Lymphoma Therapy Response Assessment with Low-Dose [¹⁸F]FDG Total-Body PET/CT

The improved sensitivity of total-body (TB) PET/CT offers the possibility of reducing injected activities. The aim of our study was to define a lower limit of reduced injected activities in [¹⁸F]FDG TB PET/CT for interim and end-of-treatment assessment of patients with lymphoma at 2 acquisition times. <b>Methods:</b> Twenty-four consecutive patients with lymphoma who were undergoing interim and end-of-treatment TB PET/CT were prospectively enrolled in this study. An [¹⁸F]FDG activity of 3.0 MBq/kg served as the reference standard (RS). Images simulating low doses of 1.0, 0.5, 0.25, and 0.125 MBq/kg were reconstructed at 1 and 2 h after injection. The coefficient of variation of the liver was assessed. Lymphoma lesions were segmented and semiquantitatively compared with the RS using the SUV. Additionally, metabolic tumor volume (MTV) for each lesion, patient-based total MTV, and total-lesion glycolysis (TLG) were analyzed. Semiquantitative parameters were normalized to the liver and blood pool by tumor-to-background ratios (TBRs) and contrast-to-noise ratios. Therapy response was assessed using Deauville criteria. <b>Results:</b> Overall, 191 lymphoma lesions were analyzed. SUV_max demonstrated a trend toward a statistically significant increase in scans with reduced activity at 1 h after injection (6.28 ± 5.87 for RS vs. 7.76 ± 6.69 for 0.125 MBq/kg; <i>P</i> = 0.07) and 2 h after injection (7.14 ± 7.16 for RS vs. 8.67 ± 7.62 for 0.125 MBq/kg; <i>P</i> = 0.13). SUV_peak, SUV_mean, MTV, and TLG did not significantly differ between the reduced injected activities and the RS. The coefficient of variation for the liver increased significantly with decreasing injected activities (<i>P</i> < 0.01). The TBR for the liver did not differ significantly, whereas the TBR for the blood pool was significantly higher only for the lowest injected activity (<i>P</i> < 0.01) at 2 h after injection. The contrast-to-noise ratio significantly decreased with reduced activities. Deauville scores did not differ significantly, up to a dose of 0.25 MBq/kg at 1 h after injection and a dose of 1.0 MBq/kg at 2 h after injection. Below this limit, we noted significantly lower Deauville scores for reduced injected activities (<i>P</i> < 0.01). <b>Conclusion:</b> Reduction of injected activities with [¹⁸F]FDG TB PET/CT for therapy response assessment in patients with lymphoma may be possible and does not result in significant differences in MTV, TBR, or TLG. SUV_max and Deauville scores were comparable to the RS to a lower limit of 0.25 MBq/kg at 1 h after injection and 1.0 MBq/kg at 2 h after injection.