BTK and post-transplant lymphoproliferative disease: At disease onset, patients exhibited marked cytopenia and an ECOG PS of 4, precluding tolerance to standard chemo-radiotherapy (5).Given prior evidence of BTK inhibitor efficacy in CNS-PTLD and the favorable CNS penetration and safety profile of low-dose orelabrutinib, reduced-dose treatment was initiated with informed consent to minimize potential toxicity.