OBJECTIVES: Radiation-induced nausea and vomiting (RINV) is commonly experienced by cancer patients undergoing radiation treatment.  Current NCCN guidelines recommend the use of the 5-hydroxytryptamine-3 serotonin receptor antagonists (5-HT3-RAs) for the prevention and treatment of RINV.  The purpose of this study was to examine the incidence of 5-HT3-RA utilization and subsequent RINV, in patients diagnosed with colon cancer and undergoing radiation treatment.  METHODS: This was a retrospective analysis using HIPAA-compliant medical and pharmacy claims in a commercially-insured U.S. patient population.  The study identified continuously enrolled adult patients diagnosed with colon cancer who received radiation treatment during the period from 4/1/2007 to 8/31/2009.  Patients receiving combination chemotherapy were excluded.  The index date was the date of initiation of radiation treatment and patients were followed for 30 days.  Prophylactic administration of a 5-HT3-RA was defined by medication on the day of or the day prior to radiation therapy.  The incidence of RINV was calculated from medical claims for an outpatient appointment, inpatient admission, or ER visit with a primary diagnosis of RINV.  RESULTS: A total of 2,400 patients were identified as meeting the inclusion and exclusion criteria for the study.  Average age of patients was 57.6 years old and 43.8% were female.  Radiation therapy included localized treatment (48.6%), total body irradiation (30.9%), or both (20.5%).  Of the patients identified, 73.9% received no 5-HT3-RA, 5.0% were treated prophylactically, and 21.1% received a 5-HT3-RA in the days after radiation therapy.  Of the patients who received the 5-HT3-RA prophylactically, 58.2% were treated with an oral formulation.  Incidence of subsequent RINV for all patients, independent of rescue medication use, was 15.2%.  CONCLUSIONS: The results showed use of the 5-HT3-RA antiemetics in patients undergoing radiation treatment for colon cancer was uncommon and prophylactic administration was relatively rare.