Specific Imaging Findings

Systemic lymphoma with intracranial spread (metastatic lymphoma, secondary CNS lymphoma – SCNSL) is characteristically extra-axial in location, typically with thin, smooth enhancement that may extend along the surface of the brain, the cranial nerves and the subependyma, which is best seen on post-contrast MR images. In addition to this leptomeningeal spread, the dura is also frequently involved with larger masses and because it is hyperdense on non-enhanced CT scans, it may mimic extra-axial hematomas and meningiomas. SCNSL avidly enhances with contrast and shows similar MR signal characteristics as the primary CNS lymphoma (PCNSL) – relatively low T2 signal and reduced diffusion on ADC maps. It frequently invades the adjacent bone, usually without destruction, so that it may not be appreciated on CT images. However, bone marrow infiltration is well seen on MRI as decreased T1 signal and brain-like intensity on DWI and ADC, along with contrast enhancement. SCNSL may extend into the brain showing intra-axial spread of enhancement that is usually accompanied with surrounding T2 hyperintense edema. Although intra-axial enhancing lesions are characteristic of PCNSL, a minority of SCNSL also present in this fashion and they show the same patterns of avid homogenous nodular and infiltrative/perivenular enhancement. Intra-axial SCNSL are usually limited to the supratentorial white matter. Perfusion studies typically reveal a relatively mild increase in cerebral blood volume in both secondary and primary CNS lymphomas.