an update of couch effect on the attenuation of megavoltage radiotherapy beam and the variation of absorbed dose in the build-up region

Purpose: fiber carbon is the most common material used in treating couch as it causes less beam attenuation than other materials. beam attenuation replaces buildup region, reduces skin-sparing effect and causes target volume under dosage. in this study, we aimed to evaluate beam attenuation and variation of build-up region in 550 txt radiotherapy couch. materials and methods: in this study, we utilized cylindrical pmma farmer chamber, dose-1 electrometer and set pmma phantom in isocenter of gantry and the farmer chamber on the phantom. afterwards, the gantry rotated 10°, and attenuation was assessed. to measure build-up region, we used markus chamber, solid water phantom and dose-1 electrometer. doing so, we set solid water phantom on isocenter of gantry and placed markus chamber in it, then we quantified the build-up region at 0° and 180° gantry angels and compared the obtained values. results: notable attenuation and build-up region variation were observed in 550 txt treatment table. the maximum rate of attenuation was 5.95% for 6 mv photon beam, at 5×5 cm2 field size and 130° gantry angle, while the maximum variation was 7 mm for 6 mv photon beam at 10×10 cm2 field size. conclusion: fiber carbon caused beam attenuation and variation in the build-up region. therefore, the application of fiber carbon is recommended for planning radiotherapy to prevent skin side effects and to decrease the risk of cancer recurrence.