Alireza Farajollahi - ۱. Drug Applied Research Center, Tabriz University of Medical Sciences, Tabriz, Iran ۲. Medical Radiation Science Research Team, Tabriz University of Medical Sciences ۳. Department of Medical Physics, Faculty of Medicine, Tabriz ۴.
narjes mohammadzadeh - Medical Physics Research Center, Tabriz University of Medical Sciences, Tabriz, Iran
Mehdi Momennezhad - Mashhad University Of Medical sciences
shahrokh naseri - Department of medical physics, Faulty of medicine, Mashhad University of Medical Sciences, Mashhad, Iran
shokoufeh mohebbi - Medical physics Department, Reza Radiation Oncology Center, Mashhad, Iran
Fateme Shahedi - Medical Physics Research Center, Mashhad University of Medical Sciences, Mashhad, Iran.
shadi mohebbi - Statistics research center, Mashhad university of Mathematical of sciences, Mashhad, Iran

Introduction: Interfractional set-up variations may cause deviation of the delivered dose from the planned dose distribution. This study aimed at calculating random and systematic set-up errors using an electronic portal imaging device (EPID) to set the optimum planning target volume (PTV) margins in patients with head and neck cancer who were under treatment with three-dimensional conformal (۳DCRT) and intensity-modulated radiotherapy (IMRT). Material and Methods: In this study, ۵۰ patients underwent ۳DCRT along with weekly electronic portal image (EPI), and daily IMRT imaging was performed on ۵۰ others. The EPIs were compared with Digitally Reconstructed Radiographs (DRRs) to quantify the systematic, random, and ۳D vector length of set-up errors in three translational directions (X, Y, Z). The PTV margins were measured utilizing International Commission on Radiation Units and Measurements report ۶۲, Stroom’s and van Herk’s models. Results: For ۳DCRT and IMRT techniques, the overall mean ۳D vector length of displacement was obtained at ۳.۹ and ۲.۷ mm, respectively. The maximum systematic and random errors were ۱.۳ and ۱.۹ mm for the IMRT technique and ۲ and ۲.۹ mm for ۳DCRT, respectively. PTV margins in the three acquisition directions were ۲-۷.۲ mm. Conclusion: It was found that a ۷ mm extension of the clinical target volume (CTV) to PTV margin ensures that ۹۰% of head and neck cancer patients have received ۹۵% of the planned dose.

۳DCRT Head and Neck IMRT Set, Up Errors