Test-Retest Variability in Lesion SUV and Lesion SUR in 18 F-FDG PET: An Analysis of Data from Two Prospective Multicenter Trials.

Quantitative assessment of radio- and chemotherapy response with 18 F-FDG whole-body PET has attracted increasing interest in recent years. In most published work, SUV has been used for this purpose. In the context of therapy response assessment, the reliability of lesion SUVs, notably their test-retest stability, thus becomes crucial. However, a recent study demonstrated substantial test-retest variability (TRV) in SUVs. The purpose of the present study was to investigate whether the tumor-to-blood SUV ratio (SUR) can improve TRV in tracer uptake. Methods: 73 patients with advanced non-small cell lung cancer from the prospective multicenter trials ACRIN 6678 ( n = 34) and MK-0646-008 ( n = 39) were included in this study. All patients underwent two 18 F-FDG PET/CT investigations on two different days (time difference, 3.6 ± 2.1 d; range, 1-7 d) before therapy. For each patient, up to 7 tumor lesions were evaluated. For each lesion, SUVmax and SUVpeak were determined. Blood SUV was determined as the mean value of a 3-dimensional aortic region of interest that was delineated on the attenuation CT image and transferred to the PET image. SURs were computed as the ratio of tumor SUV to blood SUV and were uptake time-corrected to 75 min after injection. TRV was quantified as 1.96 multiplied by the root-mean-square deviation of the fractional paired differences in SUV and SUR. The combined effect of blood normalization and uptake time correction was inspected by considering RTRV (TRVSUR /TRVSUV ), a ratio reflecting the reduction in the TRV in SUR relative to SUV. RTRV was correlated with the group-averaged-value difference (δ) in CFmean (δCFmean ) of the quantity δCF = |CF - 1|, where CF is the numeric factor that converts individual ratios of paired SUVs into corresponding SURs. This correlation analysis was performed by successively increasing a threshold value δCFmin and computing δCFmean and RTRV for the remaining subgroup of patients/lesions with δCF ≥ δCFmin Results: The group-averaged TRVSUV and TRVSUR were 32.1 and 29.0, respectively, which correspond to a reduction of variability in SUR by an RTRV factor of 0.9 in comparison to SUV. This rather marginal improvement can be understood to be a consequence of the atypically low intrasubject variability in blood SUV and uptake time and the accordingly small δCF values in the investigated prospective study groups. In fact, subgroup analysis with increasing δCFmin thresholds revealed a pronounced negative correlation (Spearman ρ = -0.99, P < 0.001) between RTRV and δCFmean , where RTRV ≈ 0.4 in the δCFmin = 20% subgroup, corresponding to a more than 2-fold reduction of TRVSUR compared with TRVSUV Conclusion: Variability in blood SUV and uptake time has been identified as a causal factor in the TRV in lesion SUV. Therefore, TRV in lesion uptake measurements can be reduced by replacing SUV with SUR as the uptake measure. The improvement becomes substantial for the level of variability in blood SUV and uptake time typically observed in the clinical context.