—  SPECIALTY CONFERENCE  —

Neuropathology

Case 5 - Glioblastoma (Epithelioid Variant)

Gregory N. Fuller
University of Texas M.D. Anderson Cancer Center
Houston, TX


Click on each slide thumbnail image for an enlarged view
Clinical History
A 49-year-old woman presented with a solitary, contrast-enhancing mass of the left frontal lobe measuring approximately 3cm in greatest diameter as assessed by MR imaging. An open biopsy was performed.


Case 5 - Figure 1 - H&E-stained tissue section shows an epithelioid neoplasm (x400).
Case 5 - Figure 2 - Diagnostic immunostain (x400)

Diagnosis
Glioblastoma (Epithelioid Variant)

Discussion

Histology
The neoplasm exhibits prominent epithelioid morphology. There is significant variation in the size of the individual tumor cells but the vast majority shows a spherical shape and lack of cytoplasmic processes. The nuclei are generally large, pleomorphic, and often display "macronucleoli." Binucleated and multinucleated tumor cells are present. The tumor is densely cellular, with little discernable background neuropil, and very little of the interface of the tumor with the surrounding brain parenchyma (which might be informative; i.e., sharp vs. infiltrative) is available for examination in this biopsy specimen.

Differential Diagnosis
The differential diagnosis is that of a malignant epithelioid neoplasm and includes metastatic melanoma, metastatic epithelioid carcinoma, metastatic epithelioid sarcoma, primary CNS rhabdoid tumor, and epithelioid glioblastoma [1, 2, 3, 4] . Other densely cellular epithelioid or "plump pink cell" gliomas might also be considered, such as gemistocytic astrocytoma, pleomorphic xanthoastrocytoma, granular cell astrocytoma, subependymal giant cell astrocytoma, lipid-rich epithelioid glioblastoma, and astroblastoma [5, 6, 7, 8, 9] ; however, most of these entities either exhibit unequivocal evidence of glial differentiation in the form of fibrillary eosinophilic cytoplasmic processes or perivascular pseudorosettes, or have other distinctive features, such as the distinctive granular cytoplasm of granular cell astrocytoma or the prominent cytoplasmic clearing of lipid-rich epithelioid glioblastoma.

Immunohistochemistry
Immunohistochemical studies performed on biopsy tissue sections yielded the following results:

Positive for vimentin, S-100 protein and GFAP
Negative for HMB-45, MART-1 and melanoma cocktail
Negative for CAM 5.2, EMA and SMA

Clinical follow-up
Over several months, despite appropriate treatment, the tumor continued to grow, with invasion of the corpus callosum and spread to the contralateral frontal lobe.

Discussion
The immunophenotypic studies confirm a diagnosis of epithelioid glioblastoma (EGM). Epithelioid glioblastoma is not recognized as a distinct entity by the current WHO Classification of Brain Tumours (WHO 2000) [10]; however, relatively pure examples are encountered and can cause considerable anxiety and diagnostic confusion if the pathologist is not familiar with the existence and features of this variant. EGMs are most commonly mistaken for metastatic amelanotic melanoma, and the mimicry can be nearly perfect, especially if the interface with the surrounding brain is not sampled [4]. Frequently, particularly in larger surgical specimens, there are areas of more typical glioblastoma in which the tumor cells have elongated cytoplasmic processes, and diffuse infiltration of the neuropil can be seen; however, in smaller biopsies these features may be absent and a high index of suspicion combined with judicious immunophenotyping is required to confirm the diagnosis. Another common pitfall is to render a diagnosis of metastatic carcinoma based on a combination of the morphology plus immunopositivity for keratins as assessed with an antibody cocktail such as AE1/AE3. A majority of high-grade astrocytomas are strongly positive for AE1/AE3 [11], as are reactive astrocytes. In fact, one bioreagent company is actively promoting AE1/AE3 as an astrocytic marker [12]. In cases in which the differential diagnosis includes both glioma and metastatic carcinoma, a better choice for a carcinoma marker would be CAM 5.2, for which only a small percentage (approximately 4%) of glioblastomas show reactivity. Aides to avoiding the diagnostic trap:

Look for areas where background neuropil is present to see if tumor cells remain in clusters or infiltrate the neuropil as single cells
Search the peripheral areas of densely cellular tumor to see if the interface between the bulk of the tumor and surrounding brain parenchyma is present, and assess the margin for sharp interface vs. diffuse infiltration
Use an appropriate immunostain panel in cases in which the H&E features are equivocal or ambiguous (e.g., GFAP, melanoma cocktail, CAM 5.2)

Morphologic variants of glioblastoma
In addition to the common "garden variety" glioblastoma, which accounts for the vast majority of cases, there are a number of less frequently encountered variants with which the surgical pathologist should be familiar to avoid misdiagnosis. Following is a brief list and description of the most common offenders:

Giant cell GBM
Giant cell glioblastomas exhibit striking pleomorphism and appear to have a distinct molecular ontogeny [10]. The chief entity in the differential diagnosis is pleomorphic xanthoastrocytoma, which shows a comparable degree of pleomorphism but generally lacks the florid microvascular proliferation, necrosis with pseudopalisading, and high mitotic rate of GBM; in addition, PXAs are characterized by prominent eosinophilic granular bodies (EGBs).

Small cell GBM
As the name implies, small cell glioblastomas occupy the opposite end of the cellular size spectrum compared to giant cell glioblastomas. The chief consideration in the differential diagnosis is often anaplastic oligodendroglioma. Given the greatly differing molecular alterations (EGFR amplification in small cell GBM; combined 1p/19q deletions in AO) and survival of these two groups of patients, accurate diagnosis is clearly critical.

