Atypical Teratoid/Rhabdoid Tumor of the Posterior Fossa Mimicking Medulloblastoma in an Infant: A Case Report.

Atypical teratoid/rhabdoid tumor (AT/RT) is a rare and highly aggressive central nervous system (CNS) neoplasm that predominantly affects children under three years of age. In the posterior fossa, AT/RT frequently mimics medulloblastoma (MB) on both radiological and histopathological evaluation. Owing to significant morphological overlap, a definitive diagnosis cannot be established on histology alone and requires immunohistochemical (IHC) confirmation, particularly demonstration of loss of nuclear integrase interactor 1 (INI1) expression. We report the case of a seven-month-old male infant who presented with signs of intracranial hypertension and rapidly progressive symptoms attributable to posterior fossa mass effect. Neuroimaging revealed a posterior fossa mass causing marked compression of the fourth ventricle and brainstem, resulting in obstructive hydrocephalus. Magnetic resonance imaging demonstrated heterogeneous contrast enhancement, initially suggestive of MB. Cerebrospinal fluid diversion was performed, followed by tumor biopsy and partial surgical resection. Although intraoperative findings and initial histopathological examination were suggestive of MB, subsequent IHC analysis demonstrated complete loss of nuclear INI1 expression, confirming the diagnosis of AT/RT. The patient subsequently received adjuvant chemotherapy. AT/RT should be strongly considered in infants presenting with posterior fossa tumors, particularly when radiological and histopathological features resemble MB. Early tissue diagnosis and routine use of INI1/SMARCB1 IHC are essential for accurate differentiation from morphologically similar entities, ensuring appropriate therapeutic management and prognostic assessment.