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Julie Refardt, Wouter T Zandee, Tessa Brabander, Richard A Feelders, Gaston J H Franssen, Leo J Hofland, Emanuel Christ, Wouter W de Herder, and Johannes Hofland

Sufficient expression of somatostatin receptor (SSTR) in well-differentiated neuroendocrine tumors (NETs) is crucial for treatment with somatostatin analogs (SSAs) and peptide receptor radionuclide therapy (PRRT) using radiolabeled SSAs. Impaired prognosis has been described for SSTR-negative NET patients; however, studies comparing matched SSTR-positive and -negative subjects who have not received PRRT are missing. This retrospective analysis of two prospectively maintained NET databases aimed to compare matched metastatic grade 1 or 2 SSTR-positive and –negative NET patients. SSTR-negativity was defined as having insufficient tumor uptake on diagnostic SSTR imaging. Patients that underwent PRRT were excluded. Seventy-seven SSTR-negative and 248 SSTR-positive grade 1–2 NET patients were included. Median overall survival rates were significantly lower for SSTR-negative compared to SSTR-positive NET patients (53 months vs 131 months; P < 0.001). To adjust for possible confounding by age, gender, grade and site of origin, 69 SSTR-negative NET patients were propensity score matched to 69 SSTR-positive NET patients. Group characteristics were similar, with the exception of SSTR-negative patients receiving more often chemotherapy and targeted treatment. The inferior survival outcome of SSTR-negative compared to SSTR-positive NET patients persisted with a median overall survival of 38 months vs 131 months (P = 0.012). This relationship upheld when correcting for the main influencing factors of having a higher grade tumor or receiving surgery in a multivariate Cox regression analysis. In conclusion, we showed that propensity score-matched SSTR-negative NET patients continue to have a worse prognosis compared to SSTR-positive NET patients despite receiving more aggressive treatment. Differences in tumor biology likely underlie this survival deficit.