Primary CNS Lymphoma

Primary central nervous system lymphoma (PCNSL) is an aggressive extranodal diffuse large B-cell lymphoma (DLBCL) confined to the brain, leptomeninges, eyes, or spinal cord. “Primary” means that the lymphoma arises in the central nervous system without systemic lymphoma at presentation. Most cases are of the activated-B-cell molecular subtype. The tumour’s location explains its urgency. A relatively small lesion can interfere with motor pathways, language networks, cognition, cerebrospinal-fluid circulation, or seizure thresholds. Symptoms therefore reflect both the anatomical site and the pressure or inflammation produced by the lesion. PCNSL is unusually sensitive to corticosteroids. Steroids can trigger lymphoma-cell apoptosis and cause striking short-term shrinkage on imaging. That response can look like a cure, but it is not definitive treatment. More importantly, steroid exposure before biopsy may destroy or alter the abnormal lymphoid tissue enough to leave the pathologist with a nondiagnostic specimen. The apparent “ghost tumour” is a diagnostic problem, not reassurance.