Tehran University of Medical Sciences Frontiers in Biomedical Technologies 2345-5837 0 0 2025 06 02 732 732 Mahya Badrasa Department of Medical Physics, Faculty of Medicine, Shahid Sadoughi University of Medical Sciences, Yazd, Iran Mojan Mobaraki Department of Medical Physics, Faculty of Medicine, International Campus, Shahid Sadoughi University of Medical Sciences and Health Services, Yazd, Iran Abolfazl Nickfarjam Department of Medical Physics, Faculty of Medicine, Shahid Sadoughi University of Medical Sciences, Yazd, Iran Mohammad Hasan Larizadeh Department of Radiation Oncology, Afzalipour Radiation Oncology Center, Kerman University of Medical Sciences, Kerman, Iran Nasim Namiranian Yazd Diabetes Research Center, Shahid Sadoughi University of Medical Sciences, Yazd, Iran Nima Hamzian Department of Medical Physics, Faculty of Medicine, Shahid Sadoughi University of Medical Sciences, Yazd, Iran 2023 06 15 2025 03 01 Purpose: The posterior oblique beams are increasingly common in radiotherapy techniques. The radiation beams traversing through the treatment couch would be attenuated and cause under-dosage in the tumor region. The attenuation of an IGRT carbon fiber Couch for different angles, energies, field sizes, measurement points, couch regions, and the ability of the Eclipse treatment planning system in dose prediction was investigated.   Materials and Methods: Vital Beam linear accelerator and Exact IGRT couch top from Varian were applied. At first, the couch coefficient was used to find the most attenuation angle. Then, at the most attenuation gantry angle, the attenuation measurements were performed in three measurement points of an inhomogeneous thoracic phantom using a farmer ionization chamber for three energies with six field sizes in three regions of an IGRT couch.   Results: In three regions of the IGRT couch and the angle of 130˚, the photon beam was most attenuated. The most significant difference between calculated and measured point doses was 1.855%.   Conclusion: The IGRT treatment couch in posterior oblique gantry angles decreased the dose in the measurement points due to gantry angle, field size, energy, and couch region. The Eclipse treatment planning system can sufficiently predict the tumor dose distribution. https://fbt.tums.ac.ir/index.php/fbt/article/view/732 https://fbt.tums.ac.ir/index.php/fbt/article/download/732/506