Primary large B-cell lymphoma of the central nervous system mimicking inflammatory myelitis at initial presentation.

Primary central nervous system large B-cell lymphoma (PCNS-LBCL) is a rare extranodal non-Hodgkin lymphoma that typically affects the brain parenchyma. Spinal cord involvement as an initial manifestation is extremely uncommon and poses considerable diagnostic challenges, often mimicking inflammatory demyelinating diseases. We report the case of a man in his late 50s who initially presented with steroid-responsive cervical myelitis, resulting in a provisional diagnosis of multiple sclerosis. Despite treatment with corticosteroids and ofatumumab, the patient experienced neurological deterioration and developed cerebellar lesions. Stereotactic brain biopsy confirmed PCNS-LBCL. Although the initial spinal lesion may have represented a definite lymphoma, it was also considered a sentinel lesion, a non-neoplastic inflammatory precursor occasionally observed in PCNSL. This case highlights the diagnostic complexity of PCNS-LBCL presenting with isolated spinal involvement and steroid responsiveness, both of which may delay appropriate treatment. Notably, cerebrospinal fluid analysis at the time of initial presentation of spinal lesions revealed elevated interleukin-10 levels, which declined substantially following chemotherapy, correlating with clinical improvement. Clinicians should maintain a high index of suspicion for central nervous system lymphoma in cases of relapsing myelitis unresponsive to conventional immunotherapy and consider timely biopsy when clinically feasible. Heightened awareness of sentinel lesions may facilitate early recognition and improve outcomes in patients with PCNSL.