Is There an Additional Value of Using Somatostatin Receptor Subtype 2a Immunohistochemistry Compared to Somatostatin Receptor Scintigraphy Uptake in Predicting Gastroenteropancreatic Neuroendocrine Tumor Response?

Abstract

Background and Aims: It is unknown whether tumoral somatostatin receptor subtype 2a (sst_2a) immunohistochemistry (IHC) has additional value compared to somatostatin receptor scintigraphy (SRS) uptake using OctreoScan® in predicting response to peptide receptor radiotherapy using ¹⁷⁷Lu-octreotate (PRRT) in patients with gastroenteropancreatic neuroendocrine tumors (GEP-NETs). The aims of this study were: (1) to establish the percentage of sst_2a immunopositivity in GEP-NET samples of PRRT-treated patients, (2) to determine the relationship between best GEP-NET response using RECIST 1.0 criteria 1 year after PRRT and tumoral sst_2a IHC, and (3) to compare characteristics of patients with sst_2a IHC-negative and -positive tumors. Methods: All 73 consecutive patients were selected for PRRT based on a positive SRS. Radiological response was scored according to RECIST 1.0 criteria. sst_2a status was detected on tumor samples by IHC. Results: In total, 93% of GEP-NET samples showed sst_2a IHC positivity. No statistically significant relationship was observed between in vitro sst_2a expression and in vivo best GEP-NET response 1 year after PRRT (p = 0.47). Sex, primary tumor site, disease stage, ENETS TNM classification, Ki-67 index, highest serum chromogranin-A level, and highest neuron-specific enolase level were not significantly different between patients with negative and positive sst_2a tumoral IHC with the exception of age at diagnosis (p = 0.007). Conclusions: sst_2a IHC of tumor samples has no additional value compared to SRS uptake using OctreoScan® in predicting tumor response after PRRT.