The outcome of T4 locally advanced colon cancer is poor when locoregional recurrence occurred. Previous studies had showed that the aggravation of T4 colon cancer is higher than N-positive colon cancer, contributing to staging paradox that outcomes of stage IIB/C are poorer than stage IIIA colon cancer and higher locoregional recurrence of stage IIB/C colon cancer. The phenomenon cannot be explained by radicality, lymph node harvested and adjuvant chemotherapy administration. To elucidate the cancerous aggravation of T4 colon cancer, the investigators should dive into the clinicopathology, metastatic mechanism and cancer biology. The investigators hypothesize that peritoneal spreading can be one of the metastatic routes of T4 colon cancer; however, few studies have addressed the finding and deserved investigation. Previous research have found that serum epigenetic alterations and cell-free DNA can serve as a prognostic marker for stage II/III colon cancer. Therefore, the aim of this study is (1) to explore the role of ascites cfDNA as a potential biomolecular marker for locoregional recurrence; (2) to identify the risk factors of locoregional recurrence for T4 colon cancer by correlating biomolecular markers between clinicopathology, epigenomic alterations in blood and ascites.