ABSTRACT
Hypointense lesions, or T2 hyperintense lesions that do not contrast enhance on MR scan, are occasionally found in both immunocompetent and immunosuppressed patients, but they are more common among the immunosuppressed population.16,30 Ringenhancing lesions similar to those of toxoplasmosis are characteristic of PCNSL in the AIDS patient, making the differential diagnosis difficult, whereas ring enhancement is uncommon in the immunocompetent population. In addition, the lesions are more frequently located in the subcortical white matter in AIDS patients. Perfusion MRI suggests that most PCNSLs have a low cerebral blood volume, similar to lower-grade gliomas and in contrast to high-grade gliomas.29 PET scanning reveals a hypermetabolic lesion but this is non-specific and not helpful in the immunocompetent population; however it may help distinguish tumor from toxoplasmosis (hypometabolic) in AIDS patients.31 Single-photon emission tomography may help distinguish PCNSL from other lesions.32 In PCNSL uptake of 123IMP is delayed, whereas it occurs early in other brain tumors. MRS may also help.33
