Preoperative Sequential Short-course Radiation Therapy and FOLFOX for Locally Advanced Rectal Cancer (SOLAR)

Inclusion Criteria: Asian ECOG (Eastern Cooperative Oncology Group) performance status 0-2 Pathologically confirmed rectal cancer (rectal cancer located 10 cm or low from the anal verge in pelvis MRI) Histologic type: adenocarcinoma, mucinous carcinoma, and signet ring cell carcinoma Locally advanced rectal cancer with one or more of the following factors based on pelvis MRI: cTanyN1-2, cT3-4N0, or presence of extramural vascular invasion MRI findings such as pelvic lymph node metastasis, anal sphincter invasion, and T4b are not included in the exclusion criteria, which cases will be enrolled by the researcher's discretion Patients with appropriate organ (bone marrow, kidney, liver) function A person who understands the study and willing to provide informed consent Exclusion Criteria: (one of the following criteria) Colon cancer or rectal cancer located more than 10 cm from the anal verge Stage I rectal cancer (clinical stage cT1-2N0) Clinically or pathologically diagnosed distant metastasis (cTanyNanyM1) Familial adenomatous polyposis Hereditary nonpolyposis colorectal cancer History of chemotherapy or radiotherapy within 6 months History of colorectal cancer or other type of malignancy within 5 years (except for cured nonmelanoma skin or in situ cervical cancer) Comorbidities that make it difficult to undergo chemotherapy or radiotherapy Bone marrow suppression with neutrophil count <2 ×109/L or platelet count <100 ×109/L prior to the first chemotherapy Peripheral sensory neuropathy with functional impairment (grade 2 or higher) Severe renal dysfunction (GFR ≤30ml/min by Wright or Cockroft formula) Severe hepatic dysfunction Genetic problems such as galactose intolerance, Lapp lactase deficiency or glucose-galactose malabsorption Taking tegafur, gimeracil, and oteracil potassium complex and those within 7 days of discontinuation Taking sorivudine or brivudine Significant heart disease or myocardial infarction within the last 6 months Hereditary diseases or history of coagulopathy Central nervous system disorders with disability or mental disorders Pregnant or lactating women Currently participating in other clinical trials or receiving research medication Unhealed wounds, fractures, peptic ulcer, abscesses in the abdominal cavity Active gastrointestinal bleeding Active infections requiring antibiotics for injection Emergency Surgery History of hypersensitivity to the drugs in study protocol Patients with non-malignant tumor diseases Dihydropyrimidine dehydrogenase deficiency Not willing to participate