Diffuse Leptomeningeal Glioneuronal Tumors: A New Entity?

Abstract

The peculiar radiological and pathological findings of four pediatric cases admitted to the University Hospital of Padua between 1990 and 2007 are described. In all cases, the contrast‐enhanced head and spine magnetic resonance images revealed thickened and abnormally enhancing subarachnoid spaces particularly at the level of basal cisterns and interhemispheric fissure. Furthermore, small cystic lesions scattered throughout the brain and mainly in the cerebellum were also visible. All patients were missing a well‐defined intraparenchymal mass, although during the follow‐up a small intramedullary lesion appeared within the cervical spine in two and subsequently in the frontal horn of the left lateral ventricle in one of those. All patients presented an indolent long‐term follow‐up. Histologically, the tumors were composed by a monotonous population of cells arranged in straight lines or in small lobules. The cells were characterized by round to oval nuclei with finely granular dispersed chromatin, inconspicuous nucleoli with oligodendrocyte‐like features. The morphological and immunohistochemical findings suggested in all cases a “glioneuronal commitment” of the tumors. Because of the unique similar clinical and neuroradiological characteristics, we propose this small series of tumors as a new possible distinct pathological and clinical entity.

INTRODUCTION

Glioneuronal tumors are a group of primary brain neoplasms of relatively recent acquisition in the World Health Organization (WHO) classification of central nervous system (CNS) tumors which has been recently expanded (10). The histogenesis of these tumors is unclear, but an origin from multipotent precursors capable of divergent differentiation has been suggested (10).

Glioneuronal tumors are characterized by a biphasic neurocytic and glial population. The neuronal component consists of synaptophysin‐positive neurocytes with round nuclei and clear cytoplasm sometimes intermingled with neurons and intermediated‐size “ganglioid” cells, whereas the glial component exhibits features of glial fibrillary acidic protein (GFAP)‐positive astrocytes.

This article summarizes the clinical, imaging and neuropathologic features of four diffuse leptomeningeal tumors which cannot easily be classified in the currently used CNS WHO classification, but have the histological criteria to be considered as glioneuronal tumors. These four tumors are characterized by similar histological appearance, mixed immunohistochemical profile combining neuronal/glial markers and widespread tumor cell invasion of the leptomeninges without evidence of a primary intraparenchymal mass. We believe that similar cases have been already published in the literature, but otherwise called diffuse leptomeningeal oligodendroglioma 1, 2, 3, 5, 6, 7, 8, 18) or neurocytoma/gangliocytoma (21). Because of the unique similar clinical and neuroradiological characteristics, we propose to consider this group of neoplasms as a new possible distinct pathological and clinical entity in the group of glioneuronal tumors.

MATERIALS AND METHODS

Cases were retrieved from the institutional files of the authors. One case had been previously reported as spinal low‐grade neoplasm with diffuse leptomeningeal dissemination (11). Pathology reports and the histological slides were reviewed. Immunohistochemical analysis was performed using the standard avidin–biotin–peroxidase method (19). The primary antibodies used, and their respective dilutions, are listed in Table 1.

Table 1.

Primary antibodies and results of immunohistochemical stains.

Antibody	Source	Dilution	Treatment	#1	#2	#3	#4
Synaptophysin	Dako, Glostrup, Denmark	1:400	EDTA 30'	+	+	+	+
GFAP	Dako, Glostrup, Denmark	1:2500	EDTA 30'	+/−	+/−	+/−	+/−
Neurofilaments	Dako, Glostrup, Denmark	1:6	−	−	−	−	−
Neu‐N	Chemicon Int., Temecula, Calif.	1:100	−	NA	NA	+	NA
S100	Immunotech, Marseille, France	1:1500	EDTA 30'	+	+	+	+
EMA	Dako, Glostrup, Denmark	1:500	−	−	−	−	−
MIB1	Dako, Glostrup, Denmark	1:100	EDTA 30'	<1%	0%	<1%	<1%

Fluorescence in situ hybridization (FISH) was performed on formalin‐fixed, paraffin‐embedded tissues of one of the considered cases (case #3), as previously described (4). Briefly, 4 µm tissue sections were hybridized with LSI^® 1p36/1q25 and LSI^® 19q13/19p13 probes (Vysis‐Abbott, Downers Grove, Ill.). Analysis was carried out with a fluorescence Zeiss microscope Axioskop 2 Plus (Carl Zeiss, Göttingen, Germany) equipped with single band filters. For each probe set, 60 nonoverlapping nuclei were enumerated, and results were reported as the ratio between number of 1p/1q and 19p/19q signals. Cut‐off values for designation of deletion were reported following Smith recommendations (17).

