Comparison of two region of interest definition methods for metabolic response evaluation with [¹⁸F]FDG-PET

In therapy response monitoring by [¹⁸F]2-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 (ΔMR(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.

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.

In CRC, only ROI(fixed volumes) based ΔMR(glu) showed significant predictive ability. In NSCLC, both techniques showed significant predictive ability. During multivariate analysis, ROI(fixed volumes) determined ΔMR(glu) was an independent predictor for both overall and progression free survival in NSCLC whereas ROI(fixed thresholds) determined MRglu was not. After dichotomization at the median ΔMR(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).

ROI(fixed volumes) based ΔMR(glu) shows better correlation with survival than ΔMR(glu) determined from a ROI(fixed thresholds).