en Medical Physics Medical Physics تحقيق بديع Original Research <div style="text-align: justify;">Background: This study aims to investigate the effect of reference dose calculation grid size (RDCGS) on gamma passing rate (GPR) for patient-specific quality assurance of intensity-modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT). Materials and Methods: A total of 20 patients were retrospectively selected. Both IMRT and VMAT plans were generated for each patient. Reference dose distributions for gamma analysis were calculated with RDCGS of 1&ndash;5 mm at intervals of 1 mm. Dose distributions were measured using MapCHECK2 and ArcCHECK dosimeters. Both global and local gamma analyses with gamma criteria of 3%/3 mm, 2%/3 mm, 2%/2 mm, and 2%/1 mm were performed with various RDCGS. Results: As the RDCGS increased from 1 mm to 5 mm, the average global GPRs with 2%/2 mm for VMAT with MapCHECK2 and ArcCHECK decreased by 9.3% and 5.9%, respectively. The average local GPRs decreased by 14% and 11.7%, respectively. For IMRT, the global GPRs decreased by 4.8% and 6%, respectively, whereas the local GPRs decreased by 10.5% and 8.6%, respectively. The effect of the RDCGS on the GPRs became larger when performing local gamma analysis as well as when applying small distance-to-agreement (DTA). As the RDCGS increased, the average changes in the GPR per mm of DTA change increased regardless of the type of radiotherapy, detector, or gamma analysis. Conclusion: For an accurate verification of the IMRT and VMAT plans, it is recommended that the reference dose distribution must be calculated with the smallest possible RDCGS.</div> Dose calculation grid, gamma analysis, intensity-modulated radiation therapy, patient-specific QA, volumetric modulated arc therapy. 521 530 http://ijrr.com/browse.php?a_code=A-10-2055-39&slc_lang=en&sid=1 M. Chun 7900319475328460016017 7900319475328460016017 No Department of Radiation Oncology, Seoul National University Hospital, Seoul, Republic of Korea J.I. Kim 7900319475328460016018 7900319475328460016018 No Department of Radiation Oncology, Seoul National University Hospital, Seoul, Republic of Korea D.H. Oh 7900319475328460016019 7900319475328460016019 No Department of Radiation Oncology, Chung-Ang University Hospital, Seoul, Republic of Korea; H.G. Wu 7900319475328460016020 7900319475328460016020 No Department of Radiation Oncology, Seoul National University Hospital, Seoul, Republic of Korea J.M. Park leodavinci@naver.com 7900319475328460016021 7900319475328460016021 Yes Department of Radiation Oncology, Seoul National University Hospital, Seoul, Republic of Korea