, frequent calcification, aberrant CD34 expression, and MAPK pathway alterations [9]. In our PLNTY case, the tumor showed small monotonous round nuclei, perinuc... |
Central neurocytoma also emerged as a meaningful mimic in our cohort. The uniform small round cell morphology, perinuclear halo-like clearing, and delicate capillary network of central neurocytoma may closely resemble oligodendroglioma, particularly when the intraventricular location is not fully appreciated or when bi... |
Supratentorial ependymoma was another diagnostically informative entity. Clear cell ependymoma is a recognized mimic of oligodendroglioma because of its small round cells, clear-cell appearance, perinuclear halo-like change, calcification, and delicate vasculature [18,19]. In our case, oligodendroglioma-like areas coex... |
The metastatic renal cell carcinoma cases further demonstrate that oligodendroglioma-like morphology is not restricted to primary neuroepithelial tumors. Clear cell renal cell carcinoma metastasis may mimic oligodendroglioma through clear cytoplasm, perinuclear clearing, and a rich delicate vascular network. This overl... |
Neuroimaging also remains an important component of the differential diagnosis in tumors with oligodendroglioma-like morphology. In the present cohort, tumor location and radiological context were particularly helpful for selected entities, such as intraventricular localization in central neurocytoma, cortical-based le... |
A key contribution of the present study is the comparison of initial microscopic impressions with final integrated diagnoses. The most common initial impressions were low-grade glial tumor and high-grade glial tumor, but several cases were initially considered as DNET, ependymoma, astroblastoma, oligodendroglioma, dysp... |
The diagnostic contribution of ancillary tests was therefore central to this study. IDH mutation and 1p/19q status excluded true oligodendroglioma and supported astrocytic lineage in IDH-mutant diffuse gliomas. ATRX and p53 supported astrocytic differentiation in selected cases. TERT promoter mutation supported gliobla... |
Based on these findings, we propose a diagnostic reappraisal workflow rather than a purely theoretical differential diagnostic algorithm. The workflow begins with recognition of oligodendroglioma-like morphology on routine sections, followed by documentation of the initial microscopic impression and targeted integrated... |
This study has several limitations. First, it is retrospective and single-center in design, with a relatively small cohort. Second, the cohort is diagnostically heterogeneous, which reflects real-world neuropathology practice but limits formal statistical comparison between individual entities. Third, molecular testing... |
## 5. Conclusions |
Oligodendroglioma-like morphology should be regarded as a diagnostic pattern rather than a tumor-specific feature. In this cohort, it generated a broad range of initial microscopic impressions and was ultimately associated with diverse final integrated diagnoses, including diffuse glioma, circumscribed glioma, glioneur... |
2064be5840291c203657e5fb9091ba77115141c8fea97e9473db1a67f582101b pmc13564585.txt |
PMCID: PMC13605746 |
TITLE: Combined Interhemispheric and Endoscopic Endonasal Resection of a Rare Olfactory Schwannoma with Preservation of Olfactory Function |
LICENSE: Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license. |
ABSTRACT: Simple SummaryOlfactory groove schwannomas (OGSs) are exceptionally rare intracranial tumors representing a surgical challenge. We describe a case of OGS removed using a combined transcranial interhemispheric and endoscopic endonasal approach supported by the use of mixed reality to achieve total resection an... |
## 1. Introduction |
Schwannomas are benign nerve sheath tumors that arise from Schwann cells and most often involve the vestibular, trigeminal, or facial nerves [1]. Those arising at the olfactory groove are among the rarest, accounting for less than 1% of intracranial schwannomas [1], with fewer than 80 cases documented to date and almos... |
The clinical picture tends to be vague. Headache and a reduction or loss of smell are the usual presenting complaints, whereas visual symptoms emerge only when the tumor extends posteriorly and exerts mass effect on the optic apparatus [6,7]. One point stands out: despite involving the cranial nerve subserving smell, o... |
Most of these lesions are solitary, unilateral, supratentorial masses located in the anterior cranial fossa. On imaging, these tumors usually appear as well-circumscribed, contrast-enhancing, extra-axial lesions in the anterior cranial fossa [6,10]. However, features such as heterogeneous enhancement, cystic components... |
