A Durable Remission Following Pseudo-Progression in Tirabrutinib Treatment for Relapsed Primary Central Nervous System Lymphoma: A Case Study.

Tirabrutinib (TIR) is a second-generation, Bruton's tyrosine kinase inhibitor (BTKi) recently developed for the treatment of relapsed and refractory primary central nervous system lymphoma (PCNSL). However, little data are available regarding potential immunomodulatory effects of TIR on PCNSL due to its rarity and aggressive tumor behavior. Here, we report the first case of pseudo-progression (PSP) in a PCNSL patient treated with TIR. A 79-year-old woman had relapsed PCNSL with multiple tumor lesions in the lateral ventricles. A temporary tumor regression was observed following TIR administration. However, 7 months later, brain tumors regrew in the left lateral ventricle and in the choroid plexus of the right lateral ventricle, suggesting TIR-resistant disease progression. Despite the enlarged tumors, the patient remained asymptomatic, and re-remission was achieved by continuation of TIR monotherapy. Moreover, a durable remission for approximately 2 years was obtained without any additional therapy. This case shows that TIR can induce immunomodulatory reaction including PSP even in PCNSL, suggesting the importance of differential diagnosis of true disease progression and immune-mediated PSP based on careful clinical and radiological monitoring to avoid premature discontinuation of effective treatment.