Feasibility of different lymph node metastases delineation approaches in [¹⁸F]SiTATE PET/CT imaging in NET patients

Accurate tumor delineation in somatostatin receptor (SSTR) PET/CT is imperative for quantifying tumor burden, therapy monitoring, and response assessment in gastroenteropancreatic neuroendocrine tumors (GEPNETs). This study aimed to evaluate different SSTR PET-based tumor delineation approaches, based on a lymph node metastasis as reference standard, and determine the most robust method for volume assessment. Singular, non-bulky lymph node metastases with a short axis diameter ≥ 1.0 cm from [¹⁸F]SiTATE PET/CT scans were included. Different tumor delineation methods were applied: fixed SUV thresholds, isocontours relative to SUV_max, and individual backwards thresholding. The correlation between PET-derived volumes and CT-derived volumes (reference standard) were investigated.

An isocontour approach using 45% of SUV_max demonstrated the strongest correlation with CT-derived volumes (r = 0.874; r² = 0.764; p < 0.001), outperforming fixed SUV thresholds, e.g. SUV 4.0 (r = 0.727; r² = 0.529; p < 0.001). The application of a backwards threshold approach resulted in the identification of significant variability (CoV: 59.6% for fixed SUV, 31.3% for isocontour).

The 45% isocontour-thresholding relative to lesional SUV_max approach constitutes a robust and clinically applicable method for SSTR PET-based tumor delineation in GEPNET patients, irrespective of changes of SSTR avidity of reference tissues (e.g. liver). Further validation is required to establish its role in total tumor volume assessment and therapy monitoring.