Purpose: The aim of this study was to correlate immunohistochemical (IHC) tissue levels of SSTR1-5 with the receptor density generated from [⁶⁸Ga]Ga-DOTANOC uptake in a prospective series of NF-PNENs.

Methods: Twenty-one patients with a total of thirty-five NF-PNEN-lesions and twenty-one histologically confirmed lymph node metastases (LN+) were included in this prospective study. Twenty patients were operated on, and one underwent endoscopic ultrasonography and core-needle biopsy. PET/CT with both [⁶⁸Ga]Ga-DOTANOC and [¹⁸F]F-FDG was performed on all patients. All histological samples were re-classified and IHC-stained with monoclonal SSTR1-5 antibodies and Ki-67 and correlated with [⁶⁸Ga]Ga-DOTANOC and [¹⁸F]F-FDG PET/CT.

Results: Expression of SSTR1-5 was detected in 74%, 91%, 80%, 14%, and 77% of NF-PNENs. There was a concordance of SSTR2 IHC with positive/negative [⁶⁸Ga]Ga-DOTANOC finding (Spearman's rho 0.382, p = 0.043). All [⁶⁸Ga]Ga-DOTANOC-avid tumors expressed SSTR2 or SSTR3 or SSTR5. Expression of SSTR5 was higher in tumors with a low Ki-67 proliferation index (PI) (-0.353, 95% CI -0.654-0.039, p = 0.038). The mean Ki-67 PI for SSTR5 positive tumors was 2.44 (SD 2.56, CI 1.0-3.0) and 6.38 (SD 7.25, CI 2.25-8.75) for negative tumors.

Conclusion: SSTR2 was the only SSTR subtype to correlate with [⁶⁸Ga]Ga-DOTANOC PET/CT. Our prospective study confirms SSTR2 to be of the highest impact for SST PET/CT signal.