Use 18F-FDG PET in initial assessment of primary central nervous system lymphoma

Objectives: Primary central nervous system lymphoma (PCNSL) diagnsosis usually relies on brain biopsy and a systemic work-up to assess the absence of concomitant systemic lymphoma lesions. It can be done with a contrast enhanced computed tomography of thorax-abdomen-pelvis and bone marrow biopsy (BMB). Precise prestaging imaging plays an important role in the decision making process. In small series, some authors suggest that fluoro-deoxy-glucose positron emission tomography (FDG PET) could play a key role in initial assessment of suspected PCNSL with the detection of systemic involvement not revealed by conventional work-up.

Methods: Between 2008 and 2016, 80 patients (mean age: 62 ± 11 years, 42 men) with histologically confirmed diagnosis of brain lymphoma were explored with FDG PET/CT in two French centers. A diffuse large B-cell lymphoma was found in 96% patients. Whole body and brain PET acquisitions were performed after the injection of 4-5 MBq/kg of FDG (Gemini XLS Philips and Discovery 690 GE Healthcare). PET/CT findings were interpreted qualitatively and semi-quantitatively with the measurement of Standard Uptake Value (SUVmax), tumor/contralateral cortex ratio tumor/cerebellum ratio and tumor/hepatic uptake ratio. Event free survival (EFS) was calculated starting from the date of brain biopsy until the date of event (death or tumor progression or recurrence). The mean event free follow-up was 509 days {range 28 - 2368 days}.

Results: Six patients (7.5%) were considered having a concomitant systemic involvement. In half of them, the systemic lymphoma lesions were exclusively revealed by FDG PET/CT. It was the case for 2 patients with multiple osteomedullar lesions but negative BMB. These findings led to change the therapeutic management in 4 patients. On the brain level, most lesions were strongly hypermetabolic : SUVmax 20.8 ± 7.6 and ratio to contralateral cortex 2.7 ± 0.9. At least one-brain lesion was visible for all patients treated by corticosteroid for one week or less. The SUVmax measured in brain lesions did not yield significant prognostic value in itself. However, the absence of brain abnormal visible uptake was significantly associated with initial complete response and better event-free survival in univariate and multivariate analyses.

Conclusion: FDG-PET is an effective tool for initial staging of PCNSL and can reveal systemic involvement that requires specific treatment even after a negative conventional work-up. At the brain level the absence of hypermetabolic visible lesion is associated with better outcome but this could simply reflect the consequences of a good initial response to corticosteroid treatment between brain biopsy and beginning of the chemotherapy. Research Support: Department of nuclear medicine, Pitié-Salpétrière Hospital University of Pierre and Marie CURIE (UPMC)