Corticosteroid response and biopsy decisions in tumefactive demyelination and primary central nervous system lymphoma

Distinguishing primary central nervous system lymphoma (PCNSL) from tumefactive demyelination becomes especially difficult after corticosteroid treatment. Both conditions may improve clinically and radiologically, while corticosteroids can reduce lymphoma cellularity, alter enhancement, and change the tissue available for sampling. The biopsy literature reflects this time dependence. Meta-analysis shows a higher average risk of nondiagnostic tissue after prebiopsy exposure, yet contemporary cohorts report high diagnostic yields in selected patients when enhancement persists and current imaging guides prompt sampling. We organize the evidence around four time-linked clinical questions. Before corticosteroids (T0), clinicians establish the competing probabilities and preserve baseline anatomy. After treatment (T1), they measure the anatomical and clinical transformation. Before biopsy (T2), current imaging should determine whether a representative target remains visible, safely accessible, and sufficiently sampleable. A persistent useful target supports prompt, imaging-updated biopsy; marked regression calls for planned short-interval imaging to recover a representative target. Cerebrospinal fluid biomarkers can modify probability, although single-marker negativity is weak and composite results remain prevalence- and platform-dependent. After biopsy (T3), pathology must fit the sampled site and subsequent course. A major verified inconsistency should reopen imaging-pathology review and prompt repeat biopsy when a useful target is available. This approach treats corticosteroid response as a measurable change in diagnostic substrate and links each decision to the evidence available at that time.