How do you treat a low-grade B-cell lymphoma with plasmacytic differentiation (Waldenstrom's) confined to the CNS?  

This is a rare complication of WM called Bing Neelapu syndrome. It is hard to find a standard of care. Patients tend to have shorter responses to standard therapies such as HD MTX based regimens. Ibrutinib has shown long-term responses (Castillo et al., PMID 30523119).