Aim. In therapy response monitoring by [(18)F]-fluoro-2-deoxy-D-glucose positron emission tomography (FDG-PET), different tumor delineations are used, resulting in different values for change in glucose metabolic rate (Delta MR(glu)). We propose a technique to compare metabolic rates in a region of interest (ROI) based on fixed volumes rather than on fixed thresholds. This method involves change in lesion size. Methods. In 49 patients with colorectal carcinoma (CRC) and 50 patients with non-small cell lung carcinoma (NSCLC) scheduled for chemotherapy, FDG-PET was performed at baseline and during chemotherapy. A ROI(fixed thresholds) was determined by using a 50% threshold on both baseline and follow-up FDG-PET. A ROI(fixed) (volumes) was determined by using a 50% threshold, determined on the series with the largest tumor volume. This ROI(fixed volumes) is used on consecutive scans. Predictive effects of both methods were investigated by survival analysis for overall and progression free survival. Results. In CRC, only ROI(fixed volumes) based Delta MR(glu) showed significant predictive ability. In NSCLC, both techniques showed significant predictive ability. During multivariate analysis, ROI(fixed volumes) determined Delta MR(glu) was an independent predictor for both overall and progression free survival in NSCLC whereas ROI(fixed thresholds) determined MR(glu) was not. After dichotomization at the median Delta MR(glu), median survival ratio was higher in ROI(fixed volumes) than ROI(fixed thresholds) for CRC (overall survival: 1.78 vs 1.25, progression free survival: 1.57 vs 1.21) and NSCLC (overall survival: 2.01 vs 2.01, progression free survival: 2.93 vs 2.13). Conclusion. ROI(fixed volumes) based Delta MR(glu) shows better correlation with survivarethan Delta MR(glu) determined from a ROI(fixed thresholds)