Quantitative indices from post-PRRT SPECT/CT: an exploratory approach to early imaging biomarkers in neuroendocrine...

To assess therapeutic response in patients with neuroendocrine tumors (NETs) using quantitative post-therapy ¹⁷⁷Lu-DOTATATE SPECT/CT metrics, such as standardized uptake values (SUVs), metabolic tumor volume (MTV), and total Lutetium uptake (TLuU), and to propose a response classification system based on these indices.

In this retrospective study, we analyzed 31 lesions in 7 patients with NETs treated with four cycles of peptide receptor radionuclide therapy (PRRT) with ¹⁷⁷Lu-DOTATATE. Visual interpretation with the Krenning score (K-S) and quantitative analysis (SUVmax, SUVpeak, SUVmean, MTV, TLuU) were performed after each cycle. Volumes of interest (VOI) were segmented using two thresholds: 40% of SUVmax or a fixed cutoff of 2.4. Treatment response was expressed as response ratios (RRs) across PRRT sessions.

SUVmax and SUVpeak correlated strongly with K-S and were consistent across thresholding methods. MTV and TLuU varied by VOI definition, but the cutoff method produced more stable longitudinal assessments. Lesions with early SUV-based RR reductions at the second PRRT cycle showed continued decline across treatment. Patients were categorized as "Good Responder," "Responder," or "Non-responder" based on quantitative trends, which aligned with visual assessments and CT findings.

Quantitative post-PRRT SPECT/CT provides objective, reproducible metrics for assessing NET therapy response. A fixed SUV cutoff allows standardized volumetric assessment. Early SUV-based index declines may act as predictive biomarkers of treatment efficacy, supporting personalized theranostic strategies.