Background: Although early stage colorectal cancer (CRC) with microsatellite instability (MSI) phenotype has a more favorable prognosis, impact on outcome of MSI status in metastatic CRC (mCRC) is rarely known. We aim to explore patterns of metastatic spread and prognosis of CRC with MSI-H (high-level MSI) in comparison to MSI-L (low-level of MSI)/MSS.

Methods: Patients with mCRC who underwent testing for MSI were identified using retrospective cohort of Yonsei Cancer Center. Patients were categorized to MSI status (MSI-H vs. MSS-L/MSS). Patterns of metastatic spread, surgical resection after recurrence and outcome between two groups were compared.

Results: A total of 944 patients were evaluated. Of all, 34 patients (3.6%) had MSI-H tumors. A distinct pattenrs of metastatic spread was observed in MSI-H tumors, namely higher rates of peritoneal metastasis (41% vs 10%, P < 0.001), intraabdominal lymph node metastasis (14.7% vs 4.8%, P = 0.011), and lower rates of lung metastasis (8.8% vs 24.6%, P = 0.034), liver metastasis (20.6% vs 53.6%, P < 0.001). Patients with MSI-H tumors were less likely to receive metastectomy (24.8% vs 55.9%, P < 0.001). Median overall survival was inferior in MSI-H tumor compared with MSI-L/MSS tumor (16.3 months vs 38.1 months, P = 0.005).

Conclusions: Different patterns of metastatic spread and receipt of surgical resection in MSI-H mCRC is demonstrated in this study. In addition, MSI-H phenotype is associated with poorer survival in mCRC on the contrary to early stage disease. Future studies focusing on deeper understanding of tumor biology and therapeutic response in this rare disease entity are warranted.

Citation Format: Jee Hung KIM, Chang-gon Kim, Joong Bae Ahn, MinKyu Jung, Seung Hoon Beom, Joo Hoon Kim, Soo Jin Heo, Sang Joon Shin. Microsatellite instability in metastatic colorectal cancer (mCRC). [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 808.