575P - Prognostic value of microsatellite instability status in stage II/III rectal cancer patients who received upfront surgery

Abstract

Background: Microsatellite instability (MSI) is one of the most important prognostic factors in patients with colon cancer but the impact of MSI in rectal cancer, which is known to have a lower incidence of MSI-high (MSI-H) tumours than proximal colon cancer, has not been fully evaluated. We studied whether MSI status affects survival in stage II and III rectal cancer patients who underwent upfront curative resection. Methods: 1138 patients who had operation between February 2008 and August 2015 in a single, tertiary care center in South Korea were included and PCR-based MSI testing was performed on tumour tissue from each patient. Study endpoints were disease free survival (DFS) and overall survival (OS). Results: Among 1138 patients, 25 (2.2%) had MSI-H tumours. Compared with microsatellite stable (MSS) or MSI-low (MSI-L) tumours, MSI-H showed similar clinical characteristics including age at diagnosis, gender, tumour location and pathologic tumour stage but they were highly associated with histological grade of tumour (p = 0.005) and presence of family history of colorectal cancers (p = 0.003). The 5-year DFS rates for patients with MSI-H and MSS/MSI-L were 78.0% and 69.2%, respectively (p = 0.637), and the 5-year OS rate was 84.0% with MSI-H and 82.4% with MSS/MSI-L (p = 0.735). On multivariate Cox regression analysis, there was also no significant difference in either DFS (p = 0.855) or OS (p = 0.912) for the patients with rectal cancer based on MSI status. Conclusions: Our results suggested that MSI has no definite prognostic role in patients with rectal cancer. MSI status was associated with differentiation of tumour and family history of colorectal cancer. Legal entity responsible for the study: Department of Oncology, Asan Medical Center, Seoul, Republic of Korea Funding: None Disclosure: All authors have declared no conflicts of interest.