A peculiar histopathological form of dysembryoplastic neuroepithelial tumor with separated pilocytic astrocytoma and rosette-forming glioneuronal tumor components

Dysembryoplastic neuroepithelial tumors (DNTs) mostly display typical clinical, neuroimaging and histopathological features, but sometimes they reveal heterogeneous or non‐specific morphology, which results in diagnostic dilemmas. We present a case of a young adult with longstanding, intractable epilepsy associated with a multinodular cystic lesion in the temporal lobe. The lesion consisted of morphologically different components. In particular, a few cortical nodules displayed a specific glioneuronal element with floating neurons typically found in DNT. Two large, well‐circumscribed nodules were entirely composed of biphasic, piloid, astroglial patterns that corresponded strictly to a pilocytic astrocytoma. The well‐defined areas, which contained numerous distinct neurocytic‐like rosettes, were identical with rosette‐forming glioneuronal tumors (RGNTs). This type of neurocytic rosette was widespread within the surrounding piloid background. Some solid nodules exhibited increased cellularity, oligodendroglioma‐like elements and a focal ribbon cell arrangement. The lesion was associated with advanced reactive gliosis and foci of dysplastic changes in the adjacent cortex. The clinico‐radiological and main histopathological features were consistent with a diagnosis of a complex variant of DNT composed of pilocytic and rosette‐forming glioneuronal components. Although both piloid tissue and rosette‐like formations have been occasionally mentioned in DNT lesions, the present case of DNT was unique in its well‐circumscribed, separate pilocytic and RGNT nodules. We concluded that it represented an unusual, mixed pilocytic/RGNT variant of DNT.

DNT components; dysembryoplastic neuroepithelial tumor; neurocytic rosettes; pilocytic astrocytoma; rosette forming glioneuronal tumor

Dysembryoplastic neuroepithelial tumors (DNTs) are benign, mixed, neuronal‐glial neoplasms that occur in children and young adults and are typically associated with a long history of chronic, often medically intractable, partial seizures with secondary generalization.[1]

DNT represents a distinct clinico‐pathological entity, first described by Daumas‐Duport et al.,[1] which was included into the group of neuronal and mixed neuronal‐glial tumors in the 2007 WHO Classification of Tumors of the Nervous System.[2] , [3] This tumor typically displays specific clinical and neuroimaging characteristics;[4] , [5] , [6] however, it sometimes displays a morphological pattern with the main features of conventional types of gliomas.[7] , [8] In particular, the diagnosis of a non‐specific histopathological form of DNT remains controversial, because it resembles various glial neoplasms and typically lacks specific glioneuronal elements.[7] Occasionally, tumors have been reported to exhibit a mixed pattern, with features of DNT and additional neuronal or glioneuronal components. Two of these mixed tumors include the mixed glioneuronal tumor, with combined morphological features of DNT and the rosette‐forming glioneuronal tumor (RGNT).[9] , [10]

Here, we describe a unique case of a young adult with longstanding intractable epilepsy and a multinodular intracortical DNT lesion of diverse morphology, including pilocytic and RGNT‐like separated nodules.

A 22‐year‐old man was referred to the Neurosurgical Department od Warsaw Medical University with a long history of medically intractable epilepsy. Upon admission, the patient was in good general condition and the neurological examination showed no abnormalities. He had experienced secondary, generalized, tonic‐clonic seizures from the age of 3 years. The epileptic seizures were not well controlled, despite simultaneous treatment with levetiracetam, tiagabine and clonazepam.

The first MRI scan was performed when the patient was 10 years old, due to an episode of transient, right‐sided hemiparesis. The T1‐weighted images revealed a well‐defined hypointense lesion in the left temporal lobe, which was not enhanced after a contrasting agent was injected (Fig. [NaN] A,B). The lesion was assumed to be due to a transient ischemic attack. The follow‐up MRI studies were carried out every few years and the neuroradiological images of the lesion remained stable until the age of 22.

A preoperative MRI was performed 12 years after the initial examination. The T1‐weighted images revealed a cortico‐subcortical lesion the same size as the original lesion in the left temporal lobe. However, preoperative MRI also showed focal contrast enhancement inside the lesion (Fig. [NaN] C,D). The patient underwent left pterional craniotomy. After opening the dura, an enlargement of the middle temporal gyrus was noted. Electrocorticography (ECoG) revealed epileptiform discharges in the anterior and middle parts of the left temporal lobe. A resection was performed to remove the anterior temporal lobe, including the tumor borders, followed by an amygdalohippocampectomy. A post‐resection ECoG demonstrated no abnormalities.

