Epithelioid glioblastoma
Epithelioid glioblastoma is a variant of glioblastoma (along with gliosarcoma and giant cell glioblastoma) only recently added to the WHO classification of CNS tumors as part of the 2016 update .
Terminology
Whether or not epithelioid glioblastomas are distinct from rhabdoid glioblastomas is at present unclear, however, the latter term should be avoided and is not recognized in the current WHO classification .
It is important to note that true epithelial differentiation of glioblastomas (typically squamous) is very rarely seen in adults but is distinct from epithelioid glioblastoma .
Epidemiology
Unlike run-of-the-mill glioblastomas that are usually encountered in older adults, epithelioid glioblastomas have a predilection for young adults and children .
Pathology
Epithelioid glioblastoma are WHO grade IV tumors . Interestingly, they sometimes co-exist with pleomorphic xanthoastrocytomas, however, their exact relationship is at present unclear .
Microscopic appearance
These tumors are heterogeneous with large epithelioid cells that have abundant eosinophilic cytoplasm, vesicular chromatin, and prominent nucleoli. These cells are reminiscent of melanoma cells. Rhabdoid cells are also sometimes encountered .
Immunophenotype
Immunohistochemistry demonstrates a mixture of astrocytic and epithelial markers :
Genetics
Approximately 50% of cases of epithelioid glioblastoma have BRAF V600E mutations .
Radiographic features
Epithelioid glioblastomas most frequently present as diencephalic or less frequently superficial cerebral hemispheric masses . Hemorrhage and leptomeningeal seeding are probably fairly common at the time of diagnosis .
These tumors are indistinguishable for glioblastoma on imaging, with enhancement, necrosis and diffusion restriction present in the vast majority of tumors .
Treatment and prognosis
Epithelioid glioblastomas are aggressive tumors with poor prognosis, even worse than ordinary glioblastomas . In one series the median survival was only 169 days .
These tumors also sometimes demonstrate systemic metastases, a very uncommon occurrence for other primary brain tumors .
Differential diagnosis
In children the main differential is atypical teratoid / rhabdoid tumor (AT/RT), distinguished by universal lack of INI1 expression in AT/RT .