We present the case of a large OGS with ethmoidal extension and a significant intracranial cystic component, successfully treated via a combined interhemispheric and endoscopic endonasal approach. This case highlights the diagnostic ambiguity and surgical complexity of managing such uncommon anterior skull base tumors. |
## 2. Illustrative Case |
### 2.1. Clinical Presentation and Preoperative Imaging |
A 52-year-old man with an unremarkable medical history came to attention because of transient visual disturbances that had been recurring over several months, accompanied by episodic headaches. He reported no nasal or olfactory complaints. Ten days before admission he developed an unusually severe headache, distinct fr... |
MRI revealed a mixed solid–cystic mass centered on the left olfactory groove, extending into the ethmoid sinus and nasal cavity, with mass effect on the optic apparatus and adjacent vessels. The solid portion enhanced heterogeneously and contained hemorrhagic foci, whereas the large frontal cyst showed no peripheral en... |
Lateralized olfactory testing revealed left-sided anosmia with a Sniffin’ Sticks composite TDI (threshold, discrimination, identification) score of 15 out of possible 48 points. The threshold test determines the lowest concentration of an odorant that can be perceived, with scores ranging from 1 (lowest) to 16 (best). ... |
### 2.2. Surgical Strategy and Technique |
Surgical resection was performed using a combined approach to address both the intracranial and ethmoidal components of the lesion. The choice of approach followed directly from the tumor’s anatomy. Above, the lesion was in contact with the optic apparatus and the terminal internal carotid artery and carried a high fro... |
Neuronavigation and a mixed reality overlay (Brainlab) were used. The segmented models—comprising the solid and cystic tumor components, the optic apparatus, and the adjacent vessels—were overlaid onto the operating microscope view, so that these structures remained visible within the operative field, including before ... |
The patient was placed in dorsal decubitus under general anesthesia with the head fixed by a skull clamp in a slightly flexed, neutral position. A left frontal craniotomy was performed through a bicoronal incision, providing access to the anterior cranial fossa via a left interhemispheric approach. Microsurgical dissec... |
To verify the integrity of the nasal mucosal lining, the rhinology team performed an endoscopic left ethmoidectomy. The skull base was remodeled and located at a much lower position. A 5 mm skull base defect was identified medially (Figure 3A). A middle turbinate flap was performed (Figure 3B) to add a nasal mucosal la... |
### 2.3. Histopathological Findings |
Histological evaluation revealed a biphasic spindle cell neoplasm composed of alternating Antoni A and Antoni B areas. Antoni A areas displayed elongated cells with eosinophilic cytoplasm, palisading nuclei, and Verocay bodies. Antoni B regions were looser, with microcystic architecture. Thick-walled hyalinized vessels... |
### 2.4. Postoperative Course and Follow-Up |
The postoperative course was uneventful, with no new neurological deficits. The patient was discharged home on postoperative day 6, following confirmation of complete resection on postoperative MRI, observation for delayed CSF rhinorrhoea, and adequate healing of the nasal reconstruction. The main complications anticip... |
Astonishingly, olfactory function improved at 1-month postoperatively on the left side from anosmia to hyposmia with a Sniffin’ Sticks TDI composite score of 20.5/48 (threshold = 1.5/16; discrimination = 6/16; identification 13//16) [13]. The right, unaffected side could be preserved (TDI = 34.5). Repeated measures at ... |
The case was discussed at our institutional neuro-oncology tumor board. Given the complete excision and benign histology, a clinical and radiological surveillance strategy was adopted in which the patient will be followed with contrast-enhanced MRI every 6 months for at least 5 years. Neurosurgical and ENT evaluations ... |
## 3. Discussion |
Olfactory groove schwannomas (OGSs) represent an exceptionally rare entity within the spectrum of intracranial nerve sheath tumors, accounting for less than 1% of all intracranial schwannomas [1]. Fewer than 80 cases have been documented in the literature, typically as isolated case reports or limited case series [2]. ... |
Given the absence of Schwann cells in the olfactory nerve, their origin remains a matter of debate [17]. Two prevailing theories have been proposed to explain their development [1,5,6,7,18]. The developmental hypothesis postulates that ectopic Schwann cells originate within the CNS due to aberrant migration of neural c... |
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