In 2016, the American Association of Physicists in Medicine (AAPM) has published a report of task group (TG) 100 with a completely new concept, entitled "application of risk analysis methods to radiation therapy quality management." TG-100 proposed implementation of risk analysis in radiotherapy to prevent harmful radiotherapy accidents. In addition, it enables us to conduct efficient and effective quality management in not only advanced radiotherapy such as intensity-modulated radiotherapy and image-guided radiotherapy but also new technology in radiotherapy. It should be noted that treatment process in modern radiotherapy is absolutely more complex and it needs skillful staff and adequate resources. TG-100 methodology could identify weakness in radiotherapy procedure through assessment of failure modes that could occur in overall treatment processes. All staff in radiotherapy have to explore quality management in radiotherapy safety.