Potential impact of different reconstruction algorithms on FDG PET- CT semiquantitative measurement: a correlation study between SUVmax and lesions volume

Aim: in oncological imaging the 18F-FDG tissues accumulation evaluated with PET-CT is proportional to the amount of increased glucose metab- olism. Its semi-quantitative measure (SUV) has quickly become the most common parameter used for detection, staging, restaging and therapy response evaluation in clinical oncology. The aim of this study was to compare PET-CT image reconstruction with standard iterative (IR) vs new True-X-TOF (Siemens Medical Solutions, Erlangen, Germany) method in order to assess their potential impact on quantification. Methods: we retrospectively analyzed 189 PET-CT lesions in lung cancer (92 pulmonary nodules, 67 metastatic lymph nodes) and lymphoma pa- tients (30 Hodgkin or Non-Hodgkin pathologic lymph nodes). SUVmax and volume were calculated with IR and True-X-TOF. The distribution of SUVmax, in relation to the reconstruction method used, was statistically studied by linear regression (frequency distribution curves and scatter plots). The relationship between volume and ratio between SUVmax True-X-TOF and IR was evaluated and studied with box-plots. Results: True-X-TOF over iterative SUV ratios "x" were divided into three groups, I 1.0≤x≤1.5, II 1.5<x≤2.0, III x>2.0, inside which regression correlation stays excellent (R=0.996, 0.996 and 0.993 respectively). Among the three groups a statistically significant stratification of ratios between lesion volumes and SUV ratio values was observed (p<0.001): smaller volumes are associated to greater SUV ratios while larger volumes (>2cm3) are associated to SUV ratios closer to 1. Pulmonary lesions and lymph nodes represented 70% and 30% respectively of findings in group I, and 45% and 55% in group II and III combined. Conclusions: FDG PET-CT SUV is determined by a lot of parameters, among which acquisition and recon- struction techniques can play a pivotal role. Our preliminary data, obtain- ed from a daily clinical setting, demonstrated that True-X+TOF shows greater SUV values in comparison to IR method. However, greater dif- ferential SUVmax values were identified in small lesions compared to larger ones, probably meaning that IR underestimates SUV in smaller lesions. Therefore, True-X+TOF reconstruction algorithm seems to allow more sensitive metabolic characterization of small lesions in comparison to iterative methods making the impact of that feature more relevant in staging and prognostic stratification of lymph nodes than of generally larger pul- monary lesions. Moreover our study highlights the risk to compare PET- CT scans obtained with different reconstruction methods during clinical follow-up and so the importance of standardization in PET-CT imaging. Larger clinical prospective trials are mandatory to confirm these prelimi- nary findings and to assess their potential impact in clinical practice.