Short-Course Radiotherapy Followed by Immunotherapy Combined With Chemotherapy for Microsatellite Stable Locally Advanced Rectal Cancer: A Prospective, Randomized, Controlled Phase II Clinical Trial

Microsatellite stable (MSS) locally advanced rectal cancer (LARC) remains a major therapeutic challenge despite advances in multimodal treatment. Colorectal cancer is the third most common malignancy worldwide and the second leading cause of cancer-related death. In China, rectal cancer accounts for nearly half of all colorectal cancers, with approximately 70% of patients presenting with locally advanced disease, which is associated with a high risk of recurrence and poor long-term survival. Neoadjuvant chemoradiotherapy followed by total mesorectal excision (TME) has become the standard treatment for LARC based on landmark trials such as CAO/ARO/AIO-94, NSABP-R03, and MRC-CR07, which demonstrated improved local control and reduced recurrence. However, the optimal neoadjuvant strategy remains under active investigation. Currently, long-course chemoradiotherapy and short-course radiotherapy (SCRT) are the two principal preoperative radiotherapy approaches. Long-course chemoradiotherapy achieves superior tumor downstaging and pathological complete response (pCR) rates but requires prolonged treatment duration and is associated with greater acute toxicity. In contrast, SCRT offers shorter treatment time, lower cost, and reduced toxicity. Importantly, delayed surgery after SCRT can significantly enhance tumor regression and achieve pCR rates comparable to those of long-course chemoradiotherapy. The development of total neoadjuvant therapy (TNT) has further transformed rectal cancer management by moving systemic chemotherapy to the preoperative setting, thereby improving treatment compliance, tumor response, and organ preservation. Among TNT strategies, consolidation chemotherapy after radiotherapy appears to provide superior tumor regression compared with induction chemotherapy. Immune checkpoint inhibitors targeting the PD-1/PD-L1 pathway have demonstrated remarkable efficacy in mismatch repair-deficient (dMMR)/microsatellite instability-high (MSI-H) rectal cancer.