Neoadjuvant chemoradiation therapy combined with immunotherapy for microsatellite stable ultra-low rectal cancer (CHOICE II): study protocol of a multicentre prospective randomised clinical trial.

introductionNeoadjuvant chemoradiotherapy (nCRT) could bring tumour shrinking and downstaging and increase the probability of organ preservation for patients with low rectal cancer. But for ultra-low rectal cancer, there is little possibility for organ preservation. Immunotherapy has been shown to have significant survival benefits in microsatellite instability-high patients but poor response in microsatellite stable (MSS) patients. Studies have demonstrated that radiotherapy and immunotherapy have synergistic effects in cancer treatment. There is no existing evidence about the clinical efficacy of immunotherapy combined with nCRT for patients with MSS ultra-low rectal cancer. METHOD AND ANALYSIS: This trial is an open-labelled multicentre prospective randomised controlled trial (NCT05215379) with two parallel groups and allocation ratio 1:1 (nCRT+immunotherapy vs nCRT group). Eligible participants will be aged 18-75 years, with a desire for anus preservation, confirmed cT ETHICS AND DISSEMINATION: This trial was approved by the Ethics committee of Changhai Hospital and other medical centres (Grant number:CHEC2022-118). The results of this study will provide further insight into the clinical efficacy of immunotherapy in combination with nCRT in patients with MSS ultra-low rectal cancer. TRIAL REGISTRATION NUMBER: NCT05215379.