Impact of local tumor-to-background uptake ratio on PET metabolic response assessment

Positron Emission Tomography Response Criteria in Solid Tumors (PERCIST) quantifies changes in radiotracer uptake to assess therapeutic response in cancer. However, the accuracy of these quantifications depends on imaging parameters, tumor size, and the local tumor-to-background uptake ratio (local-TBR). In this study, 'background' refers to the surroundings of the lesion rather than a standardized reference tissue. A NEMA Image Quality phantom was filled with 18F-FDG at varying sphere-to-background ratios to simulate clinical scenarios corresponding to PERCIST-defined thresholds for partial metabolic response (-30%) and progressive metabolic disease (+30%). Positron emission tomography (PET)/computed tomography imaging revealed that measured uptake changes systematically underestimated the true ±30% differences, particularly in smaller spheres. These findings indicate a potential source of systematic error in PET-based tumor response assessment, which may influence clinical interpretation. Further studies are recommended to investigate the effects of varying imaging parameters, tumor types, and clinical settings to improve the robustness of PERCIST-based evaluations.