Case: AP267
Contributor's Comment: From this EQAP exercise, it appears that atypical teratoid/rhaboid tumor (AT/RT) is still an underrecognized entity, with a significant proportion of participants diagnosing the tumor as "ependymoma".
As defined in the WHO classification, AT/RT is a highly malignant CNS tumor predominantly manifesting in young children, typically containing rhaboid cells, often with primitive neuroectodermal cells and with divergent differnetiation along epithelial, mesenchymal, neuronal or glail lines. At the genetic level, there is inactivation of the INI1/hSNF5 gene in virtually all cases (and thus immunostaining for INI1 to demonstrate lack of expression is an excellent way to support the diagnosis in the correct context).

The tumor can be supratentorial or infratentorial. The morphology is highly variable, and sometimes only one component is seen, especially in small biopsies.The rhabdoid cells may or may not be classical in appearance (the current case is composed almost exclusively of rhabdoid cells). Some cases are composed almost entirely of small primitive neuroectodermal cells.
Epithelial and mesenchymal components may be absent. Because of the varied morphology, a high index of suspicion is required for the correct diagnosis. Consider this possibility for any difficult-to-classify or primitive neuroectodermal tumor of the CNS in young patients, and order INI1 immunostaining for confirmation.