Information about treatment and follow‐up, and written informed consents were requested from treating physicians.

RESULTS

Case series

The main radiological single‐case characteristic descriptions and patients’ outcome are summarized in Table 2.

Table 2.

Clinicoradiological features of the analyzed cases.

Case no.	Age (year)/Sex	Radiology: CT and MRI findings at the time of diagnosis	Outcome	Length of follow‐up (years)
1	4/Male	Tetraventricular communicating hydrocephalus; enlargement of the sella turcica; marked progressive cortical and subcortical cystic involvement of the cerebellum, brainstem and spinal cord; diffuse cerebral and spinal leptomeningeal enhancement	Stable disease	2
2	3/Female	Tetraventricular communicating hydrocephalus; marked progressive cortical and subcortical cystic involvement of the cerebellum, basal temporal and frontal lobes, brainstem and spinal cord; lobulated intramedullary enhancing lesion (C7–D5); diffuse cerebral and spinal leptomeningeal enhancement	Death at 9 years of age	6
3	3/Female	Tetraventricular communicating hydrocephalus; marked progressive cortical and subcortical cystic involvement of the cerebellum, basal frontal and temporal lobes, brainstem and spinal cord; small intramedullary enhancing lesion (C3); diffuse cerebral and spinal leptomeningeal enhancement	Stable disease	6
4	13/Male	Tetraventricular communicating hydrocephalus; enlargement of the sella turcica and the internal auditory canals; moderate cortical and subcortical cystic involvement of the cerebellum; diffuse cerebral and spinal leptomeningeal enhancement; thalamic calcifications on brain computed tomography	Stable disease	1.5

Case 1

This is a 4‐year‐old boy admitted to the Paediatric Department of the University Hospital of Padua after a 5‐day history of dizziness associated with accidental falls, vomiting and somnolence. His family and personal medical history were unremarkable. The patient underwent a brain and spine magnetic resonance imaging (MRI) which revealed a tetraventricular communicating hydrocephalus with an enlarged sella turcica and widening of the posterior fossa subarachnoid spaces. Moreover, there were small non‐enhancing cysts scattered all over the brain and spinal cord, in particular in the cerebellum, brainstem and conus medullaris (Figure 1A). After the administration of gadolinium, a diffuse leptomeningeal enhancement surrounding the brain and the spinal cord was noted but without evidence of a spine or brain mass (Figure 1C–H). The patient underwent an immediate ventricular–peritoneal shunt (VPS) placement. The search of most common infective markers in cerebrospinal fluid (CSF) was negative. A brain and spinal follow‐up MRI showed an increase of the cysts, spreading most anteriorly in the medial temporal lobes. The child was treated with systemic chemotherapy (temozolamide for 6 months), and at present is still alive with stable disease after 24 months from the first clinical presentation.

Figure 1.

Representative neuroradiological features of the described tumors at initial presentation. A,B. Axial T2 images show cystic lesions diffusely scattered throughout the surface of the cerebellum, brain stem, mesial temporal lobes and basal frontal lobes. C. Axial and D. coronal contrast‐enhanced T1 images demonstrate thickening and enhancement of the leptomeninges. A tetraventricular communicating hydrocephalus is also present. E. Spinal sagittal T2 image reveals small cystic lesions involving the posterior surface of the conus midollaris (black arrows). F. Corresponding contrast‐enhanced T1 image demonstrates a marked nodular and linear leptomeningeal enhancement involving the spinal cord and the cauda equina; the intra‐axial cysts do not enhance. Spinal axial. G. T2 and H. contrast‐enhanced T1 images at cervical level show a small superficial leptomeningeal enhancing nodule (white arrow) and an underlying intramedullary cyst (open arrow). I. Sagittal contrast‐enhanced T1 images depict a small intramedullary lesion posterior to C3‐C4 (white arrow) and a marked cerebral and spinal leptomeningeal enhancement coexisting with a tetraventricular communicating hydrocephalus and an empty sella. Small non‐enhancing cysts are scattered throughout the cerebellar and pontine surface. (A,C,D–H case #1; B and I, case #3).

