Abstract
Aim: Standardized uptake value (SUV) normalized by body weight is affected by amount of body fat. Higher (overestimated) SUV is measured in normal tissues and lesions in obese patients than the patients with normal body mass index (BMI). SUV calculated/normalized by lean body mass (LBM, fat free body mass) (SUVLBM or SUL) instead of total weight is recommended to provide more accurate SUV results. Given importance of a quantitative PET parameter particularly when comparing PET studies, in this study we aimed to re-visit and re-assess the effect of obesity on SUV, to measure SUL in patients with normal BMI and obese patients and also to test the effect of amount of LBM on SUL. Methods: 18F-fluorodeoxyglucose (FDG) whole body images of adult patients were analyzed retrospectively. We measured both maximum SUV (SUVmax) and mean SUV (SUVmean), calculated/normalized by total body weight, in the blood pool and liver in patients with normal BMI (18.5-24.9) and high BMI (30 and greater) (obese). In all patients we calculated the LBM via an equation using patient’s height and weight and corrected all the SUVs to SULs (SULmean and SULmax). Mean ± SD SUVs and SULs were compared in various circumstances. Scatter plots were generated for weight and SUV-SUL differences. Results: Mean ± SD SUVmean in the liver and blood pool was significantly higher in obese patients (30 patients) compared to patients with normal BMI (20 patients) (4.1 ± 0.6 and 3.2 ± 0.6 versus 3.0 ± 0.5 and 2.4 ± 0.4, p<0.001). Mean SULmean (in liver and blood pool) was significantly lower than mean SUVmean, in all patients which was approximately 75% of SUVmean in patients with normal BMI and 55% of SUVmean in obese patients (p<0.001). Mean SULmean (in the liver and blood pool) was approximately the same in obese and normal BMI patients, in a subgroup of patients with different amount of LBMs (p>0.05). The SUV-SUL difference was significantly higher in obese patients than those in patients with normal BMI (p<0.001). Above statistical results were also the same when SUVmax and SULmax were compared. Conclusion: SUV (calculated/normalized by weight) overestimates the metabolic activity in all patients which is more significant in obese patients than the patients with normal BMI. SUL is not affected by body weight and the amount of LBM.
