Comparative evaluation of SUV, tumor-to-blood standard uptake ratio (SUR), and dual time point measurements for assessment of the metabolic uptake rate in FDG PET

Background: We have demonstrated recently that the tumor-to-blood standard uptake ratio (SUR) is superior to tumor standardized uptake value (SUV) as a surrogate of the metabolic uptake rate K-m of fluorodeoxyglucose (FDG), overcoming several of the known shortcomings of the SUV approach: excellent linear correlation of SUR and K-m from Patlak analysis was found using dynamic imaging of liver metastases. However, due to the perfectly standardized uptake period used for SUR determination and the comparatively short uptake period, these results are not automatically valid and applicable for clinical whole-body examinations in which the uptake periods (T) are distinctly longer and can vary considerably. Therefore, the aim of this work was to investigate the correlation between SUR derived from clinical static whole-body scans and K-m-surrogate derived from dual time point (DTP) measurements. Methods: DTP F-18-FDG PET/CT was performed in 90 consecutive patients with histologically proven non-small cell lung cancer (NSCLC). In the PET images, the primary tumor was delineated with an adaptive threshold method. For determination of the blood SUV, an aorta region of interest (ROI) was delineated manually in the attenuation CT and transferred to the PET image. Blood SUV was computed as the mean value of the aorta ROI. SUR values were computed as ratio of tumor SUV and blood SUV. SUR values from the early time point of each DTP measurement were scan time corrected to 75 min postinjection (SURtc). As surrogate of K-m, we used the SUR(T) slope, K-slope, derived from DTP measurements since it is proportional to the latter under the given circumstances. The correlation of SUV and SURtc with K-slope was investigated. The prognostic value of SUV, SURtc, and K-slope for overall survival (OS) and progression-free survival (PFS) was investigated with univariate Cox regression in a homogeneous subgroup (N = 31) treated with primary chemoradiation. Results: Correlation analysis revealed for both, SUV and SURtc, a clear linear correlation with K-slope (P < 0.001). Correlation SUR vs. K-slope was considerably stronger than correlation SUV vs. K-slope (R-2 = 0.92 and R-2 = 0.69, respectively, P < 0.001). Univariate Cox regression revealed SURtc and K-slope as significant prognostic factors for PFS (hazard ratio (HR) = 3.4/P = 0.017 and HR = 4.3/P = 0.020, respectively). For SUV, no significant effect was found. None of the investigated parameters was prognostic for OS. Conclusions: Scan-time-corrected SUR is a significantly better surrogate of tumor FDG metabolism in clinical whole-body PET compared to SUV. The very high linear correlation of SUR and DTP-derived K-slope (which is proportional to actual K-m) implies that for histologically proven malignant lesions, FDG-DTP does not provide added value in comparison to the SUR approach in NSCLC.