Repeatability of SUV in Oncologic 18F-FDG PET

Abstract

Quantitative analysis can potentially improve the accuracy and consistency of ¹⁸F-FDG PET, particularly for the assessment of tumor response to treatment. Although not without limitations, SUV has emerged as the predominant metric for tumor quantification with ¹⁸F-FDG PET. Growing literature suggests that the difference between SUVs measured before and after treatment can be used to predict tumor response at an early stage. SUV is, however, associated with multiple sources of variability, and to best use SUV for response assessment, an understanding of the repeatability of the technique is required. Test-retest studies involve repeated scanning of the same patient on the same scanner using the same protocol no more than a few days apart and provide basic information on the repeatability of the technique. Multiple test-retest studies have been performed to assess SUV repeatability, although a comparison of reports is complicated by the use of different methodologies and statistical metrics. This article reviews the available data, addressing issues such as different repeatability metrics, relative units, log transformation, and asymmetric limits of repeatability. When acquired with careful attention to protocol, tumor SUV has a within-subject coefficient of variation of approximately 10%. In a response assessment setting, SUV reductions of more than 25% and increases of more than 33% are unlikely to be due to measurement variability. Broader margins may be required for sites with less rigorous protocol compliance, but in general, SUV is a highly repeatable imaging biomarker that is ideally suited to monitoring tumor response to treatment in individual patients.

Keywords: FDG; PET; SUV; repeatability; reproducibility; standardized uptake value.

FIGURE 1.

Bland–Altman plots showing SUV_max difference data (●) in both original units (A) and relative units (B), as function of mean. Relative difference data are consistent with normal distribution, and Kendall τ (0.026, P = 0.89) indicates that magnitude is not proportional to mean. Dashed lines show limits of repeatability given by [−RC, +RC]. In B, data have also been plotted using secondary y-axis (×). Close agreement between red and blue data points illustrates that scales on left and right y-axes are substantially similar. (Data are from Heijmen et al. (27), kindly provided by the author.)

FIGURE 2.

Simulated test–retest difference data. SUV₁ and SUV₂ were random samples drawn from normal distribution with coefficient of variation of 12%. One thousand pairs of random samples were generated, corresponding to the different noise realizations shown on x-axis. (A) Differences between SUV₁ and SUV₂ are shown relative to their average. DSD was 16.7% (MAPD, 13.5%). Dashed lines indicate 95% limits of repeatability that are symmetric about zero [−33%, +33%]. (B) Differences are shown relative to single baseline value (SUV₁). Asymmetric limits of repeatability are marked at [−28%, +39%].

FIGURE 3.

Summary of SUV repeatability results. wCV was inferred from published data and is shown separately for SUV_max, SUV_mean, and SUV_peak. Labels by each data point refer to publications noted in Table 1. Dashed horizontal lines indicate mean wCV for each SUV type: 10.96%, 9.98%, and 9.60% for SUV_max, SUV_mean, and SUV_peak, respectively.