Purpose: We aimed to evaluate the clinical characteristics of microsatellite instability in early gastric cancer. Materials and Methods: The microsatellite instability status of resected early gastric tumors was evaluated using two mononucleotide repeat markers (BAT25 and BAT26) and three dinucleotide repeat markers (D5S346, D2S123, and D17S250). Tumors with instability in two or more markers were defined as microsatellite instability-high (MSI-H) and others were classified as microsatellite stable (MSS). Results: Overall, 1,156 tumors were included in the analysis, with 85 (7.4%) classified as MSI-H compared with MSS tumors. For MSI-H tumors, there was a significant correlation with the female sex, older age, tumor location in the lower gastric body, intestinal histology, lymphovascular invasion (LVI), and submucosal invasion (P<0.05). There was also a trend toward an association with lymph node (LN) metastasis (P=0.056). In mucosal gastric cancer, there was no significant difference in MSI status in tumors with LN metastasis or tumors with LVI. In submucosal gastric cancer, LVI was more frequently observed in MSI-H than in MSS tumors (38.9% vs. 25.0%, P=0.027), but there was no difference in the presence of LN metastases. The prognosis of MSI-H tumors was similar to that of MSS tumors (log-rank test, P=0.797, the hazard ratio for MSI-H was adjusted by age, sex, pT stage, and the number of metastatic LNs, 0.932; 95% confidence interval, 0.423-2.054; P=0.861). Conclusions: MSI status was not useful in predicting prognosis in early gastric cancer. However, the frequent presence of LVI in early MSI-H gastric cancer may help guide the appropriate treatment for patients, such as endoscopic treatment or limited LN surgical dissection.

Purpose: The role of microsatellite instability is well established in advanced gastric cancer. However, its clinical implication has not been well addressed in early gastric cancer. The purpose of this study is to evaluate the clinical characteristics of microsatellite instability in early gastric cancer. Methods: Patients who underwent gastrectomy with curative intent in Severance hospital from January, 2005 to December, 2010 and diagnosed as early gastric cancer (pT1a/b) were enrolled in this study. Remnant gastric cancer and patients who received chemotherapy before operation were excluded. Microsatellite instability status was evaluated by two mononucleotide repeat markers (BAT25 and BAT26) and three dinucleotide repeat markers (D5S346, D2S123, and D17S250) and instability in two or more markers was defined as miscrosatellite instability high (MSI-H), otherwise classified as microsatellite stable (MSS). Results: Of 1156 tumors that included the final analysis, 85 (7.4%) were MSI-H. MSI-H cancer was related to frequent female gender (54.1% vs. 32.8%, p&lt;0.001), older age (63.4 ± 10.65 vs. 56.7 ± 11.50, p&lt;0.001), lower body (81.2% vs. 59.7%, p=0.002), intestinal histology (63.5% vs. 48.2%, p=0.005), lympho-vascular invasion (25.9% vs. 13.3%, p=0.001), submucosal invasion (63.5% vs. 48.3%, p=0.007), and trend of lymph node metastasis (17.6% vs. 10.8%, p=0.056) compared to MSS type. Lymph node metastasis and lymph-vascular invasion was not differed by MSI status in mucosal gastric cancer (3.2% vs. 2.3%, p=0.755 and 3.2% vs. 2.3%, p=0.755, respectively). In sumbucosal gastric cancer, however, lympho-vascular invasion was frequently observed in MSI-H tumor (38.9% vs. 25.0%, p=0.027) but there was no difference in lymph node metastasis (25.9% vs. 19.9%, p=0.298). When we compared the disease free survival by MSI status, the prognosis of MSI-H tumor was similar compared to that of MSS tumor (log-rank test p=0.797, adjusted Hazard ratio of MSI-H by age, sex, pTstage and number of metastatic LNs: 0.932 [95% confidence interval: 0.423-2.054, p=0.861]). Conclusions: MSI status could not be a useful biomarker in early gastric cancer to predict prognosis of it. However, frequent lympho-vascular invasion of MSI-H early gastric cancer would be a warning that careful patient's selection for endoscopic treatment or limited lymph node dissection for surgery is necessary. Citation Format: Yoon Young Choi, Dong Gyu Kim, Taeil Son, Hyoung-Il Kim, Jae-Ho Cheong, Woo Jin Hyung, Sung Hoon Noh. Clinical implication of microsatellite instability in early gastric cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 2650.