Background and Aims: It is unknown whether tumoral somatostatin receptor subtype 2a (sst2a) 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 sst2a 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 sst2a IHC, and
(3) to compare characteristics of patients with sst2a 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. sst2a status was detected on tumor samples by IHC. Results: In total, 93% of GEP-NET samples showed sst2a IHC positivity. No statistically significant relationship was observed between in vitro sst2a 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 sst2a tumoral IHC with the exception of age at diagnosis (p = 0.007).
Conclusions: sst2a IHC of tumor samples has no additional value compared to SRS uptake using OctreoScan® in predicting tumor response after PRRT.

Peptide receptor radiotherapy using 177Lu-octreotate, Somatostatin subtype 2a expression, Tumor response, Gastroenteropancreatic neuroendocrine tumors,
Neuroendocrinology: international journal for basic and clinical studies on neuroendocrine relationships
Erasmus MC: University Medical Center Rotterdam

van Adrichem, R.C.S, Kamp, K, van Deurzen, C.H.M, Biermann, K, Feelders, R.A, Franssen, G, … de Herder, W.W. (2016). Is there an Additional Value of Somatostatin Receptor Subtype 2A Immunohistochemistry over Somatostatin Receptor Scintigraphy Uptake in Predicting Gastroenteropancreatic Neuroendocrine Tumor Response?. Neuroendocrinology: international journal for basic and clinical studies on neuroendocrine relationships, 103(5), 560–566. doi:10.1159/000441604