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Purpose: To investigate influences of reconstruction algorithms and count statistics variation on quantification and treatment response assessment in cancer patients, by using a large field of view-FOV scanner. Methods: 54 cancer patients underwent PET/CT scan: 1) at baseline: 1.5 min/FOV, reconstructed by ordered-subset expectation maximization + point-spread-function-OSEM-PSF and bayesian penalised-likelihood-BPL algorithm 2) at restaging: 2 min/FOV, reconstructed also at 1.5 and 1 min/FOV, using OSEM-PSF and BPL. SUL (lean-body mass SUV) peak and max were measured for each target-lesion (n = 59). Differences in quantification obtained from datasets with different reconstruction algorithms and different time/FOV were evaluated. For any pair of PET datasets, metabolic response was assessed by using SULpeak, with a threshold of 30% in variation considered as significant. Results: Both at baseline and restaging, SULpeak and max values were higher in BPL reconstructions than in OSEM-PSF (p < 0.0001). SULpeak at different time/FOV reconstructions showed no statistically significant differences both with OSEM-PSF and BPL; SULmax depended on acquisition time (p < 0.05). In 56/59 lesions (95%) therapy response was concordant regardless count statistics variation and reconstruction algorithm; 2/59 (3%) showed different responses according to count statistics, both for OSEM-PSF and BPL; in 1/59 lesion (2%) response was different depending on reconstruction algorithm used. Conclusions: BPL provided higher SULpeak and max than OSEM-PSF. With a large FOV/high sensitivity scanner, variation of time/FOV in restaging PET scans gave stable and reproducible results in terms of SULpeak, both for OSEM-PSF and BPL. Thus, metabolic response defined by SULpeak variation proved to be quite independent from count statistics

Although breathing movement can introduce heavy bias in both image quality and quantitation, currently there is lack of interest for the respiratory motion management in PET/CT imaging. The respiratory gating technology has shown significant benefits on image quality, reducing motion related artifacts and increasing quantification. Indeed, many papers have demonstrated that there is a consistent increase of the Standardized Uptake Value (SUV) in gated PET images when compared to ungated ones, particularly for the small lesions located both in liver and in lung. Beside the increase in quantification, respiratory gating can also increase sensitivity, specificity and accuracy of PET/CT. Finally, gated PET/CT can be used for radiation therapy planning, reducing the uncertainty in target definition, optimizing the volume to be treated and reducing the possibility of "missing" during the dose delivery. New technologies, able to define the movement of lesions and organs directly from the PET sinogram, can solve some problems (i.e.: extended acquisition time, radiation exposure) that currently are limiting the clinical use of gated PET/CT. If these new technical improvements for motion compensation will be clinically validated, gated technique could be applied routinely in any PET/CT scan.