Molecular characterization of appendiceal goblet cell carcinoid

231 Background: Goblet cell carcinoid (GCC) is a distinct subtype of appendiceal neoplasm that exhibits unique clinical and pathologic features. There are a few reports focusing on the molecular differences between GCC and other appendiceal tumors such as adenocarcinoma and neuroendocrine tumor (NET). Methods: A total of 495 appendiceal tumor samples (53 GCCs, 428 adenocarcinomas and 14 NETs) were tested with Next-Generation Sequencing (NGS) on a 592-gene panel and immunohistochemistry (IHC). Microsatellite instability (MSI) / mismatch repair (MMR) status was tested with a combination of NGS, IHC and fragment analysis. Tumor mutational burden (TMB) was evaluated by NGS, and PD-L1 were tested by IHC (SP142). Molecular characteristics of GCCs are compared with those of adenocarcinomas and NETs, using Chi-square test. Results: The top five genes with most frequent mutation rate in GCCs were TP53 (24.0%), ARID1A (15.4%), SMAD4 (9.4%), KRAS (7.5%) and CHEK2 (4.0%). Compared to adenocarcinomas, GCCs showed significantly lower mutation rates in KRAS (7.5% vs 60.4%), GNAS (3.8% vs 34.4%), APC (1.9% vs 11.7%), while significantly higher mutation rates in CDH1 (3.8% vs 0.7%), CHEK2 (4.0% vs 0.3%), CDC73 (2.0% vs 0.0%), ERCC2 (2.0% vs 0.0%) and FGFR2 (1.9% vs 0.0%). Compared to NETs, GCCs showed significantly lower mutation rate in KRAS (7.5% vs 28.6%), APC (1.9% vs 28.6%), BRCA2 (0.0% vs 7.1%) and FANCA (0.0% vs 7.1%), with all p < 0.05. In GCCs, MSI-H/dMMR, TML-high (> 17mut/Mb) and PD-L1 expression were seen in 0.0%, 0.0% and 2.0%, respectively. No significant difference was observed in these immune-related markers’ frequency, compared to adenocarcinomas and NETs. Conclusions: GCCs had considerably distinct mutational profile compared to appendiceal adenocarcinomas and NETs. Understanding these molecular characteristics may be critical for a development of effective treatment strategy in GCC.