Usefulness of Positron Emission Tomography Using 18F-FDG Performed At Diagnosis and During Initial Chemotherapy for Imaging Primary Central Nervous System Lymphoma: A 24 Immunocompetent patient's Study.

Primary central nervous system lymphoma (PCNSL) is a rare brain tumor potentially curable by chemotherapy alone or a combination of chemotherapy and radiation therapy. At staging, gadolinium-enhanced magnetic resonance imaging (MRI) is the standard method to evaluate CNS lesions and computed tomography (CT) is perform at diagnosis to detect the presence of a systemic disease. The initial tumor response to therapy is usually assessed by MRI. At present, there are no established imaging markers of prognosis in patients with PCNSL. Positron emission tomography (PET) using ¹⁸F-Fluorodeoxyglucose (FDG) is routinely used for the initial staging and the evaluation of treatment response in systemic Diffuse Large B-cell Lymphoma (DLBCL). In PCNSL, the clinical relevance of positron emission tomography FDG PET/CT is not well known. The aim of our study was to determine the added value of FDG PET/CT in the management of PCNSL performed at diagnosis and during initial treatment to assess whether it could predict the outcome of PCNSL patients.

Patients and methods: From august 2008 to may 2011, we enrolled 24 consecutive PCNSL immunocompetent patients with histological proven DLBCL who underwent FDG PET/CT before specific treatment. The mean age of patients was 63.7 years (range, 51.7–78.8). Follow-up FDG PET/CT examinations were performed in 14 of them (58%) after 2 cycles of chemotherapy and in patients who relapsed. All PET images were acquired 1 hour after FDG injection and interpreted qualitatively and semi quantitatively by 2 nuclear medicine physician. The maximum standard uptake value (maxSUV) corrected to body weight and injected FDG activity was measured for each patient into the most hypermetabolic CNS lesion (TmaxSUV). The results were compared to the clinical and conventional imaging data. The correlation between TmaxSUV and respectively the Progression Free Survival (PFS) and Overall Survival (OS) was statistically analysed. We also evaluated for 15 patients, the correlation between the Ki67 index on tumors and TmaxSUV.

All the patients presented with brain lesions. The spinal cord was also involved in 1 of them. The sensitivity of FDG PET/CT for the detection of CNS lesions was 91.7 %. Two out of 24 patients were considered as false negative. The mean TmaxSUV was 13.9 +/− 9.3 (range, 4.9–38). FDG PET/CT found systemic spread of lymphoma in 2 patients (8.3%). Follow-up ranged from 3.2 to 33.8 months (mean, 20.6 months). After 2 cycles of chemotherapy, FDG PET/CT was considered as negative in 14/14 patients whereas gadolinium-enhanced MRI showed lesions with residual contrast-enhancement in 7 of them. Four patients who were PET- and MRI + after 2 cycles of chemotherapy relapsed exclusively into the CNS (n=2) and/or outside (n=2). FDG PET/CT showed all the sites of relapse. At last follow-up, 18 patients were alive and six died of progressive disease. No correlation between TmaxSUV at diagnosis and PFS (P =.15), OS (P =.14) and Ki67 index was respectively observed.

Although the physiologic glucose metabolism in normal brain tissue is high, FDG PET/CT has a good sensitivity to detect PCNSL. FDG PET/CT could be useful to detect any systemic spread of PCNSL at staging and is able to diagnose disease relapse. Pretreatment Tmax SUV is not correlated with PFS and OS in our study. Moreover, FDG PET/CT seems not to be reliable for the prediction of relapse when it is performed after 2 cycles of chemotherapy. Others type of TEP tracers need to be study in PCNSL for response assessment and the prediction of patient's outcome.

No relevant conflicts of interest to declare.