Magnetic resonance images from a patient with PCNSL. A T1-weighted, axial, postcontrast scan (left) demonstrates intense, homogenous enhancement of the tumor in the region of the left caudate nucleus. An axial T2/FLAIR scan at the same anatomical level (right) demonstrates hyperintense signal surrounding the tumor, reflecting vasogenic cerebral edema. (Courtesy Priscilla K. Brastianos, M.D.)