The postoperative course was uncomplicated. The patient was discharged to return home on the 9th day post‐surgery in good condition. At 44 months follow‐up, the patient was seizure‐free, without neurological deficits. No tumor recurrence was evidenced on MRI images up to 39 months after surgery (Fig. [NaN] E,F). To date, neither radiotherapy nor chemotherapy has been required.

Biopsy tissue was fixed in 10% formalin, embedded in paraffin blocks, and stained with HE. Immunohistochemistry was performed on paraffin‐embedded specimens, labeled with the ABC method, with 3‐3′diaminobenzidine (DAB) as a chromogen. The primary antibodies included anti‐GFAP (polyclonal, dilution 1:5000), anti‐synaptophysin (dilution 1:200), anti‐neurofilament protein (dilution 1:100), anti‐S‐100 protein (polyclonal; dilution 1:800), and anti‐Ki67 antigen (clone MIB‐1, dilution 1:100). All antibodies were from Dako, Glostrup, Denmark.

Microscopically, the intracortical cystic lesion was composed of distinct multiple nodules of diverse morphology (Fig. [NaN] A). Several cysts were also visible in the subcortical white matter. Single neurocytic rosettes were found in their surroundings (Fig. [NaN] B).

Only a few, well‐circumscribed cortical nodules exhibited a glioneuronal element specific to DNT (Fig. [NaN] A). Some large neurons appeared to “float” in the microcystic/myxoid matrix (Fig. [NaN] B). The vertically oriented axons were lined by small cells with round nuclei and clear cytoplasms, which corresponded to S‐100 protein positive oligodendrocyte‐like cells (Fig. [NaN] C). Similar oligodendrocyte‐like cells were also arranged around small vessels, creating distinct, perivascular pseudorosettes (Fig. [NaN] D). Some areas with increased cellularity exhibited an oliodendroglioma‐like appearance with a characteristic pattern of delicate capillaries (Fig. [NaN] E,F). Several large, well‐circumscribed nodules were composed entirely of compact piloid astroglial tissue with a microcystic and myxoid background (Fig. [NaN] A), eosinophilic granular bodies (EGB) (Fig. [NaN] B) and a few Rosenthal fibers. The pilocytic cells were strongly immunoreactive for GFAP (Fig. [NaN] C). This morphological picture was identical to the biphasic pattern of a pilocytic astrocytoma.

Additionally, some areas comprised structures formed by neurocytes arranged around an eosinophilic neuropil core that closely mimicked the rosette glioneuronal element typical for a RGNT (Fig. [NaN] D). They were embedded in a GFAP‐positive background (Fig. [NaN] E) and exhibited slight synaptophysin immunoreactivity within the neuropil core (Fig. [NaN] F). Similar neurocytic rosettes were scattered through areas that appeared to be DNT (Fig. [NaN] G) and piloid tissues (Fig. [NaN] H).

The lesion was associated with advanced reactive gliosis in the adjacent cerebral cortex and white matter. Focal dysplastic disorganization of the cerebral cortex cytoarchitecture was observed, which corresponded to focal cortical dysplasia, type 3C, in the adjacent brain tissue. Heterotopic neurons were observed in the white matter.

DNTs are rare neoplasms with characteristic clinical and morphological features.[11] , [12] , [13] DNT is typically located in the supratentorial cortex, involving mostly the temporal and frontal lobes. The intracortical topography is readily demonstrated with MRI, which is very useful for the diagnosis of DNT in cases with inconclusive pathologic findings.[4] Very rarely, DNT lesions occur in an extracortical location, most often in the periventricular region.[14] , [15] , [16] There are also reports of DNTs located in the cerebellum[17] , [18] , [19] and brainstem.[20]

DNT is generally considered a benign, stable lesion with an indolent clinical course and favorable prognosis, even after subtotal surgical resection without adjunctive therapy.[1] However, it is important to completely resect epileptogenic lesions to achieve a seizure‐free outcome.[21] , [22] , [23] Occasionally, DNTs in children may behave aggressively, with re‐growth of the primary or residual lesion.[24] Malignant transformations might be suspected when dynamics of the DNT lesion are documented on MRI findings.[25]

