Small bowel irradiation results in epithelial cell loss and consequently impairs function and metabolism. A metabolic end product of small bowel enterocytes is plasma citrulline. The investigators evaluate the correlation between plasma citrulline level, dose-volume histogram of small bowel, and small bowel toxicity grade by Common Terminology Criteria for Adverse Events version 4.0 (CTCAE ver.4.0) to investigate whether citrulline can be used as a biomarker for quantifying radiation-induced epithelial cell loss.

Inclusion Criteria:

• Histologically confirmed cancer patients
• Appropriate liver, renal, and bone marrow function for radiotherapy
• Willing to provide informed written consent
• At least 20 years old

Exclusion Criteria:

• prior abdominopelvic radiation therapy or chemotherapy or abdominal surgery
• Any contraindication to radiotherapy (i.e. Severe connective tissue disorder, etc.)
• Prior or simultaneous history of other malignancy
• On medication for small bowel disease or CTCAE 4.0 Grade 1 or higher toxicity before radiation
• Any treatment delay more than 1 week during radiotherapy
• No radiotherapy due to any other reason except small bowel toxicity