Spindle cell GBM
Spindle cell glioblastoma should be distinguished from gliosarcoma and from fibrosarcoma, although all three have grave prognoses. Spindle cell GBM shows at least focal positivity for GFAP and S-100 protein, and does not produce an extracellular reticulin matrix lattice. In contrast, the sarcoma component of gliosarcoma and fibrosarcomas lack expression of GFAP and S-100 protein, and show a dense and finely intermeshed reticulin pattern.

Bland cell GBM
OK, I made this term up. But it reflects a not uncommon problem that many pathologists have in diagnosing those glioblastomas whose cellular constituents are fairly monotonous and nondescript. The fact is that not all glioblastomas are strikingly pleomorphic tumors; some exhibit a rather bland cellular uniformity. This can be somewhat unsettling if not previously encountered; however, in the setting of a diffuse astrocytoma, no matter how cytologically bland or uniform, the presence of mitotic figures, vascular proliferation and/or necrosis warrants a diagnosis of glioblastoma.

Inflammatory GBM
Some glioblastomas show prominent foci of acute inflammatory cell (polymorphonuclear leukocyte) infiltrates [13]. This can be confusing and potentially lead to misdiagnosis as abscess or inflammatory disorder. Close examination of the glial component will usually reveal pleomorphism that is far in excess of that seen in reactive astrogliosis. Some glioblastomas show macrophage infiltration. The prominent presence of macrophages in a CNS lesion is suggestive of demyelinative or inflammatory disease and these disorders must be excluded first; however, macrophages can also be seen in glioblastoma. As with acute inflammatory cell infiltrates in glioblastoma, close examination of the glial component will usually reveal the true nature of the lesion.

Myxoid GBM
Both anaplastic astrocytomas and glioblastomas occasionally exhibit a striking myxoid background that may suggest sarcoma or other myxoid tumor type. The two simply keys to correct diagnosis are an awareness of this variant and the use of a GFAP immunostain if there is uncertainty about the diagnosis.

Epithelioid GBM
Epithelioid glioblastoma is one of the most convincing poseurs. Many examples arise superficially in the temporal or frontal lobe and the superficial location combined with a relatively circumscribed appearance mimics meningioma or metastasis on MR images. In its relatively rare pure form, the resemblance to metastatic amelanotic melanoma is striking and often requires immunophenotyping for confirmation of the diagnosis. In other cases, the epithelioid tumor cells constitute only one component, with transition areas to classical fibrillary glioblastoma and areas of unequivocal diffuse infiltration of brain parenchyma making diagnosis easy. Pure epithelioid glioblastomas are usually positive for GFAP; however, this may be only focal, and large areas of the neoplasm may show reactivity only for vimentin. This is another factor that tends to increase the chances of misdiagnosis if only small tissue samples are obtained by biopsy.

Lipid-Rich Epithelioid GBM
This variant shares with epithelioid glioblastoma a distinctive epithelioid cellular morphology, dense cellularity, frequently focal circumscription from the surrounding brain parenchyma and tendency to be mistaken for a metastasis, but differs by the presence of prominent cytoplasmic lipidization, which imparts a "clear cell" or vacuolated cytoplasmic appearance. Whereas epithelioid glioblastoma strongly resembles amelanotic melanoma, lipid-rich epithelioid glioblastoma resembles balloon cell melanoma.

References

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  2. Prayson RA, Chahlavi A. Metastatic epithelioid sarcoma to the brain: palisaded necrosis mimicking glioblastoma multiforme. Ann Diagn Pathol. 6:302-6, 2002.
  3. Fuller GN, Goodman JC, Vogel H, and Ghorbani R. Epithelioid glioblastoma: a distinct clinicopathologic entity. J Neuropathol Exp Neurol 57:501, 1998 [abstract].
  4. Fuller GN and Goodman JC. Practical Review of Neuropathology. Philadelphia: Lippincott Williams & Wilkins, 2001, p154.
  5. Rosenblum MK, Erlandson RA, Budzilovich GN. The lipid-rich epithelioid glioblastoma. Am J Surg Pathol. 15:925-34, 1991.
  6. Castellano-Sanchez AA, Ohgaki H, Yokoo H, Scheithauer BW, Burger PC, Hamilton RL, Finkelstein SD, Brat DJ. Granular cell astrocytomas show a high frequency of allelic loss but are not a genetically defined subset. Brain Pathol. 13:185-94, 2003.
  7. Brat DJ, Scheithauer BW, Medina-Flores R, Rosenblum MK, Burger PC. Infiltrative astrocytomas with granular cell features (granular cell astrocytomas): a study of histopathologic features, grading, and outcome. Am J Surg Pathol. 26:750-7, 2002.
  8. Port JD, Brat DJ, Burger PC, Pomper MG. Astroblastoma: radiologic-pathologic correlation and distinction from ependymoma. Am J Neuroradiol. 23:243-7, 2002.
  9. Brat DJ, Hirose Y, Cohen KJ, Feuerstein BG, Burger PC. Astroblastoma: clinicopathologic features and chromosomal abnormalities defined by comparative genomic hybridization. Brain Pathol.10:342-52, 2000.
  10. Kleihues P, Cavenee WK, eds. Pathology and Genetics of Tumours of the Nervous System, 2nd edition. New York: Oxford University Press, 2000.
  11. Oh D, Prayson RA. Evaluation of epithelial and keratin markers in glioblastoma multiforme: an immunohistochemical study. Arch Pathol Lab Med.123:917-20, 1999.
  12. Martin CA, Badran AF. AE1/AE3 as a marker for human and mouse astrocytes. Connection (DakoCytomation) 6:4, 2003.
  13. Folkerth RD, Frosch MP, De Girolami U. "Inflammatory" gliomas: a diagnostic dilemma. J Neuropathol Exp Neurol 58:510, 1999 [abstract].