Association of microsatellite instability with prognosis in the pathologic N2 colon cancer patients treated with adjuvant FOLFOX chemotherapy.

Abstract

3568 Background: The impact of microsatellite instability (MSI) on survival has been rarely explored, with controversy, in stage III colon cancer treated with adjuvant 5-flurouracil-oxaliplatin combination (FOLFOX) chemotherapy. We evaluated association between MSI and survival in this population. Methods: We analyzed 312 patients (pts) with curatively resected stage III colon cancer treated with adjuvant FOLFOX chemotherapy. We determined MSI status using polymerase chain reaction amplification as high levels of MSI (MSI-H, ≥ 2 unstable markers), low levels of MSI (MSI-L, 1 unstable marker), and microsatellite stable (MSS, no unstable marker). Results: Of 312 pts, 8.3% showed MSI-H and they were younger (median age, 51.5 vs. 58 years, p=0.018). MSI-H tumors were more commonly located ascending colon (46.2% vs. 23.1%, p=0.002) and had poorly differentiated features (30.8% vs. 5.2%, p<0.0001). After the median follow-up of 30.9 months, 3-year relapse-free (RFS) and overall survival (OS) rates were 76.1% and 91.7%, respectively. In univariate analysis, pathologic N2 (pN2) was associated with reduced RFS (p<0.0001) and OS (p=0.002), while MSI status did not affect either RFS (p=0.254) or OS (p=0.961). In patients with pN2 tumors, however, MSI-H was associated with better survival compared with MSS/MSI-L; the RFS and OS in pts with pN2/MSI-H were comparable to those in pts with pN1 tumors (table). Conclusions: MSI status alone did not affect survival in our population. However, pts with pN2/MSI-H showed favorable survival outcomes comparable to those with pN1, and MSS/MSI-L/pN2 was associated with worst survival, implicating that MSI-H modifies the inherent worse prognosis of pN2 tumors. [Table: see text]