Total Neoadjuvant mFOLFOX6 Plus Suvemcitug and Enlonstobart With Selective Omission of Pelvic Radiotherapy for High-Risk Mid-Rectal Locally Advanced Rectal Cancer

This is a prospective, multicenter, open-label, single-arm Phase II study designed to evaluate the efficacy and safety of total neoadjuvant mFOLFOX6 combined with suvemcitug and enlonstobart in patients with MRI（Magnetic Resonance Imaging）-defined high-risk, mid-rectal, locally advanced rectal adenocarcinoma with pMMR/MSS status. All enrolled participants will receive 6 cycles of mFOLFOX6 plus suvemcitug and enlonstobart every 2 weeks. After 6 cycles, participants will undergo reassessment with pelvic MRI and systemic imaging. Participants who achieve at least 20% tumor regression on MRI without new high-risk radiologic features will continue with 2 additional cycles of mFOLFOX6 followed by radical total mesorectal excision surgery, without routine pelvic radiotherapy. Participants who do not meet the predefined regression threshold or show disease progression will receive long-course chemoradiotherapy with concurrent capecitabine, followed by 3 cycles of mFOLFOX6 and radical surgery.