Node-Sparing Modified Short-Course Radiotherapy Combined with CAPOX and Tislelizumab for Locally Advanced MSS of Middle and Low Rectal Cancer (mRCAT): An open-label, single-arm, prospective, multicentre clinical trial
Authors: Cai Cheng , Zhang Xia , Sun Xiaonan , Wang Huogang , Chen Engeng , Chen Li , Gu Benxing , Wang Jianping , Huang Xuefeng , Lao Weifeng , Wang Xiaowei , Chen Min , Ding Shubo , Du Jinlin , Song Zhangfa 
Published: September 2023

Background
Neoadjuvant chemoradiotherapy followed by total mesorectal excision is a standard treatment for locally advanced rectal cancer. Mismatch repair-deficient locally advanced rectal cancer was highly sensitive to PD-1 blockade. However, most rectal cancers are mismatch repair-proficient or microsatellite stable subtypes for which PD-1 blockade is ineffective. Radiation can trigger the activation of CD8 + T cells, further enhancing the responses of microsatellite stable rectal cancer to PD-1 blockade. Radioimmunotherapy offers a promising therapeutic modality for rectal cancer. Progenitor T exhausted cells are abundant in tumour-draining lymph nodes and play an important role in immunotherapy. Conventional irradiation fields include the mesorectum and regional lymph nodes, which might cause considerable damage to T lymphocytes and radiation-induced fibrosis, ultimately leading to a poor response to immunotherapy and rectal fibrosis. This study investigated whether node-sparing modified short-course irradiation combined with chemotherapy and PD-1 blockade could be effective in patients with microsatellite stable locally advanced rectal cancer.
Methods
Our clinical trial investigates the safety and efficacy of node-sparing modified short-course radiotherapy combined with CAPOX and tislelizumab for microsatellite stable locally advanced middle and low rectal cancer. This phase II study will recruit 32 patients to receive node-sparing modified short-course radiotherapy (the irradiated planned target volume only included the primary tumour bed but not the tumour-draining lymph nodes, 25 Gy/5f, 5 Gy/f) followed by CAPOX and tislelizumab. CAPOX and tislelizumab will be started two days after the completion of radiotherapy: oxaliplatin 130 mg/m2 intravenous infusion, day 1; capecitabine 1000 mg/m2 oral administration, days 1–14; and tislelizumab 200 mg, intravenous infusion, day 1. There will be three 21-day cycles. TME will be performed at weeks 11–12. We will collect blood, tumour, and lymphoid specimens; perform flow cytometry and in situ multiplexed immunofluorescence detection; and analyse the changes in various lymphocyte subsets. The primary endpoint is the rate of pathological complete response. The organ preservation rate, tumour regression grade, local recurrence rate, disease-free survival, overall survival, adverse effects, and quality of life will also be analysed.
Discussion
In our research, node-sparing modified radiotherapy combined with immunotherapy probably increased the responsiveness of immunotherapy for MSS rectal cancer patients, reduced the occurrence of postoperative rectal fibrosis, and improved survival and quality of life. This is the first clinical trial to utilize a node-sparing radiation strategy combined with chemotherapy and PD-1 blockade in the neoadjuvant treatment of rectal cancer, which may result in a breakthrough in the treatment of MSS rectal cancer.
Trial registration:
This study was registered at www.clinicaltrials.gov. Trial registration number: NCT05972655. Date of registration: 31 July 2023.