Case 2

This girl was 40 months old when she presented a diffuse abdominal pain, vomiting, mild morning headaches, bilateral sixth nerve palsy and mild right‐side hemiparesis. On admission, a contrast‐enhanced head computed tomography scan was obtained and a tetraventricular hydrocephalus was documented without visible intracerebral masses. Shortly after, acute spinal tenderness and truncal rigidity led to perform a gadolinium‐enhanced spinal MRI which revealed a diffuse spinal leptomeningeal enhancement and two cystic lobulated intramedullary spinal lesions at the level of C7–D5. The child was then brought to the operating room for the surgical resection of the spinal lesion and for the positioning of a VPS. The mass was partially resected. Three months later, a cerebral MRI revealed diffuse multiple small cystic lesions over the surface of the cerebellum, brain stem, basal frontal and temporal lobes. Over the next 4 years, despite a short course of systemic chemotherapy, the patient's MRI demonstrated constant progression in size and extent of the spinal lesion, brain parenchymal cyst‐like lesions and diffuse leptomeningeal enhancement. The child died 6 years after diagnosis after a devastating course of progressive neurological deterioration. No post‐mortem examination was allowed. This case was previously reported as spinal low‐grade neoplasm with diffuse leptomeningeal dissemination (11).

Case 3

This 3‐year‐old girl, born at 29 weeks for maternal gestosis, presented with difficulties in walking and increased cranial circumference. Brain and spine MRI showed a tetraventricular communicating hydrocephalus and cystic lesions diffusely scattered throughout the cerebrellum, basal temporal and frontal lobes and on the surface of brainstem (Figure 1B). The contrast‐enhanced T1‐weighted images revealed a diffuse marked thickening of the leptomeninges, particularly in the basal cisterns, sylvian fissures and interhemispheric fissure and a small superficial lesion in the spinal cord at the level of C3 (Figure 1I). Initially, the child was treated with a 12‐month course of systemic chemotherapy. Afterward, she did fine but 2 years after stopping therapy, the child developed a solid spinal lesion which was irradiated. Subsequently, 18 months after finishing RT, she developed a left intraventricular frontal lesion which was incompletely removed. Presently, the child has just finished a course of local RT with stable diffuse leptomeningeal lesions at 6 years from diagnosis.

Case 4

A 13‐year‐old boy was admitted to our institution because of sudden awakening at night, confusional state, dysarthria and dyslalia. His medical history was unremarkable. A brain and spine MRI depicted few non‐enhancing cyst structures scattered over the cerebellar surface. Post‐gadolinium images revealed diffuse leptomeningeal thickening and enhancement particularly in the cerebral sulci, cerebellar vermis, basal prepontine and ambient cisterns, as well as within the internal auditory canals with no evidence of intraparenchymal involvement. The follow‐up MRI showed a progression of the leptomeningeal enhancement, mostly surrounding the inferior dorsal spinal cord. Systemic treatment has been always refused by the parents, and currently the child is alive with a slow asymptomatic progression of the disease after 18 months from initial diagnosis.

Pathological findings

In all these cases, a dural biopsy was performed. Minute samples characterized by a pearly opacified surface and an increased consistence were obtained. The histological samples showed thickened desmoplastic leptomeninges with sclerohyaline bands and enlarged capillary‐sized blood vessels diffusely infiltrated by a monotonous population of cells arranged in straight lines or in small lobules within a compact to loosely fibrillary stroma (Figure 2A,B,D).

Figure 2.

