ESTRO 2025 - Abstract Book

S1162

Clinical – Lower GI

ESTRO 2025

which thrombocytopenia was the most common (5.0%). While, 6.5% of grade 3 irAEs were observed. Finally, 152 patients underwent surgery, of whom 42.7% (35/82) in the iTNT group and 22.9% (16/70) in the TNT group reached pCR; 29 patients adopted W&W after clinical complete remission. The CR rate in the iTNT group was 45.6% (57/125), which was significantly higher than that in the TNT group (24.7% (23/93), P = .002). Tissue sequencing is currently underway. Conclusion: SCRT-based TNT combined with PD-1 inhibitor further improved the CR rate in pMMR/MSS LARC with good tolerance. Our phase III trial is in progress and the survival results are worth waiting for. References: 1. Jin, J., et al., Multicenter, Randomized, Phase III Trial of Short-Term Radiotherapy Plus Chemotherapy Versus Long Term Chemoradiotherapy in Locally Advanced Rectal Cancer (STELLAR). J Clin Oncol, 2022. 40(15): p. 1681-1692. 2. Xia, F., et al., Randomized Phase II Trial of Immunotherapy-Based Total Neoadjuvant Therapy for Proficient Mismatch Repair or Microsatellite Stable Locally Advanced Rectal Cancer (TORCH). J Clin Oncol, 2024: p. Jco2302261. 3. Zhang, W., et al., Preoperative short-course radiotherapy followed by chemotherapy and PD-1 inhibitor administration for locally advanced rectal cancer: A study protocol of a randomized phase II/III trial (STELLAR II study). Colorectal diseas, 26(9), 1732–1740. 929 Poster Discussion Impact of thermoradiation short course radiotherapy on locally advanced rectal cancer (LARC): Prospective phase II trial. Barbara G Salas-Salas 1 , Laura Ferrera-Alayón 1 , Antonio A. Alayón-Afonso 1 , Miguel Sanchez-Carrascal 2 , Johannes Crezee 3 , Marta Lloret Saez-Bravo 1 1 Radiation Oncology, University Hospital Dr Negrin, Las Palmas de Gran Canaria, Spain. 2 Radiophysics, University Hospital Dr Negrin, Las Palmas de Gran Canaria, Spain. 3 Radiation Oncology, Amsterdam UMC, Amsterdam, Netherlands Purpose/Objective: The aim of this study to investigate the effect of preoperative short course radiotherapy (SCRT) plus Total neoadyuvant chemotherapy (TNT-ChT), combined with locoregional deep hyperthermia (HT) on locally advanced rectal cancer (LARC) patients compared with SCRT plus TNT-ChT alone. Material/Methods: In this double-arm, prospective phase II RTHT-LARC trial (#2020-335-1), after informed consent, 61 patients with LARC were included between March 2020 and July 2023. Eligible patients were aged ≥ 18, pathologically confirmed adenocarcinoma and a magnetic resonance imaging (MRI) confirmed LARC stage cT1-2N+M0, cT3N0M0, cT3N+M0, cT4N0M0, and cT4N+M0 (Table 1). Patients were assigned to either the SCRT plus TNT-ChT alone group (n=27) or SCRT plus TNT-ChT plus HT group (n=34). All patients were treated with short course radiotherapy, 25Gy in five fractions, once a day for a week. Total neoadjuvant chemotherapy consisted of FOLFOX or XELOX for 18 weeks. Adjuvant HT was delivered twice a week, with an interval of 72 hours during SCRT. Primary endpoint was defined as a clinically relevant increase of ≥ 10% in pathological complete response (pCR) rate between both arms. Secondary endpoints were acute toxicity (≤ 3 months) according to Common Terminology Criteria for Adverse Events version 5.0 and the assessment of HT tolerance and adherence. Results: From 60 patients (98.4%) histopathological-results were obtained. 13 patients (39.5%) achieved pCR in the SCRT plus TNT-ChT plus HT group, compared to 29.5% (n=8) in the SCRT plus TNT-ChT alone, a 10% clinical relevant difference. There was no significant difference in acute toxicities between groups (P=1.00) and no significant difference in grade Keywords: rectal cancer, radiotherapy and immunotherapy