There are three specific histological variants of DNT, known as simple, complex and non‐specific.[7] , [26] Simple and complex forms of DNT are readily identified, but the diagnosis of the non‐specific variant remains controversial. The simple DNT type is composed mostly or entirely of a specific glioneuronal element formed by vertically oriented axons, which are lined by small oligodendrocyte‐like cells, and they contain so‐called “floating neurons”. The “floating” neurons were originally considered as a constant cellular component of the specific glio‐neuronal element of DNT.[1] However, not all DNTs show the presence of “floating” neurons. On the other hand, a recent immunohistochemical and morphometric study have suggested that neurons in the simple form of DNT are in fact entrapped pre‐existing neurons.[27] There are controversies also related to the so‐called oligodendroglioma‐like cells (OLCs) of DNT. As the glial nature of OLCs has been postulated, DNT might be regarded as a pure glial tumor, corresponding to the diagnosis of non‐infiltrating oligodendroglioma (grade I).[27] , [28] Such consideration seems to confirm a close link between DNT and oligodendroglioma but it requires further molecular studies.

The complex variant of DNT is characterized by multinodular architecture, and it includes nodules that resemble astrocytomas, oligodendrogliomas and oligoastrocytomas. This variant also displays a specific glioneuronal element and foci of cortical dysplastic disorganization. The glioneuronal element strongly supports the diagnosis of DNT, but it is not always present.[29] The recently introduced “non‐specific form” of DNT does not contain this peculiar element. Also, it histologically resembles a conventional glioma or glioneuronal tumor.[30] In those cases, the diagnosis of DNT should be based mainly on clinico‐neuroradiological criteria.[7] Of course, such a claim may raise reasonable controversy; thus the diagnosis of nonspecific DNT is not widely accepted. However, such a diagnosis might be considered in special cases of young adults with long‐term epilepsy. The diagnosis of tumor with indolent clinical behavior allows the avoidance of aggressive medical treatment. The patient must remain under careful post‐operative care, including frequent MRI control studies.

A diagnosis of DNT should also consider various other tumors with neuronal and glial components that are included in a heterogeneous group of mixed neuronal‐glial tumors of the CNS.[8] , [31] , [32] Occasionally, unusual compound tumors have been reported, which show features characteristic of DNT and pleomorphic xanthoastrocytoma[33] or ganglioglioma.[34] A recent study described also a mixed glioneuronal tumor with histological features of DNT and RGNT.[9] , [10]

In the present case, both clinical and histopathological features fulfilled the criteria for the diagnosis of a DNT. The patient was young, had a long clinical history of early onset, experienced partial seizures with secondary generalization, but did not have other neurological deficits or progression. The control neuroimaging studies demonstrated a stable cortical temporal lobe lesion and only the last preoperative MRI evidenced a slight focal contrast enhancement.

Histopathologically, the tumor described here exhibited a distinct, multinodular architecture. We observed diagnostically important, specific glioneuronal elements of DNT, accompanied by various glial and glio‐neuronal nodules, which corresponded to the complex form of DNT. A few, well‐circumscribed nodules were composed entirely of pilocytic astroglial tissue with a biphasic pattern; they also contained EGB and Rosenthal fibers, identical to a pilocytic astrocytoma. Previous studies have only occasionally described a DNT with predominant pilocytic components[35] and those were most frequently observed in recurrent lesions.[36] , [37] Additionally, the present case displayed well‐defined areas where neurocytic cells were arranged around a synaptophysin‐positive neuropil; those areas were identical to the rosette‐forming glioneuronal elements found in RGNT. Similar rosette‐like arrangements have been occasionally detected in DNT, but to date, only two cases studies have described DNTs with distinct RGNT elements.[9] , [10] The first case study reported findings from an autopsy of an older man with a glioneuronal tumor of the third ventricle composed of multiple foci with histological features of DNT and RGNT. The lesion was considered as a “collision” of two independent tumors of their unusual anatomic location, or alternatively, as a DNT variant that exhibited rosette formations more characteristic of a RGNT.[10] The second case study reported a mixed glioneuronal tumor that exhibited histological features of a DNT and a RGNT; however, these two different components were intermingled with no obvious boundaries.[9] The lesion was considered an unusual variant of DNT that coexisted with a RGNT in the same tumor. In contrast, in the present study, the tumor displayed quite well‐separated nodules composed of distinct components that corresponded to a pilocytic astrocytoma and a RGNT. Moreover, the background of piloid features predominated throughout the entire lesion.

