Specific Imaging Findings

Primary CNS lymphoma (PCNSL) most commonly presents as a homogenous, well-defined intra-axial mass, hyperdense on nonenhanced CT and of low to isointense T2 signal (primarily due to high cellularity). The typical lesions show dense homogenous enhancement and very low diffusion with characteristic dark appearance on ADC maps. PCNSL may also manifest with a predominantly perivascular, ill-defined infiltrative spread pattern. Associated vasogenic edema and mass effect are usually present. PCNSL primarily involves the deep brain structures, periventricular regions, corpus callosum and septum pellucidum with tendency to spread along the subependymal white matter. Lesions may be multiple and leptomeningeal spread can be observed. However, PCNSL may also present with necrotic and even hemorrhagic lesions, primarily in immunocompromised, usually HIV-positive patients. Contrast enhancement can also vary and, in very rare cases, it may even be completely absent, more frequently after steroid treatment. Vasogenic edema and mass effect can sometimes also be minimal. Perfusion imaging shows increased rCBV; however, lower than with high-grade gliomas or metastases. FDG PET and SPECT reveal high metabolic activity of PCNSL. Rare spontaneously fluctuating lesions with changes of shape, size and location have been reported.

Pertinent Clinical Information

Clinical presentation is nonspecific, related to infiltration of brain structures or mass effect, frequently with relatively minor symptoms considering the size of the lesion. Prognosis is generally poor and disease rapidly progressing, especially in HIV-positive patients. Due to its infiltrative characteristics, MRI tends to underestimate the burden of disease. Body FDG PET may disclose a systemic site of malignancy in some patients.