Intraparenchymal clear cell ependymoma.

Clear cell ependymoma (CCE) is an uncommon variant of ependymoma having a predilection for the supratentorial region. Histologically, it bears an uncanny resemblance to oligodendroglioma, central neurocytoma, hemangioblastoma and metastasis from clear cell carcinoma. Here, we report a rare case of clear cell ependymoma in a 45-year-old male, which histomorphologically resembled anaplastic oligodendroglioma on intraoperative smears, frozen section and routine light microscopy. Immunohistochemistry, however, helped to arrive at the correct diagnosis. Unlike other clear cell tumors of the brain parenchyma, CCE is known to follow an aggressive course and, hence, obtaining a correct diagnosis is imperative since it has a direct therapeutic and prognostic connotation.

Introduction

Ependymomas are slow growing tumors of childhood and young adults, accounting for approximately 2–9% of all neuroepithelial tumors and 6–12% of all intracranial tumors in children. Most cases are infratentorial in location, with fourth ventricle and cervical spine being the favored sites. Supratentorial parenchymal ependymomas arising outside the ventricular system are rare entities and is encountered more in the pediatric age group.[1]

Of the different variants of ependymomas, cellular and papillary are most common, while clear cell and tanycytic ependymomas are uncommon and constitute rare case reports or small series only. Unlike most ependymomas, clear cell ependymomas (CCE) are associated with an aggressive behavior and early recurrence despite gross total resection, and needs more vigorous management.

We report a rare case of an intraparenchymal clear cell ependymoma in an adult male, which mimicked anaplastic oligodendroglioma on cytology and histology, thus necessitating the usage of immunohistochemistry for definitive diagnosis.

Case Report

A 45-year-old right-handed male presented with partial seizures involving right upper limb and right side of face with progressive aphasia of three months duration. He was a known diabetic on regular medication and satisfactory control. There was no history of headache or dimness of vision. He was afebrile and hemodynamically stable. His neurological examination revealed normal higher mental functions along with Broca's aphasia. There was no associated sensory, cerebellar or gait abnormality.

Magnetic resonance imaging (MRI) and computerized tomography (CT) scan showed a mass lesion involving the left frontotemporal region and adjoining insular cortex along with areas of calcification, necrosis and solid elements. The lesion caused significant mass effect with compression of the left lateral ventricle with perilesional edema. The patient underwent left parietal craniotomy and debulking surgery. Intraoperatively, a fleshy moderately vascular soft tumor was seen with ill-defined margins within the brain parenchyma.

Microscopy

Intraoperative cytology (touch imprint and squash preparations) showed a hypercellular smear composed of large number of delicate vascular channels along with multiple clusters of monomorphic round to oval cells containing fine fibrillary processes. Most of the tumor cells had eosinophilic cytoplasm with high nuclear cytoplasmic ratio and conspicuous nucleoli. Cells were devoid of nuclear pleomorphism and appreciable mitotic activity. Occasional reactive astrocytes were noted [Figure1a–d]. Based on this, a diagnosis of low-grade glial tumor favoring oligodendroglioma was offered. However, owing to the presence of focal ill-formed peritheliomatous distribution of tumor cells a differential diagnosis of ependymoma was contemplated.

Figure 1

(a-d): Photomicrograph of cytology smears showing a hypercellular smear (a: Pap, ×100) composed of clusters of monomorphic round to oval cells containing fine fibrillary processes (b: MGG ×200). Focal ill-defined peritheliomatous arrangement present (c: MGG ×100). Most of the tumor cells had eosinophilic cytoplasm with a high N:C ratio and conspicuous nucleoli (d: MGG ×400)

Histopathological examination

All the tumor bits were processed and 5-mm thick sections were cut from routine formalin-fixed, paraffin-embedded tissue, and stained by hematoxylin and eosin (H and E) stain. Immunohistochemistry (IHC) was done using streptavidin-biotin immunoperoxidase technique (Envision Kit, M/s. Dakopatts, Denmark) using monoclonal antibodies to glial fibrillary acidic protein (GFAP), vimentin, S-100 protein, and epithelial membrane antigen (EMA), while the proliferation index was evaluated using a MIB-1 antibody (all antibodies were prediluted and obtained from M/s. BioGenex, USA).

Microscopic examination of the resected specimen showed a clear cell tumor with cells predominantly arranged in sheets and nests, without any unequivocal areas of perivascular pseudorossettes or ependymal rosettes. Individual tumor cells were round to oval with clear cytoplasm, having round hyperchromatic nuclei and inconspicuous nucleoli. Focally, cells showed mild anisonucleosis, nuclear atypia and increased mitosis (4-6/hpf). Areas of reactive gliosis, calcification and endothelial proliferation were also present. There was no area of necrosis in the tumor. Based on this a diagnosis of anaplastic oligodendroglioma (WHO grade III) was opined [Figures2a–d].

