Introduction: The accuracy of patient positioning plays an important role for radiotherapy tasks. Short and automated Quality Assurance (QA) programs are becoming one of the challenging tasks in Radiotherapy. The current study, investigates the accuracy of treatment couch positioning with four degrees of freedom (4DoF) using a fast and accurate method based on an image acquired using an electronic portal imaging device (EPID). Materials and Methods: A Siemens medical linear accelerator equipped with an a-Si EPID was used to capture portal images. An accurate EPID QA phantom (PTW EPID QC Phantom®) and a proper in-house algorithm were used. The phantom has different sections to assess EPID image quality. Preliminary tests including Mechanical performance of the EPID positioning, EPID sensitivity and the code response from image processing point of view were investigated through several deviations deliberately created to evaluate translational and rotational displacements of treatment couch. Loading effect on treatment couch was also investigated. Results: The results of EPID preliminary tests and in-house code reliability were within tolerances expected. The mean deviations of tests for (4DoF) treatment couch were 0.25°±0.10˚, 0.428±0.112 mm, 0.428±0.112 mm and 0.286±0.112 mm for rotational, longitudinal, lateral and vertical shifts, respectively. The results of incline caused by a 130kg loading on the couch showed that the proposed method is a reliable and fast to find the uncertainties occurred for treatment couch positioning. Conclusion: As a part of linac QA program, treatment couch tests including translational and rotational motions under unloaded/loaded conditions were performed. The corresponding errors for table positioning including minimum detectable rotational and translational displacements, and couch sagging effect under loaded conditions were evaluated using an EPID. Creating one major test for the whole assessments for couch checks, fast and accurate and no labour intensive procedure, and electronic portal image instead of film and other imagers can be addressed as advantages of the method proposed.

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