Background: The incidence trend of esophagogastric junction (EGJ) adenocarcinoma in Japan has not been sufficiently investigated. Little is known about the microsatellite instability (MSI) status of this tumor. Summary: Previously published studies analyzing the trend of EGJ adenocarcinoma in Japan were reviewed. And a trend of surgically resected cases (Siewert type I-III) utilizing a retrospective multicenter cohort of 379 patients from 4 academic institutions in Japan investigated. Although an increasing trend in the last 2 reports was considered controversial, our cohort demonstrated a growing number of EGJ adenocarcinoma cases between 2006 and 2013. This trend was evident, especially in Siewert type I cases. In the previous 16 studies that performed MSI testing, MSI-high tumors ranged 0-8.3%, though there were no fixed microsatellite markers on EGJ adenocarcinoma. In a recent comprehensive genetic analysis by The Cancer Genome Atlas, MSI testing using the following 7 markers, BAT25, BAT26, BAT40, D2S123, D5S346, D17S250 and TGFR-II showed a favorable correlation with hypermutated tumors. We performed MSI testing using 6 of those markers, except TGFR-II, on 206 cases from one institution, and detected 15 cases (7.3%) with MSI-high. The prevalence of MSI-high was 0% in Siewert type I, 7.6% in type II, and 16.7% in type III. Key message: The number of surgically resected EGJ adenocarcinoma cases gradually increased, and MSI-high was infrequent in Siewert type I-II tumors in our Japanese cohort. Considering MSI-high as a predictive biomarker for emerging immune checkpoint inhibitors, MSI status is becoming more beneficial in EGJ adenocarcinoma.
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