Utility of FDG-PET in predicting the histology of relapsed or refractory lymphoma.

18F- fluorodeoxyglucose-positron emission tomography (FDG-PET) is a valuable prognostic tool in modern lymphoma care. In this study, we explored the utility of quantitative FDG-PET parameters in predicting the histology of suspected relapsed/refractory (R/R) lymphoma. We retrospectively analyzed 290 FDG-PET scans performed for suspected R/R lymphoma. FDG-PET parameters measured were maximum and mean Standardized Uptake Value (SUVMax, SUVMean), Total Metabolic Tumor Volume and Total Lesion Glycolysis (TLG). Receiver operating characteristic curve analysis was used to obtain the optimal thresholds that best discriminate 1) benign versus R/R lymphoma, 2) indolent versus aggressive non Hodgkin lymphoma (NHL) and 3) aggressive transformation of indolent NHL. We found that while all four FDG-PET parameters discriminated R/R lymphoma from benign histology, TLG was the best performing parameter (optimal cut-off ≥245, sensitivity 63%, specificity 86%, PPV 97%, NPV 30%, AUC 0.798, P<0.001). SUVMax discriminated aggressive from indolent NHL with modest accuracy (optimal threshold ≥15, sensitivity 46%, specificity 79%, PPV 82%, NPV 38%, AUC 0.638, P<0.001). In patients with prior indolent NHL, SUVMax was a modest predictor of transformation (optimal cut-off 12, sensitivity 71%, specificity 61%, PPV 50%, NPV 78%, AUC 0.676, P 0.006). Additionally, SUVMax ≥25 and an increase in SUVMax from baseline ≥150% were highly specific (96% and 94% respectively). These FDG-PET thresholds can aid identification of suspected R/R lymphoma cases with higher likelihood of R/R disease and aggressive transformation of indolent NHL, guiding the necessity and urgency of biopsy.