Repeatability of ¹⁸F-FDG PET/CT total metabolic active tumor volume and total tumor burden assessment in NSCLC patients: impact of acquisition, image reconstruction and delineation method.

Objectives: In this study we investigated the effects of uptake time, acquisition protocol, image reconstruction settings and delineation methods on total metabolic active tumor volume (MATV) and total tumor burden (TTB = MATV × SUV_mean summed across all lesions) and their repeatability for ¹⁸F-FDG PET/CT studies in non-small cell lung cancer (NSCLC) patients. Methods: Ten stage III and IV NSCLC patients underwent 2 baseline ¹⁸F-FDG PET/CT studies on separate days. At each time point, scans were obtained at both 60 and 90 minutes post injection. All PET/CT data were reconstructed using EARL compliant (EARL) and resolution modeling with point spread function (PSF) protocols. The reconstructed images were assessed with four semi-automated VOI segmentation methods: isocontours at SUV = 2.5, SUV = 4.0, 41% of SUV_max and a contrast adapted 50% of SUV_peak method. In addition, a consensus VOI was derived on the agreement across the predefined VOIs based on the majority vote method, i.e., when a voxel was selected by 2 or more of those VOIs (MV2). The total MATV and TTB and their repeatability were then calculated for each reconstructed dataset using all of the 5 methods. The results were evaluated using Pearson’s correlations, box-plots and Bland-Altman analyses. The obtained delineation of all cases were visually inspected (by an experienced observer) and qualitatively ranked into four classes: success, under or overestimation of volume and failure. Results: The best success rate was obtained using MV2 at 60min tracer uptake with success on 90% cases, independent of reconstruction settings. For those scans, there was perfect correlation between EARL and PSF derived MATV (r² = 1.00, p = 0.92) at a mean MATV difference of -4.49% and repeatability coefficient (RC = 1.96 × SD) of 20.95%. The MATV test-retest (TRT) mean difference was 0.35% (RC = 24.07%) and -3.73% (RC = 20.39%) for EARL and PSF, respectively. Yet, best repeatability was obtained with 90min uptake scans (with a success on 88% of cases, independent of reconstruction settings), having a mean MATV TRT difference of 1.15% (RC = 6.78%) for EARL and 3.38% (RC = 15.57%) for PSF. The same trend was seen for TTB, with repeatability coefficients of 11.88% for EARL and 16.66% for PSF. Most importantly, MV2 was never the worst segmentation, independent of tracer uptake time and image reconstruction settings (total success of 89%), while 11% of the cases showed an overestimated MATV. The performance of MV2 was not affected by lesion size or location. Conclusion: The use of a consensus delineation approach (MV2) seems to be useful for accurate and reliable MATV and TTB assessments in ¹⁸F-FDG PET/CT studies in NSCLC patients and seems to be least affected by variations in tracer uptake time and image reconstruction settings. This reliability is especially important for multicenter and retrospective studies, since these PET/CT scans usually have been collected with different protocols and scanners from different vendors and generations. Further research is needed to explore the use and robustness of a consensus delineation approach for ¹⁸F-FDG PET/CT MATV and TTB assessments in other diseases.