A Prospective, Multicenter Randomized Controlled Study of the Application of Preoperative FOLFOXIRI Chemotherapy Combined With Lateral Lymph Node Dissection in Low- and Medium-lying Rectal Cancer With Lateral Lymph Node Metastasis

The goal of this clinical trial is to compare in newly diagnosed patients with resectable rectal cancer with lateral lymph node metastasis. The main questions it aims to answer are: * Evaluate the effectiveness and safety of Forfoxiri combined with lateral lymph node dissection, and optimize the treatment plan for rectal cancer patients with resectable lateral lymph node metastasis. * Using multiple omics techniques to identify and confirm specific molecular markers for chemotherapy response, lateral lymph node metastasis, and distant metastasis in rectal cancer, in order to achieve the goal of precise treatment of lateral lymph node metastasis Participants will * receive three cycles of chemotherapy with FOLFOXIRI regimen, and the specific dose was: irinotecan 150mg/m2, d1, Oxaliplatin 85mg/m2, d1, 5-Fu, 2400mg/m2, continuously pumped for 46 hours, repeated for 14 days. * After 3 cycles of chemotherapy, rectal MRI reexamination was performed, and the efficacy was evaluated using the RECIST method. * For cCR or cPR, another 2 cycles of chemotherapy will be performed, and CT and rectal MRI evaluations will be performed again after treatment. Radical rectal cancer surgery and LLND were performed 4-6 weeks after the last administration (unilateral or bilateral LLND was determined based on the metastasis of LLN before treatment). * RECIST evaluates patients with SD, and if MRI evaluates rectal cancer patients without rectal fascia involvement (MRF -), they will undergo two cycles of triple drug combination chemotherapy. After treatment, CT and rectal MRI evaluations will be performed again. Rectal cancer radical surgery+LLND will be performed 4-6 weeks after the last administration. * If MRF+is unable to complete R0 resection or RECIST evaluation of PD patients, in order to ensure the treatment effect of the patients, preoperative long-term synchronous radiotherapy and chemotherapy should be performed. Preoperative concurrent chemoradiotherapy regimen: Capecitabine (1650mg/m2/day, twice orally); Radiotherapy (pelvic cavity 2 Gy/dose, GT50 Gy); Intensity modulated radiation therapy is used for radiotherapy. CT and rectal MRI evaluations are performed again 8-12 weeks after radiotherapy, followed by rectal cancer radical surgery and LLND. If distant metastasis occurs during treatment, a second-line chemotherapy regimen can be chosen. Researchers will compare another group (Capecitabine (1650mg/m2/day, oral administration twice) was used for long-term concurrent chemoradiotherapy before operation in all patients; Radiotherapy (pelvic cavity 2 Gy/dose, GT50 Gy). Intensity modulated radiation therapy is used for radiotherapy. CT and rectal MRI evaluation are performed again 8-12 weeks after radiotherapy, followed by rectal cancer radical surgery+LLND (unilateral or bilateral LLND is determined based on the LLN metastasis before treatment)) to see if the experimental group is superior to the control group in terms of 3-year disease free survival, local recurrence rate, lateral lymph node positivity rate, and 5-year overall survival rate.