Couch Displacement Effects on Volumetric Modulated Arc Therapy Delivery and Verification of Simplified Couch Structure

Attenuation properties of the iBEAM evo carbon fiber couch were modeled with a commercial Monte Carlo treatment planning system, Monaco (Elekta AB, Stockholm, Sweden), and we evaluated the dosimetric effect due to the couch displacement on the prostate volumetric modulated arc therapy (VMAT) treatment plans. The couch shape and dimensions were set according to the vendor specifications and a cone-beam computed tomography scan. The accuracy of the couch model was validated by comparison with ionization chambers and treatment planning system calculated dose. In order to evaluate the couch displacement effects on prostate VMAT plans, phantom position on the couch was shifted from the original position by a simulation in Monaco and measurements with the ionization chamber and films were obtained. For a 5 cm shift, the deviation in dose at the isocenter was less than 1.0% and the dose distribution showed a good agreement with the original (no-shifted) plan. Our findings for the phantom lateral displacement suggest that the positioning accuracy will have no clinical significance on prostate VMAT plans when the iBEAM evo couch is adopted. The insensitivity of the couch displacement indicated that the couch dimension can be simplified in the couch modeling calculation in the Monaco treatment planning system.