Liver and blood pool SULs normalized by CT based lean body mass is more reliable than those by predictive formulas

Objectives: PERCIST recommends using SUV normalized by lean body mass (SUL) as a strong response parameter. The LBM is often estimated by various predictive formulas in clinical practice. However, errors is substantial with formulas. The aim of study was to introduce a method for estimating lean body mass (LBM) by limited field of view (FOV) CT from PET/CT, and compare the results with LBM estimates of various formulas. SULpeak in normal liver and blood pool with different methods will also be compared.

Methods: A total of 52 patients who received whole-body PET/CT examinations (the arms were maintained above the head) were retrospectively retrieved. The body composition could be calculated based on threshold of CT attenuation. LBM determined was defined in 7 ways by CT (LBM_CT1-7) and 2 ways predictive equations (LBM_F1-2). All results were compared with the measurement of LBM from whole-body CT (LBM_CT1, reference standard) described in the literature. Liver SUV normalized by LBM was calculated using LBM from each method (SUL_CT1-7, SUL_F1-2). Agreement between methods was assessed by Bland-Altman analysis. Percentage difference and percentage error were also calculated.

Results: The intraclass correlations (ICC) of liver and blood pool SULpeak measurement evaluated from different limited FOV of CT compared with the reference standard were excellent. The ICC is 0.997, 0.996, 0.994, 0.993, 0.979 and 0.978 for SUL_CT2-7, only 0.792 and 0.762 for LBM_F1-2. The best ICC being obtained for the largest FOV, from eye to 20cm under ischia: ICC, 0.997; 95% confidence interval [95% CI], 0.993-0.999; P < 0.0001, is much better than the ICC obtained with the mathematic formulas (the best ICC for a mathematic formula was 0.7792; 95% CI, 0.543-0.899; P ,<0.0001). Moreover, the analysis with the Bland-Altman plot showed that the differences in mean lean masses between the studied technique and the reference standard was the smallest for the proposed technique (for the largest FOV, mean difference -0.1 kg with the narrowest 95% CI [−1.5 to 1.3 kg]). SULpeak of both liver and blood pool normalized by LBM from CT has smaller SDs (0.42 vs 0.35, and 0.36 vs 0.28).

Conclusions: The present study demonstrated substantial errors in individual SUL_Fs compared with SUL_CTs as a reference value. Normalization of SUV by LBM determined by CT rather than formulas is a useful approach to reduce errors in individual SUL_Fs, even with limited FOV.