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2)Graduate School of Health Sciences, Showa University, Kanagawa, Japan3)Department of Radiation Oncology, Faculty of Medicine, Juntendo University, Tokyo, Japan4)Department of Radiation Oncology, Shonan Kamakura General Hospital, Kanagawa, Japan5)Department of Radiological Technology, Faculty of Health Science, Juntendo University1)Department of Radiation Oncology, Graduate School of Medicine, Juntendo University, Tokyo, JapanHiroyuki WATANABE1, 2), Satoru SUGIMOTO3), Toru KAWABATA1), Jun TAKATSU3), Kyoichi KATO2), Keisuke SASAI1)Hironori NAGATA4), Chie KUROKAWA3, 5), Keisuke USUI3, 5), Tatsuya INOUE3), Corresponding author: Hiroyuki WatanabeDepartment of Radiation Oncology, Graduate School of Medicine, Juntendo University2-1-1 Hongo, Bunkyo-ku, Tokyo, 113-8421, JapanTEL: +81-3-6426-3055　FAX: +81-3-3784-8404　E-mail: watanabehi@cmed.showa-u.ac.jp〔Received　Jan. 14, 2022〕〔Accepted　Jun. 1, 2022〕J-STAGE Advance published date: Aug. 2, 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.JMJ22-0003-OAObjectives: In radiation therapy, the field-in-field (FIF) technique is used to prevent the administration of unnecessarily high doses to reduce toxicity. Recently, the FIF technique has been used for whole brain irradiation (WBI). Using the FIF technique, the volume that receives a higher than prescribed dose (hotspot) can be largely reduced; however, the treatment planning requires time. Therefore, to reduce the burden on the treatment planners, we propose a semiautomatic treatment planning method for the FIF technique.Methods: In the semiautomatic FIF technique, hotspot regions in a treatment plan without the FIF technique are identified three-dimensionally, and beams with blocks that cover the hotspot regions using a multileaf collimator (sub-beams) are automatically created. The sub-beams are added to the original plan, and weights are assigned based on the maximum dose of the original plan to decrease the doses in the hotspot regions. This method was applied to 22 patients previously treated with WBI, wherein treatment plans were originally created without the FIF technique.Results: In the semiautomatic FIF plans, the hotspots almost disappeared. The dose to 95% of the volume and the volume receiving at least 95% of the prescribed dose in the planning target volume decreased by only 0.3% ± 0.2% and 0.0% ± 0.1%, respectively, on average compared with those in the original plan. The average semiautomatic FIF processing time was 28 ± 4 s.Conclusions: The proposed method reduced the hotspot regions with a slight change in the target coverage.Key words: automation, radiotherapy treatment planning, whole brain irradiationJuntendo Medical Journal2022. 68(4), 375-386Original ArticlesSemiautomatic Treatment Planning for the Field-in-field Technique IntroductionWhole brain irradiation (WBI) is one of the main treatments for patients with brain metastases and is also performed for prophylactic cranial irradia-tion as an adjuvant therapy for patients with small cell lung cancers1-3). WBI is usually performed using opposed lateral fields. Treatment with opposed lateral fields generates a hotspot in the frontal lobe, where the dose is considerably higher than the prescribed dose2, 4). Recently, the uniformity of the target dose has been improved by introducing advanced techniques, such as intensity-modulated radiation therapy. The International Commission on Radiation Units and Measurements (ICRU) recommends that the dose in the planning target 375in Whole Brain Irradiation