FINAL DIAGNOSIS : CELLULAR SCHWANNOMA
Schwannoma is infrequently located intracranially, forming approximately 8% of all primary intracranial neoplasms with an affinity for the cerebellopontine angle region . Cellular schwannoma, described by Woodruff in 1981 , is considered a variant of schwannoma and comprising about 10% of all schwannomas .
The most common location of cellular schwannoma is the paravertebral region with the sacral site constituting 64% of all neoplasms . The intracranial location accounts for 8% of all cellular schwannomas. The retrobulbar region is an extremely uncommon site for this tumor and to the best our knowledge, our current case forms the fourth case after those described by Casadei et al  and Huang et al .
Cellular schwannoma is generally encapsulated and sometimes associated with a nerve . On histology, unlike classical schwannoma, cellular schwannoma discloses a markedly increase in cellularity, comprising fascicles of spindle cells which can occasionally be associated with herringbone or storiform pattern. Compact and hypercellular fascicles recapitulating Antoni-A areas can be identified. Although classical Verocay bodies are seldom identified, there may be occasional suggestion of palisades [3-6]. Antoni-B areas are, however, not prominently featured . The spindle cells may exhibit mild nuclear atypia [4, 6]. Thick-walled blood vessels usually displayed in classic schwannoma are also present in the cellular variant [3-4]. Microscopic areas of necrosis may be observed in cellular schwannoma, but geographic necrosis seen in malignant peripheral nerve sheath tumor (MPNST) is absent [3,6,7]. Mitotic activity usually does not exceed 4 per 10 high power fields [3-5] although in plexiform cellular schwannoma in childhood, a mitotic index of up to 31 per 10 high power fields has been reported .
Cellular schwannoma is strongly positive for S100 protein and vimentin . Glial fibrillary acidic protein (GFAP) is variably positive [3-6]. The striking feature seen ultrastructurally is the presence of numerous tightly packed intertwined cell processes  as identified in our case. Primitive intercellular junctions and cell membranes abutting stroma and covered by well developed continuous and occasionally duplicated basement membranes, are also present. Luse bodies may be absent.
In the retrobulbar region, a diagnosis of meningioma may be considered, since the tumor cells in a cellular schwannoma may form whorls . However the presence of areas resembling Antoni A in association with a negative staining for EMA militates against a diagnosis of meningioma. Glial tumors enter the differential diagnosis given the findings of GFAP positivity within the tumor. However careful discernment of Antoni A-like areas and ultrastructural findings on electron microscopy in conjunction with the clinical and radiological findings favor a diagnosis of cellular schwannoma in our case.
Cellular schwannoma may be mistaken for malignant peripheral nerve sheath tumor (MPNST) due to its association with increased cellularity, nuclear atypia, mitotic activity, necrotic foci, bony erosion, and tumor recurrence [3-6]. However, a MPNST  is usually more cellular, associated usually with a higher degree of anaplasia, lacks the thick walled hyalinised blood vessels and usually demonstrates only focal S-100 positivity on immunohistochemistry. In the orbital region, a solitary fibrous tumor has been confused for a schwannoma .
While metastases and death have not been described in cellular schwannoma [3-6], recurrences in incompletely resected cellular schwannoma have been documented, especially in sites where complete tumor resection is difficult to achieve (e.g. intraspinal or intracranial) [3, 4]. In such instances, mitotic count significantly correlates with the incidence of tumor recurrence  and close follow up is advocated.
Contributed by Hong Wui Tan, MRCPath, Seng Geok Nicholas Goh, FRCPA, Wai Ming Yap, FRCPath, Khoon Leong Chuah, FRCPA