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Verification of Extended Source-To-Imager Distance (SID) Correction for Portal Dosimetry
Prog. Med. Phys. 2018;29(4):137-142
Published online December 31, 2018
© 2018 Korean Society of Medical Physics.

Jaeman Son*, Jung-in Kim*,†,‡, Jong Min Park*,†,‡,§, Chang Heon Choi*

*Department of Radiation Oncology, Seoul National University Hospital, Biomedical Research Institute, Seoul National University Hospital, Institute of Radiation Medicine, Seoul National University Medical Research Center, Seoul, §Center for Convergence Research on Robotics, Advanced Institutes of Convergence Technology, Suwon, Korea
Correspondence to: Chang Heon Choi ( 
Tel: 82-2-2072-4151  Fax: 82-2-2650-4151
Received November 20, 2018; Revised December 7, 2018; Accepted December 10, 2018.
This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License ( which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
This study aimed to evaluate and verify a process for correcting the extended source-to-imager distance (SID) in portal dosimetry (PD). In this study, eight treatment plans (four volumetric modulated arc therapy and four intensity-modulated radiation therapy plans) at different treatment sites and beam energies were selected for measurement. A Varian PD system with portal dose image prediction (PDIP) was used for the measurement and verification. To verify the integrity of the plan, independent measurements were performed with the MapCHECK device. The predicted and measured fluence were evaluated using the gamma passing rate. The output ratio was defined as the ratio of the absolute dose of the reference SID (100 cm) to that of each SID (120 cm or 140 cm). The measured fluence for each SID was absolutely and relatively compared. The average SID output ratios were 0.687 and 0.518 for 120 SID and 140 SID, respectively; the ratio showed less than 1% agreement with the calculation obtained by using the inverse square law. The resolution of the acquired EPIDs were 0.336, 0.280, and 0.240 for 100, 120, and 140 SID, respectively. The gamma passing rates with PD and MapCHECK exceeded 98% for all treatment plans and SIDs. When autoalignment was performed in PD, the X-offset showed no change, and the Y-offset decreased with increasing SID. The PD-generated PDIP can be used for extended SID without additional correction.
Keywords : Portal dosimetry, PDIP, Extended SID, VMAT

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