Case Report: Cerebellar ALK+ ALCL: diagnostic challenges and therapeutic innovation.

Primary anaplastic lymphoma kinase (ALK)-positive anaplastic large cell lymphoma (ALCL) of the central nervous system (CNS) is an exceedingly rare T-cell malignancy, with only a few adult cases reported worldwide. To our knowledge, this represents the first reported adult case of cerebellar ALK+ ALCL from China, highlighting both diagnostic challenges and therapeutic innovations in neuro-oncology. We present a 35-year-old male with subacute cerebellar syndrome initially misdiagnosed as infectious encephalitis due to overlapping clinical, radiological, and cerebrospinal fluid (CSF) findings. Progressive neurological deterioration despite empiric antimicrobial therapy prompted histopathological evaluation, which confirmed ALK+ ALCL with a high Ki-67 index (60%). Targeted therapy with the CNS-penetrant ALK inhibitor alectinib induced rapid clinical improvement and significant radiographic regression. This case highlights the diagnostic pitfalls of CNS ALCL, emphasizes the necessity of early biopsy in atypical lesions, and demonstrates the transformative potential of molecularly targeted therapies for rare neuro-oncological malignancies.