Evaluation of Table Transmission with Different Couch Tops Available at the Clinic
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PO-GePV-P-8 (Sunday, 7/10/2022) [Eastern Time (GMT-4)]
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Purpose: Modern external beam radiotherapy treatment planning requires inserting a couch model for appropriate couch attenuation. Modern linear accelerators are equipped with multiple tabletops and inserts for different treatments. For couch tops with rails, the position of the rails can be important. Other couch inserts can also be used such as the BoS Headframe or S-frame. It is important to quantify the transmission through different tabletops that we use clinically and compare it against the calculated values for comparison and quantification of the uncertainty imparted by these different couch models in a treatment plan.
Methods: We scanned a solid water phantom and several treatment plans were generated with a 4x4 cm2 posterior-anterior field and different combinations of beam energy and couch placement. The same dose-calculation algorithm was used as for clinical patient plans to determine dose to an ion chamber per 100 MU. We then obtained measurements for each calculated scenario and compared the measurements against the calculated values.
Results: The results indicate significant attenuation through the kVue couch rails for the measurements and calculations. However, when the rails are out of the field, the calculated relative dose difference between the Varian Exact couch and the QFix Calypso kVue couch is only up to 1%. Typically, we do not insert any couch models for treatment calculations on head, neck, and brain plans. The BoS frame measurements show attenuation of up to 2.5%, while the S-frame shows attenuation of only up to 0.9%.
Conclusion: The clinical physics team should do appropriate measurements and commission the couch parameters when acquiring couch accessories. In addition, it is advisable to use the proper couch models for treatment plans and give necessary instructions and training to the planning and treatment team about couch inserts and couch accessory positioning to avoid uncertainties in treatment delivery.
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