Infantile posterior fossa atypical teratoid/rhabdoid tumor: navigating diagnostic complexity and molecular paradigms

Atypical teratoid/rhabdoid tumor (AT/RT) is an exceptionally aggressive, SWI/SNF-deficient embryonal neoplasm that poses profound diagnostic and therapeutic challenges, particularly in infants. We present the case of a female infant under 1 year of age harboring a massive, cystic-solid AT/RT of the posterior cranial fossa. The clinical trajectory was complicated by rapid-onset obstructive hydrocephalus, necessitating urgent cerebrospinal fluid diversion prior to gross total microsurgical resection. Definitive diagnosis was established via comprehensive neuropathological evaluation, highlighting a robust rhabdoid histomorphology, an elevated Ki-67 index, and the pathognomonic complete loss of SMARCB1 (INI1) nuclear expression. This report meticulously details the morphological and immunohistochemical profiling required to differentiate AT/RT from medulloblastoma and ependymoma in the infantile demographic. Furthermore, we contextualize these findings within the 2021 WHO Classification of CNS Tumors, discussing the imperative of identifying specific molecular subgroups (TYR, SHH, MYC) and the necessity of screening for Rhabdoid Tumor Predisposition Syndrome. Finally, we review contemporary adjuvant strategies, emphasizing the transition from conventional neurotoxic regimens toward targeted epigenetic therapies, such as EZH2 inhibition, in the pediatric population.