Characteristic radiographic features of PCNSL on magnetic resonance imaging. (A) A T1 axial, postgadolinium image depicts a periventricular contrast-enhancing lesion with near-uniform contrast enhancement, vasogenic edema and mass effect, in displacement of the lateral ventricles. Lesional contrast enhancement using MRI is used for response assessment. (B) A flair signal abnormality demonstrates the extent of vasogenic edema. (Courtesy Soonmee Cha, MD, University of California–San Francisco).