Node-Sparing Hypofractionated Radiotherapy Plus Chemotherapy and PD-1 Inhibitor in pMMR/MSS High-Risk Locally Advanced Colon Cancer: A Prospective, Randomized, Phase II Trial

Most colorectal cancers are microsatellite stable (MSS) or mismatch repair-proficient (pMMR) subtypes, which show limited efficacy to PD-1 inhibitors. Radiotherapy can enhance the release of tumor-associated antigens, thereby improving the responsiveness of pMMR/MSS colorectal cancers to PD-1 blockade. Tumor-draining lymph nodes (TDLNs) are critical sites where PD-1 inhibitors exert their antitumor effects; however, previous studies have reported that direct radiation-induced damage and fibrosis may impair lymphatic drainage and antitumor immunity. Early reports have demonstrated a remarkable pathological complete response (pCR) rate of 77.8% with lymph node-sparing short-course radiotherapy (25 Gy in 5 fractions) in locally advanced rectal cancer. In metastatic colorectal cancer, single-fraction high-dose irradiation (6-8 Gy) has been shown to induce robust abscopal effects. Based on these findings, our study aims to evaluate whether lymph node-sparing hypofractionated radiotherapy (25 Gy/5F or 24 Gy/4F) followed sequentially by chemotherapy and PD-1 blockade can increase the pCR rate, improve tolerability, and ultimately enhance outcomes in patients with pMMR/MSS high-risk locally advanced colon cancer.