Patterns of recurrence by microsatellite instability in colorectal cancer

501 Background: Although colorectal cancer (CRC) with microsatellite instability (MSI) has a more favorable prognosis than microsatellite stable (MSS) CRC, patterns of recurrence has not been precisely investigated. We aim to explore patterns of recurrence and prognosis of CRC with MSI-H (high-frequency MSI) in comparison to MSI-L (low-frequency of MSI)/MSS. Methods: Patients with stage I-III colorectal cancer who received complete surgical resection were included using retrospective cohort of Yonsei Cancer Center. Patients were categorized to MSI status (MSI-H vs. MSS-L/MSS). Patterns of recurrence, disease free survival (DFS) and overall survival (time from diagnosis to death as OS1 and time from recurrence to death as OS2) between two groups were compared. Results: A total of 2944 patients were evaluated. Of all, 263 patients (8.9%) has MSI-H tumor. Patients with MSI-L/MSS tumor experienced more systemic recurrence events than patients with MSI-H tumor (13.1% vs. 4.6%). In patients who experienced recurrence, MSI-H tumor has tendency toward young age, proximal location and poorly differentiated histology. MSI-H tumor is more likely to recur as peritoneal metastasis (33.3% vs. 8.0%, p-value 0.002) and lymph node metastasis (25.0% vs. 7.7%, p-value 0.033). In contrast, MSI-L/MSS tumor is more likely to recur as lung metastasis (37.1% vs. 0.0%, p-value 0,008). There was no difference in DFS [hazard ratio (HR) = 0.753, p-value 0.241] and OS1 [HR = 1.486, p-value 0.203]. However, OS2 was marginally inferior in MSI-H tumor [HR = 1.779, p-value 0.064]. Conclusions: Patterns of recurrence in MSI-H CRC is different from that of MSI-L/MSS and modification of surveillance strategy in MSI-H CRC should be considered. In addition, prognosis of recurred MSI-H CRC is not superior than MSI-L/MSS and prognostic value of MSI in this circumstance should be evaluated using larger sample size.