Representative histopathological features of the described tumors. A. Thickened leptomeninges diffusely infiltrated by neoplastic cells. B. Tumors were composed by a monotonous population of round cells arranged in straight lines. C. Pseudopapillary pattern of neoplastic growth. D. Cuboidal glial cells intermingled with neurocytes and ganglioid cells. E. Intratumoral neural pseudorosettes were also observed. F. Oligodendrocyte‐like honeycomb appearance. G. Strongly cytoplasmatic synaptophysin‐immunoreactive tumor cells. H. Nuclear Neu‐N‐immunoreactive neurocytic component. Original magnifications 10×, 20× and 40×. (A, case #1; B, case #4; C–H, case #3).

Cells were characterized by round to oval nuclei with finely granular dispersed chromatin, inconspicuous nucleoli with clear oligodendrocyte‐like features with perinuclear haloes (Figure 2B). No mitosis, necrosis, calcifications, lymphoid infiltration, myxoid changes or endothelial vascular proliferation were observed. No Rosenthal fibers, nor rosettes or pseudorosettes were detected.

Results of immunohistochemical stains are summarized in Table 1. In general, tumors showed diffuse reactivity for synaptophysin and S100, patchy reactivity for GFAP and negative for neurofilaments or epithelial membrane antigen. Proliferation index, as percentage of MIB1‐positive cells [MIB1 labeling index (MIB1 L. I.)], was always less than 1%.

Only in case #3, after a first dural biopsy (performed in 2002), we obtained a significative sample of tissue from the lesion appeared on the inner surface of the lateral ventricle frontal horn (2007). The analyzed sample was composed by a biphasic architecture. The more differentiated part of the tumor was abutting in the ventricle lumen and composed by uniform small cuboidal cells with round nuclei and scant clear cytoplasm intermingled with “ganglioid” cells occasionally arranged in perivascular pseudorosettes or pseudopapillary structures (Figure 2C–E). Additional features include fibrillary areas mimicking neuropil and rare foci of microvascular proliferation of the capillary‐sized blood vessels. The inner part of the tumor showed anaplastic histological features with increased cellularity and a diffuse honeycomb pattern of growth. The neoplastic oligodendrocyte‐like cells, diffusely infiltrating the brain parenchyma (Figure 2F), and showed mild polymorphism with hyperchromatic nuclei. Endothelial proliferation in the branching capillaries was evident. No tumoral necrosis was observed. An increased mitotic activity (three mitotic figures ×10 high‐power field) with an MIB1 L. I. higher than 5% was detected. FISH analysis revealed deletion of 1p, whereas 19q was intact.

A significant number of nuclei were immunopositive for Neu‐N. Tumor cells were also diffusely synaptophysin positive (Figure 2G,H). Scattered cells were GFAP positive.

DISCUSSION

The main clinical neuroradiological and histological similarities of the tumors affecting these four pediatric patients deserve special considerations.

The neuroradiological appearance of these tumors is very characteristic (Figure 1). On T1 post‐gadolinium sequences, a thick and diffuse leptomeningeal enhancement, designing the surface of the brain and the basal cisterns, similar to that described in tuberculous meningitis, was documented in all of the patients. The spinal cord was always involved with a diffuse linear or nodular leptomeningeal enhancement.

A peculiar and specific neuroradiological finding was the presence of multiple small cysts scattered over the surface of the cerebellum, brainstem, spinal cord, medial temporal lobes and inferior frontal lobes. The cystic lesions were numerous and pronounced; nevertheless, in one case, they were small and have to be carefully sought. The cysts were hyperintense on T2 images and iso‐hypointense on T1 and FLAIR images. These MR signal characteristics might be related to both a serous or mixoid content of the cysts, but solid evidence is missing; indeed, it remains unclear what these cysts may represent.

The neuroradiological progression of these lesions was variable, and the relationship with treatment was difficult to explore, because of the exiguity number of cases. In any case, the disease presented with relatively indolent behavior. Some of these cysts showed a progressive enlargement and extension from the surface of the brain into the cerebral parenchyma with an inhomogeneous central contrast enhancement.

A small solid enhancing lesion appeared during the follow‐up in two of our cases, extending from the surface to the internal part of the spinal cord, supporting the hypothesis of neoplasm dissemination via the Virchow–Robin perivascular spaces. The subsequent development of a discrete C6–C7 spinal lesion, 5 years after the presentation of the symptoms, in an 8‐year‐old boy who initially presented with only a diffuse leptomeningeal enhancement has been reported by Armao et al (1). The post‐mortem examination revealed that the lesion was caused by tumor cell infiltration and expansion into the perivascular spaces.