RGNT represents a separate clinico‐pathologic entity with evidence of glial and neuronal differentiation.[2] , [38] , [39] , [40] It is a low‐grade, slowly growing neoplasm that occurs in young adults and shows indolent behavior, but no tendency for recurrence. It was originally described as a tumor of the fourth ventricle that involves the cerebellum; however, tumors with similar morphology were also described in the pineal gland, hypothalamus and spinal cord.[41] , [42] Its histology consists of neurocytes and distinctive rosettes associated with a glial component. Because neuronal and glio‐neuronal tumors are similar in biological behavior and morphology, they are thought to share a common ontogeny and biology.[43]

The differential diagnosis of our case ought to include a cerebral example of RGNT. However, the presented tumor should be considered as a DNT due to its distinct multinodular architecture, documented both on MRI and histopathological studies. Although RGNT might be cystic and might contain DNT‐like areas with a linear arrangement of OLCs, it usually does not demonstrate the “floating” neurons.

The current case described a DNT with the combined histological features of a DNT, a pilocytic astrocytoma and a RGNT. These findings supported the notion that DNTs have heterogeneous morphology and some features that overlap with other neuronal‐glial tumors. Thus, compound tumors with predominantly pilocytic and rosette‐forming glioneuronal patterns might be alternatively regarded as an additional histological variant of a DNT. Both DNTs and RGNTs are slowly growing neoplasms that preferentially affect young adults, and both are composed of distinct neurocytic and glial components. Their histogenesis remains unresolved, but it should be considered that these glioneuronal entities may have a common origin.

Graph: MRI. (A,B) The first MRI scan. A well‐defined hypointense lesion on T1‐weighted images in the left temporal lobe (white arrows), without enhancement after contrast injection. (C,D) The preoperative MRI; 12 years after initial examination. T1‐weighted images. A cortico‐subcortical lesion in the left temporal lobe of the same size (white arrows) with a focal enhancement after contrast administration inside the lesion (black arrows). (E,F) Follow‐up MRI obtained 16 months after the surgery. No tumor remnant or recurrence was found.

Graph: Histopathology. (A) Intracortical cystic lesion composed of distinct multiple nodules of diverse morphology. HE. (B) Several cysts in the subcortical white matter with a few neurocytic rosettes (arrows). HE. Bars: A, 500 μm, B, 100 μm.

Graph: Histopathology of cortical nodules. (A) Cortical nodule that resembles a specific glioneuronal element of a dysembryoplastic neuroepithelial tumor (DNT). HE. (B) A few large neurons, which appear to “float” within a myxoid matrix, HE. (C) S‐100 protein positivity in small oligodendrocyte‐like cells. (D) Perivascular arrangement of oligodendrocyte‐like cells. HE. (E) Oliodendroglioma‐like appearance with characteristic pattern of delicate capillaries. HE. (F) Nodule with increased cellularity and oliodendroglioma‐like appearance. HE. Bars: A, C, D, 100 μm, B, E, F, 50 μm.

Graph: Histopathology and immunohistochemistry of pilocytic and rosette‐forming glioneuronal tumor (RGNT) components. (A) Glial nodule composed entirely of compact piloid astroglial tissue with a microcystic and myxoid background. HE. (B) Pattern of pilocytic astrocytoma with an eosinophilic granular body (arrow). HE. (C) Pilocytic cells with strong immunoreactivity for GFAP. (D) Area comprised of structures formed by round neurocytes arranged around an eosinophilic neuropil core, typical for a RGNT. HE. (E) GFAP immunoreactivity in an astroglial background. (F) Synaptophysin expression in the neuropil core of the rosettes. (G) Distinct neurocytic‐like rosette in the piloid tissue with Rosenthal fibres. HE. (H) Neurocytic rosettes surrounded by glioneuronal element of dysembryoplastic neuroepithelial tumor (DNT). HE. Bars: A, C, 100 μm, B, D‐H, 50 μm.