The purpose of this study is to determine the maximum tolerated dose and safety of S-1 plus radiotherapy for patients with rectal cancer

This stage I study is designed to evaluate the appropriate dose of S-1 plus radiotherapy for patients with R0 resection of rectal cancer for stage II-III patients(AJCC 7th). To evaluate safety, data on adverse events will be collected from the time of enrollment until withdrawn of complete whole study.

Inclusion Criteria:

• R0 resection of histologically proved stage II/III rectal cancer;

• 18-75 years old;

• No previous radiotherapy or chemotherapy for rectal cancer;

• Performance status of ECOG 0,1;

• Adequate organ function defined as below:

  i. WBC ≥ 4,000/mm^3 ii. ANC ≥ 1,500/mm^3 iii. Hemoglobin ≥ 10g/dL iv. Platelet ≥ 100,000/m^3 v. Total bilirubin ≤ 1.5ULN vi. AST/ALT ≤ 1.5ULN vii. Serum creatinine ≤ 1.5ULN or creatinine clearance rate ≥ 60ml/min、Urea nitrogen ≤ 1.5ULN viii. Protein in urine dipstick test<1+; if the test result >1+ ,total protein in urea must <500mg within 24 hours

• Able to receive oral administration

• Informed consent

Exclusion Criteria:

• Hypersensitive to S-1 or its excipients
• Pulmonary fibrosis or interstitial pneumonitis found within 28 days prior to registration
• Significant co-morbid medical conditions, including, but not limited to, heart failure, renal failure, hepatic failure, hemorrhagic peptic ulcer, mechanical or paralytic ileus, or poorly controlled diabetes
• Received any investigational drug or anti-cancer agent
• Pregnant or lactating female or pregnancy test positive
• Severe mental disorder
• Judged ineligible by physicians for participation in the study due to safety concern.