Primary Central Nervous System Lymphoma: A Case Report of the Neurological Great Mimicker.

Primary central nervous system lymphoma (PCNSL) is a rare lymphoproliferative disorder that primarily affects immunocompromised individuals. It presents with diverse neurological phenotypes, making its diagnosis challenging and frequently delayed. Clinical heterogeneity is compounded by the limitations of available diagnostic methods, and the use of corticosteroids can mask imaging and histological characteristics, further increasing diagnostic delay. We describe a case of a 68-year-old woman with a history of Hashimoto's thyroiditis who presented with rapidly progressive dementia (RPD). Initial testing revealed positive anti-thyroglobulin antibodies, leading to a suspicion of steroid-responsive encephalopathy associated with autoimmune thyroiditis. She was treated with corticosteroids, without clinical improvement. Brain magnetic resonance imaging (MRI) showed supratentorial hyperintense lesions on T2-weighted fluid-attenuated inversion recovery (T2/FLAIR) sequences without diffusion restriction. Additionally, a nodular cerebellar lesion exhibited contrast enhancement and restricted diffusion. Cerebrospinal fluid (CSF) analysis, including cytology and flow cytometry, was unremarkable. The first brain biopsy performed four months after corticotherapy was inconclusive, revealing a non-specific inflammatory pattern. Only a second brain biopsy, performed nine months after corticosteroid therapy, confirmed the diagnosis of PCNSL.