One-day dual-tracer examination in neuroendocrine neoplasms: a real advantage of low activity LAFOV PET imaging

Somatostatin receptor (SSTR)-PET is crucial for effective treatment stratification of neuroendocrine neoplasms (NENs). In highly proliferating or poorly differentiated NENs, dual-tracer approaches using additional [¹⁸F]FDG PET can effectively identify SSTR-negative disease, usually requiring separate imaging sessions. We evaluated the feasibility of a one-day dual-tracer imaging protocol with a low activity [¹⁸F]FDG PET followed by an SSTR-PET using the recently introduced [¹⁸F]SiFAlin-TATE tracer in a long axial field-of-view (LAFOV) PET/CT scanner and its implications in patient management.

Twenty NEN patients were included in this study. Initially, a low activity [¹⁸F]FDG PET was performed (0.5 ± 0.01 MBq/kg; PET scan 60 min p.i.). After 4.2 ± 0.09 h after completion of the [¹⁸F]FDG PET, a standard activity of [¹⁸F]SiFAlin-TATE was administered (3.0 MBq/kg; PET scan 90 min p.i.). To ensure the quantification accuracy of the second scan, we evaluated the potential impact of residual [¹⁸F]FDG activity by segmenting organs with minimal physiological SSTR-tracer uptake, such as the brain and myocardium, and assessing the activity concentrations (ACTs) of tumor lesions. Residual tumor lesion ACTs of [¹⁸F]FDG were calculated by factoring fluorine-18 decay, identifying a maximum residual ACT of 15% (R15%). To account for increased [¹⁸F]FDG trapping over time, higher residual ACTs of 20% (R20%) were considered. These simulated [¹⁸F]FDG ACTs were compared with those measured in the second PET scan with [¹⁸F]SiFAlin-TATE. The influence of the dual-tracer PET/CT results on therapeutic strategies was evaluated.

[¹⁸F]FDG cerebral uptake significantly decreased in the subsequent SSTR-PET (mean uptake [¹⁸F]FDG: SUV_mean 6.0 ± 0.4; mean uptake in [¹⁸F]SiFAlin-TATE PET: SUV_mean 0.2 ± 0.01; p < 0.0001); with similar results recorded for the myocardium. Simulated residual [¹⁸F]FDG ACTs represented only a minimal percentage of ACTs measured in the tumor lesions from the second PET scan (R15%: mean 5.2 ± 0.9% and R20%: mean 6.8 ± 1.2%), indicating only minimal residual activity of [¹⁸F]FDG that might interfere with the second PET scan using [¹⁸F]SiFAlin-TATE and preserved semi-quantification of the latter. Dual-tracer PET/CT findings directly influenced changes in therapy plans in eleven (55%) of the examined patients.

LAFOV PET scanners enable a one-day dual-tracer protocol, providing diagnostic image quality while preserving the semi-quantification of two ¹⁸F-labeled radiotracers, potentially simplifying the assessment of tumor biology and improving the clinical patient management while reducing logistical challenges. Additionally, low-activity PET imaging facilitates one-day dual-tracer PET examinations.