ESTRO 2025 - Abstract Book

S1161

Clinical – Lower GI

ESTRO 2025

837

Proffered Paper Preoperative Short-course Radiotherapy Followed by Chemotherapy and PD-1 Inhibitor for Locally Advanced Rectal Cancer: Phase II Results of STELLAR II Yuan Tang 1 , Haoyue Li 1 , Lichun Wei 2 , Ning Li 3,4 , Wenjue Zhang 5 , Feiyan Deng 5 , Yufei Lu 6 , Zifa Lei 7 , Xianyu Meng 2 , Shunan Qi 1 , Yongwen Song 1 , Wenwen Zhang 1 , Hao Jing 1 , Gong Li 8 , Shixin Liu 9 , Zheng Jiang 10 , Jianqiang Tang 10 , Zheng Liu 10 , Haitao Zhou 10 , Shulian Wang 1 , Chen Hu 11 , Yexiong Li 1 , Jing Jin 1,5 1 State Key Laboratory of Molecular Oncology and Department of Radiation Oncology, National Cancer Center/National Clinical Research Center for Cancer/Cancer Hospital, Chinese Academy of Medical Sciences and Peking Union Medical College (PUMC), Beijing, China. 2 Department of Radiation Oncology, First Affiliated Hospital of Air Force Medical University, Xi'an, China. 3 Department of Radiation Oncology, National Cancer Center/National Clinical Research Center for Cancer/Cancer Hospital, Chinese Academy of Medical Sciences and Peking Union Medical College (PUMC), Beijing, China. 4 Department of Radiobiology, Shanxi Province Cancer Hospital/ Shanxi Hospital Affiliated to Cancer Hospital, Chinese Academy of Medical Sciences/Cancer Hospital Affiliated to Shanxi Medical University, Taiyuan, China. 5 Department of Radiation Oncology, National Cancer Center / National Clinical Research Center for Cancer / Cancer Hospital & Shenzhen Hospital, Chinese Academy of Medical Sciences and Peking Union Medical College, Shenzhen, China. 6 Department of Radiation Oncology, Cancer Hospital of Henan Province, Zhengzhou, Henan, China. 7 Tumor Radiotherapy Department, First Affiliated Hospital of Air Force Medical University, Xi'an, China. 8 Department of Radiation Oncology, Beijing Tsinghua Changgung Hospital, School of Clinical Medicine, Tsinghua University, Beijing, China. 9 Department of Radiation Oncology, Jilin Provincial Cancer Hospital, Changchun, China. 10 Department of Colorectal Surgery, National Cancer Center/National Clinical Research Center for Cancer/Cancer Hospital, Chinese Academy of Medical Sciences and Peking Union Medical College (PUMC), Beijing, China. 11 Division of Quantitative Sciences, Sidney Kimmel Comprehensive Cancer Center, Johns Hopkins University School of Medicine, Baltimore, USA Purpose/Objective: For LARC, previous STELLAR study has shown that a total neoadjuvant therapy (TNT) paradigm of short-course radiotherapy (SCRT) followed by 4 cycles chemotherapy can achieve non-inferior DFS, higher complete response (CR) and better OS compared to chemoradiotherapy. Recent studies have indicated that, PD-1/PD-L1 inhibitors have strong synergistic potential with radiotherapy and may help improving the CR rate. Thus, the STELLAR II trial (NCT05484024) aims to investigate whether SCRT-based TNT combined with PD-1 inhibitor (iTNT) could further improve prognosis. Material/Methods: This was a multicenter, randomized, phase II/III trial for pMMR/MSS LARC in China. Eligible patients were randomly assigned to the iTNT or TNT group . Patients in the iTNT group received SCRT (5 Gy× 5), followed by 4 cycles of CAPOX and PD-1 inhibitor (sintilimab), while the TNT group receiving the same therapy but without sintilimab. After neoadjuvant therapy, resection or W&W strategy was planned based on efficacy evaluation, then 2 cycles of same regimen as before were recommended. The primary endpoints were CR rate for phase II and 3y-DFS for phase III. The outcomes of phase II trial are presented. Results: From 2022 August to 2023 November, 218 eligible patients were randomized, and of whom 125 and 93 patients were treated according to the iTNT and TNT groups, respectively. The median age was 59 years (range, 21-74), and 72.5% were men. Most of them had T3-4 (95.9%), N+ (94.0%) and lower location tumors (62.8%), with 53.7% MRF+ and 46.3% EMVI+. All patients completed SCRT. The 4-cycle completion rates were 87.1% (iTNT group) and 95.6% (TNT group). In the iTNT group, 82.3%, 92.7%, 98.4% and 100% of patients received ≥4,3,2 and 1 cycle of sintilimab, respectively. The rates of grade 3-4 treatment-related toxicities were 31.2% (iTNT group) and 19.4% (TNT group), of