PCNSL at relapse with periventricular and subependymal invasion. Common patterns of relapse are within the vitreoretinal compartment and subependymal growth into the periventricular space, leading to spread within CSF pathways. (A and B) T1 axial postgadolinium images of PCNSL that progressed after 3 cycles of HD-MTX: Contrast-enhancing disease is evident, involving the anterior commissure of the corpus callosum. (C) Brain autopsy, obtained 1 week after the MRI, confirmed periventricular DLBCL with subependymal spread.