An investigation of the relation between tumor-to-liver ratio (TLR) and tumor-to-blood standard uptake ratio (SUR) in oncological FDG PET

BACKGROUND: The standardized uptake value (SUV) is the nearly exclusive means for quantitative evaluation of clinical [18F-]fluorodeoxyglucose (18F-FDG) positron emission tomography (PET) whole body investigations. However, the SUV methodology has well-known shortcomings. In this context, it has been recognized that at least part of the problems can be eliminated if tumor SUV is normalized to the SUV of a reference region in the liver (tumor-to-liver [TLR] ratio). In recent publications, we have systematically investigated the tumor-to-blood SUV ratio (SUR) for normalization of tumor SUVs which in our view offers principal advantages in comparison to TLR. The aim of this study was a comprehensive comparison of TLR and SUR in terms of quantification of tumor lesions. METHODS: 18F-FDG PET/CT was performed in 424 patients (557 scans) with different tumor entities prior to radio(chemo)therapy. In the PET images, SUV(max) of the primary tumor was determined. SUV(liver) was calculated in the inferior right lobe of the liver. SUV(blood) was determined by manually delineating the aorta in the low-dose CT. TLR and SUR were computed and scan time corrected to 60 min p.i. (TLR(tc) and SUR(tc)). Correlation analysis was performed for SUV(liver) vs. SUV(blood), TLR vs. SUR, SUV(liver)/SUV(blood) vs. SUV(blood),SUR(tc)/TLR vs. SUR(tc), and SUR(tc)/TLR(tc) vs. SUR(tc). Variability of the respective ratios was assessed via histogram analysis. The prognostic value of TLR and TLR(tc) for distant metastases-free survival (DM) was investigated with univariate Cox regression in a homogeneous subgroup (N = 130) and compared to previously published results for SUV and SUR(tc). RESULTS: Correlation analysis revealed a linear correlation of SUV(liver) vs. SUV(blood) (R (2)=0.83) and of TLR vs. SUR(tc) (R(2)=0.92). The SUV(liver)/SUV(blood) ratio (mean ± s.d.) was 1.47 ± 0.18. For the SUR(tc)/TLR ratio, we obtained 1.14 ± 0.21 and for the SUR(tc)/TLR(tc) ratio 1.38 ± 0.17. Survival analysis revealed TLR and TLR(tc) as significant prognostic factors for DM (hazard ratio [HR] = 3.3 and HR = 3, respectively). Both hazard ratios are lower than that of SUR(tc) (HR = 4.1) although this reduction does not reach statistical significance for the given limited group size. HRs of TLR and SUR(tc) are both significantly higher than HR of SUV (HR = 2.2). CONCLUSIONS: Suitability of the liver as surrogate of arterial tracer supply for SUV normalization via TLR computation is limited. Further studies in sufficiently large patient groups are required to better characterize the relative performance of SUV, TLR, and SUR in different settings.