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Toru KAWABATA1, 2), Satoru SUGIMOTO3), Chie KUROKAWA3, 4), Keisuke USUI3, 4), Tatsuya INOUE3), Hironori NAGATA5), Hiroyuki WATANABE1, 6), Keisuke SASAI1, 3)2)Varian Medical Systems, Tokyo, Japan6)Graduate School of Health Sciences, Showa University, Kanagawa, Japan5)Department of Radiation Oncology, Shonan Kamakura General Hospital, Kanagawa, Japan3)Department of Radiation Oncology, Faculty of Medicine, Juntendo University, Tokyo, Japan1)Department of Radiation Oncology, Graduate School of Medicine, Juntendo University, Tokyo, Japan4)Department of Radiological Technology, Faculty of Health Science, Juntendo University, Tokyo, JapanUsing an Electronic Portal Imaging Device Cine Mode590Corresponding author: Satoru SugimotoDepartment of Radiation Oncology, Faculty of Medicine, Juntendo University2-1-1 Hongo, Bunkyo-ku, Tokyo, 113-8421, JapanTEL: +81-3-5802-1230　FAX: +81-3-3816-0958　E-mail: ssugimot@juntendo.ac.jp〔Received　Apr. 18, 2022〕〔Accepted　Aug. 29, 2022〕J-STAGE Advance published date: Nov. 18, 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.JMJ22-0014-OAObjectives: To develop a method for detecting jaw positions during jaw-tracking delivery to ensure an accurate delivery of radiation to patients using an electronic portal imaging device (EPID) in the cine mode on a linear accelerator for radiotherapy.Materials: A bidirectional picket fence (BPF) plan was used in a novel application to detect jaw positions during jaw-tracking delivery. In the BPF plan, jaws tracked multileaf collimator (MLC) apertures. The irradiated patterns were acquired by an EPID in the cine mode.Methods: The upper- and lower-half leaves in the MLC moved in opposite directions to facilitate detection of jaw positions on EPID images. A picket-fence-like image was created by summing all acquired cine images and evaluated to detect MLC leaf positions.Results: Jaw positions determined on the cine images were compared with those expected from the delivered BPF plan. The absolute differences (average ± 1 standard deviation) were 0.16 ± 0.19 mm for the X1 jaw and 0.11 ± 0.16 mm for the X2 jaw. The maximum error in the MLC leaf positions detected in the picket-fence-like pattern were 0.11 mm.Conclusions: Jaw positions during jaw-tracking delivery were identified using the cine EPID images and could be determined within an accuracy better than 0.5 mm. The BPF plan is also available as a picket fence test and can determine the MLC leaf positions to an accuracy better than 0.5 mm.Key words: radiotherapy, quality assurance, jaw-tracking technique, multileaf collimatorJuntendo Medical Journal2022. 68(6), 590-598Original ArticlesDevelopment and Evaluation of Jaw Position Detection Method in Jaw-tracking Delivery IntroductionRadiation therapy (RT) using linear accelerators (linacs) can deliver volumetric modulated arc therapy (VMAT), which simultaneously allows delivery of a concentrated dose to a target and spare dosing to surrounding normal tissues1). VMAT is a dynamic arc irradiation with the complex modulation of the gantry speed, the shape of irradiation fields formed by multileaf collimators (MLCs), and dose rate.Linacs move collimator jaws located at the upstream of the MLC to block the interleaf trans-mission in VMAT, which is termed the jaw-tracking technique (JTT). The JTT can reduce doses deliv-ered to normal tissues surrounding a target volume2). Several researchers reported that compared with