Purpose: The purpose of this study was to estimate the clinical utilization of radiation therapy (RT) in Korea between 2009 and 2013. Materials and Methods: We analyzed open claims data from the Health Insurance Review and Assessment Service. The subjects were patients who had diagnostic codes C00-C97 or D00-D48 according to the 10th revision of the International Classification of Diseases, with procedure codes indicating RT treatment. Results: The total number of patients who received RT in 2009, 2010, 2011, 2012, and 2013 were 45,571, 49,593, 54,671, 59,172, and 61,485, respectively. Among them, the total numbers of male and female patients were 20,780/24,791 in 2009, 22,711/26,882 in 2010, 24,872/29,799 in 2011, 27,101/32,071 in 2012, and 27,941/33,544 in 2013. The five cancers that were most frequently treated with RT between 2009 and 2012 were breast, lung, colorectal, liver, and uterine cervical cancers. However, the fifth most common cancer treated with RT that replaced uterine cervical cancer in 2013 was prostate cancer. The three leading types of cancer among the male patients were lung, colorectal, and liver cancers, whereas in female patients, they were breast, uterine cervical, and lung cancers. The type of cancer most commonly treated by RT was cancer of the central nervous system in patients aged 20 years or less, breast cancer in patients aged 30-50 years, and lung cancer in patients aged 60 years or more. Conclusion: Data from this study provided the clinical utilization of RT in Korea between 2009 and 2013.