Conversely to secondary meningeal spread from gliomas, carcinomas or lymphomas, repeated CSF examinations in all patients did not show any malignant cells, although CSF protein was elevated. It is of interest that in none of the similar cases described in the literature, there was documentation of cytological diagnosis of the tumor cells in CSF 1, 2, 3, 5, 7, 18, 21). This fact could be explained by the entrapment of tumor cells by the dense desmoplastic reactive fibrous tissue.

Histologically, these tumors were composed by cells characterized by round to oval nuclei with finely granular dispersed chromatin and inconspicuous nucleoli with clear oligodendrocyte‐like cytoplasm. These histological findings might have favored the diagnosis of oligodendrogliomas and oligodendrogliomatosis in some of the similar cases presented in the literature 1, 2, 3, 5, 6, 7, 8, 18). Moreover, the diagnosis of oligodendrogliomas was achieved only on cytologic criteria, that is, clear cytoplasm and round nuclei caused by the lack of specific markers for oligodendrogliomas. As previously described in a case of diffuse leptomeningeal oligodendroglioma (3), we found the deletion of 1p in one of our cases. This deletion is neither pathognomonic of oligodendroglioma (4) nor particularly frequent in pediatric cases that usually do not show 1p/19q co‐deletions 9, 14, 15). In oligodendrogliomas, synaptophysin immunoreactivity is usually caused by residual parenchyma and is frequently seen at the infiltrating tumor borders. In our cases, the constant immunohistochemical profiles observed (i.e. the positive reactivity for synaptophysin and Neu‐N) strongly suggest a glioneuronal commitment of the neoplasm.

Also, neurocytomas and dysembrioplastic neuroepithelial tumor (DNT) show similar histological/immunophenotypical profile (21), but in our cases a common characteristic was the absence of a primary neoplastic mass in contrast with the pathological evidence of the other glioneuronal tumors (i.e. DNT, extraventricular neurocytoma, papillary glioneuronal tumor and rosette‐forming glioneuronal tumor) (10). A possible explanation about the origin of these diffuse leptomeningeal tumors could be isolated groups of glioneuronal progenitor cells entrapped in the context of the leptomeninges during the primitive migration. These embryonal cells could be able of divergent differentiation with neuronal, oligodendroglial and astrocytic features 12, 16, 20). In fact, in the literature there have been described cases of morphologically classic oligodendroglioma with neurocytic rosettes or neurocytomas arboring 1p/19q deletion 12, 13). The description of these entities suggests a histogenetic overlap between oligodendroglioma and extraventricular neurocytoma (12), and further supports the existence of a new “superfamily” of tumors with oligodendroglial and neurocytic potential in which our series of diffuse leptomeningeal glioneuronal tumors could be included.

Interestingly, in the other similar cases presented in the literature, but considered as diffuse leptomeningeal oligodendrogliomas, the immunohistochemical profiles are quite variable and sometimes inconsistent which could be related to the glioneuronal nature of the described neoplasms, and further indicate the difficulty to classify these types of tumors.

Three of four patients are alive up to 2 years of follow‐up, following minimal to no clinical intervention, and these data suggest these tumors as neoplasm with a slow progressive and quite indolent course. However, in case #3, the subsequent appearance of a bulking neoplastic intraventricular lesion with anaplasia, high mitotic index and focal vascular endothelial proliferation suggests a potential aggressive biological transformation.

In conclusion, we hypothesized that the tumors affecting the children we described represent a new nosological entity characterized by: (i) intense enhancement of subarachnoidal space with cystic lesions; (ii) diffuse leptomeningeal infiltration by glioneuronal cells without a primary mass; and (iii) quite indolent course. For these reasons, this group of neoplasms could be descriptively named “diffuse leptomeningeal glioneuronal tumors.” Further studies and larger validation are needed to test our hypothesis and to consider “leptomeningeal glioneuronal tumors” as a distinct nosological entity in the SNC tumor classification.