Figure 2

(a-g) Photomicrograph of histology sections of clear cell ependymoma showing sheets of clear cells (a: H and E, ×200) with large areas of calcification (b: H and E ×100) and focal peritheliomatous pattern (c: H and E, ×100) and brisk mitotic activity (d: H and E, ×400). IHC highlighted GFAP-immunoreactive cell processes in the perivascular location (e: GFAP, ×100) with tumor cells displaying immunopositivity for vimentin (f: vimentin, ×400) and EMA (g: EMA, ×400)

However, owing to the presence of an ill-formed peritheliomatous distribution of cell, along with vascular channels with endothelial proliferation, IHC was requisitioned, which showed immunoreactivity for GFAP, strong positivity for vimentin and dot positivity for EMA [Figures2e–g]. MIB-1 labeling index was 12%. This confirmed the diagnosis of intraparenchymal clear cell ependymoma (WHO grade III).

Discussion

In 1983 Kawano et al, reported a cystic tumor with clear cells having a central round nuclei and perinuclear halo, which on immunohistochemistry and electron microscopy revealed to be ependymal in nature.[2] Subsequently, they encountered three more similar cases, which they termed as ‘clear cell ependymoma’.[2] CCE was subsequently included in the World Health Organization classification in 1993, as a distinct variant.

Most cases of CCE have been reported to affect the infratentorial compartment, with predilection for the fourth ventricle and cervical spine. Intraparenchymal location in the supratentorial compartment of this rare variant of ependymoma is unusual and is known to pose significant diagnostic challenge, even on histology.[1-6]

Patients of supratentorial ependymomas show focal neurological deficits, seizures and intracranial hypertension, similar to the present case, while spinal ependymomas generally present with motor and sensory deficits.[1]

Though intraoperative cytology has proved to be of immense value in central nervous system tumors, but ependymal tumor owing to its rarity may pose significant diagnostic challenge, especially if it occurs in an intraparenchymal location and is of an anaplastic variety.[7] A retrospective study evaluating 4100 cases of intraoperative cytologic preparations noted an overall accuracy in diagnosis (excluding grading) of 95%, with the most frequently misdiagnosed lesions in decreasing order being ependymoma, glioblastoma, metastatic carcinoma, oligodendroglioma, meningioma and astrocytoma.[8] However, since an ependymoma is managed by gross total resection, it is important to differentiate it from other glial tumors.

Cytology of a low-grade ependymoma is typically characterized by monomorphic round cells with salt-and-pepper chromatin, and conspicuous micronucleoli within a fibrillary background. Arrangement of tumor cells encasing blood vessels, and variably separated by perivascular cell-free zones of fibrillary processes (perivascular pseudorosettes) are diagnostic of ependymal tumor, especially in the clinical context of a ventricular or periventricular location.[9] However, in case the tumor has an intraparenchymal location, occurs in an adult, contains cells having an eosinophilic cytoplasm and sparse cytoplasmic processes, distinguishing it from a glial tumor may become difficult, as was evidenced in this case. Further, the problem gets multiplied in high-grade ependymoma where despite the presence of high vascularity, the perivascular arrangement of cells may not be readily apparent on cytology.[9]

Intraparenchymal CCE may continue to pose diagnostic dilemma even on conventional histology where it is characterized by sheets of uniform tumor cells with perinuclear halo, centrally located rounded nuclei, and occasional perivascular pseudorosettes.[10] Presence of an arborizing vasculature and large areas of calcification, as present in the index case, may simulate an oligodendroglial tumor, while the clear cell morphology warrants differentiation from hemangioblastoma, oligodendroglioma, neurocytoma, or metastasis of renal cell carcinoma.

Detailed workup with a panel of immunohistochemistry markers consisting of GFAP, vimentin, S-100 and EMA is essential for proper diagnosis.[1] On IHC, ependymal tumors are characterized by GFAP-immunoreactivity in the fibrillary matrix, especially in the perivascular location, along with a cytoplasmic punctate, dot-like EMA positivity.[1] Ultrastructural morphology consisting of intracellular intermediate filaments, desmosomes, occasional cilia, and microvilli present within microlumina and on the cell surface are diagnostic of ependymal differentiation.[1] Recently studies have also reported that the chromosomal aberrations in supratentorial clear cell ependymomas are different from that of other ependymomas and may have a bearing on prognosis.[4]

Conclusion

Supratentorial ependymoma is a rare entity and the clear cell variant in this location, is exceptional. Published reports of cytology of CCE are sparse in the available literature. Owing to the unusual location and morphology, identifying this entity may be a diagnostic challenge not only on cytology during intraoperative consultation but also on routine histology. Since proper diagnosis has a therapeutic and prognostic bearing, workup of these cases need to be supplemented by a complete panel of immunohistochemical markers for an accurate diagnosis.