To evaluate the efficacy and feasibility of external beam radiotherapy(EBRT) for duodenal adenocarcinoma in an 84-year-old female who underwent EBRT(2.2 Gy/d for a total dose of 46.2 Gy) using helical tomotherapy(HT). Toxicity was evaluated on the National Cancer Institute’s common toxicity criteria(CTCAE 3.0). The patient completed the treatment without G3-G4 toxicity. After 22-mo follow-up, she is alive and well, in complete remission with no late side effects. HT seems to be feasible and effective for duodenal adenocarcinoma in old to very old patients.