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2)Department of Radiation Therapy, Misugikai Satou Hospital, Osaka, Japan1)Department of Radiation Oncology, Juntendo University, Graduate School of Medicine, Tokyo, JapanCorresponding author: Keisuke Sasai (ORCID: 0000-0002-9962-6497)Department of Radiation Oncology, Juntendo University, Graduate School of Medicine2-1-1 Hongo, Bukyo-ku, Tokyo 113-8421, JapanE-mail: ksasai@juntendo.ac.jp355th Triannual Meeting of Juntendo Medical Society “Farewell Lectures of Retiring Professors”〔Held on Mar. 30, 2022〕〔Received　Jun. 8, 2022〕〔Accepted　Jun. 16, 2022〕J-STAGE Advance published date: Aug. 15, 2022Copyright © 2022 The Juntendo Medical Society. This is an open access article distributed under the terms of Creative Commons Attribution License (CC BY), which permits unrestricted use, distribution, and reproduction in any medium, provided the original source is properly credited. doi: 10.14789/jmj.JMJ22-0025-R　In the present review, I provide an overview of the development of radiation therapy and short history of the Department of Radiation Oncology, Juntendo University. I also emphasize the importance of radiation therapy as a major treatment modality for cancers.　Radiation therapy is a standard treatment for malignant tumors. It aims to deliver a sufficient radiation dose to a target volume to eradicate tumor cells or relieve symptoms of disease. Therapy can achieve good results in many types of cancers. Although radiation therapy sometimes causes undesirable adverse events, it is generally less invasive than other treatment modalities and does not alter the shape and function of healthy organs. When the author joined this field in 1981, radiation therapy techniques were highly primitive; however, during the past 42 years, treatment has advanced rapidly with the development of computer science, mechanical techniques and instrumentation. Currently, patients can be treated with precise radiation techniques, including intensity-modulated radiation therapy, image-guided radiation therapy, stereotactic irradiation, and brachytherapy. We also introduced a new treatment planning system that uses not only anatomical but also metabolic imaging, which permits correct delineation of the target volume. Therefore, it is crucial to stay up to date with advances and developments in rapidly emerging technologies to maintain high-quality treatment. The Department of Radiation Oncology at Juntendo University (Tokyo, Japan) is still small; however, it is gradually expanding and conducting research in both clinical and basic fields. It is the author’s hope that many young investigators will join this field in the future.332Juntendo Medical Journal2022. 68(4), 332-338Special ReviewsIntroductionRadiation therapy is a standard treatment for malignant diseases. It is a treatment modality that aims to deliver a sufficient radiation dose to a target volume to eradicate tumor cells or relieve symp‑toms of disease.In the United States, approximately one-half of patients with cancers undergo radiation therapy; however, only one-quarter of such patients are irradiated in Japan1). There are several reasons for this low frequency of radiation therapy. For example, Key words: radiation therapy, radiation oncology, image-guided radiation therapy, intensity modulated radiation therapy, biological target volumethe number of cancers that are not candidates for radiation therapy, such as gastric malignancy, are higher than those in the United States or in Euro‑pean countries. Additionally, as a result of the destructive atomic bombings of Hiroshima and Nagasaki (Japan), and the Fukushima nuclear catastrophe, many Japanese generally fear radia‑tion and radiotherapy. In the present review, I provide an overview of the development of radiation therapy and short history of the Department of Radiation Oncology, Juntendo University (Tokyo, Japan). I also empha‑My 42-year Experience in Radiation OncologyKeisuke SASAI1, 2)