Mild Metabolism Increase on Posttreatment 18 F-FDG PET may Indicate Recurrence of Primary CNS Lymphoma

In immunocompetent patients, primary central nervous system lymphoma (PCNSL) typically presents with homogeneously enhancing brain lesions and high 18 F-FDG avidity, ranging from 2 to 5 times higher than that of healthy tissue at baseline. We present the case of a 34-year-old woman in whom PCNSL recurrence was questioned due to the appearance of mildly increased glucose metabolism (1.3 times higher than normal contralateral tissue) and FLAIR hypersignals, despite the absence of contrast enhancement or cerebrospinal fluid abnormalities. Histopathology confirmed the recurrence. This case highlights that new 18 F-FDG PET uptake during follow-up, even with lower avidity than baseline, should prompt concern for recurrence or residual disease in PCNSL.