This study explores changes in metabolic tumor volume, metabolic tumor diameter, and maximum standardized uptake value (SUVmax), for earlier and more accurate identification of lymphomas’ response to treatment using 18F- FDG PET/CT. Pre- and post-treatment PET/CT studies of 20 patients with Hodgkin disease (HL) and 7 patients with non- Hodgkin lymphoma (NHL) were retrospectively selected for this study. The diameter and volume of the metabolic tumor was determined by an in-house developed adaptive local thresholding technique based on a 50% threshold of the maximum pixel value within a region. Statistical analysis aimed at exploring associations between metabolic size measurements and SUVmax and the ability of the three biomarkers to predict the patients’ response to treatment as defined by the four classes in the European Organization for Research and Treatment of Cancer (EORTC) guidelines. Results indicated moderate correlations between % change in metabolic tumor volume and % change in metabolic tumor maximum diameter (R=0.51) and between % change in maximum diameter and % change in SUVmax (R=0.52). The correlation between % change in tumor volume and % change in SUVmax was weak (R=0.24). The % change in metabolic tumor size, either volume or diameter, was a “very strong” predictor of response to treatment (R=0.89), stronger than SUVmax (R=0.63). In conclusion, metabolic tumor volume could have important prognostic value, possibly higher than maximum metabolic diameter or SUVmax that are currently the standard of practice. Volume measurements, however, should be based on robust and standardized segmentation methodologies to avoid variability. In addition, SUV-peak or lean body mass corrected SUV-peak may be a better PET biomarker than SUVmax when SUV-volume combinations are considered.