Abstract
1415
Purpose: Background activity of non-tumorous soft tissue is often used as reference to assess tumor treatment response on 18F-FDG PET/CT. Our objective was to find the preferred site of assessment through assessing the repeatability of different background activities, which was expressed by standardized uptake value normalized to lean body mass (SUL).
Methods: Patients who have had two 18F-FDG PET/CT examinations within several days were selected for evaluation. Background SULs from aortic blood pool (ABP-SUL), liver (L-SUL) and muscle (M-SUL) were recorded. Intra-class correlation coefficients (ICCs) and Bland-Altman plots were used to evaluate the degree of repeatability between two scans. Then the intra-patient variation in SULs and factors which included blood glucose level (BGL), tracer uptake period and dose were calculated as relative changes between two scans. Finally, a linear regression model was used for analyzing all relative changes to identify which factor had correlation with SULs variability.
Results: Twenty-one patients were included. The SUL ICCs for ABP, liver and muscle were 0.735, 0.497 and 0.816 respectively. Similar results were obtained from Bland-Altman plots. From the Bland-Altman plots, 29% (6/21) L-SUL absolute variations and 29% (6/21) L-SUL relative variations were found outside the permitted variation limits by PERCIST. These numbers were 10% (2/21) and 14% (3/21), and 0% (0/21) and 10% (2/21) for ABP-SUL and M-SUL respectively. There were positive correlations between variations of L-SUL and M-SUL and that of BGL (b=0.740 and 0.430 respectively, p &It 0.05 altogether), and between variation of M-SUL and that of tracer uptake period (b=0.517, p=0.001). Conclusion: The SUL of liver is more sensitive to blood glucose level, therefore may not be suitable for the referential background. Activities within aortic blood pool and muscle are stable and should be the preferred background for sequential patient evaluation. Given the widespread nature, muscle is the most suitable and the most reproducible area for assessment of SUL.