Couch modelling for volumetric modulated arc therapy (VMAT)

The aim of this work was to investigate the impact of the accuracy of a certain couch model on the overall accuracy of the dose calculation. For this purpose two models of the Varian Exact Couch (Varian Medical Systems, Palo Alto, CA, USA) for use in the planning of volumetric modulated arc therapy (VMAT) were created in Monaco 5.1 (Elekta, Stockholm, Sweden). The first was a simplistic model, with the relative electron densities (ED) of structures based entirely on CT Hounsfield units (HU). The second couch model was obtained by assigning the appropriate EDs to the relevant couch structures. Instead of getting EDs from the CT scan using the Hounsfield units to ED conversion, the second approach was to determine the optimal ED iteratively. The values accepted for the second couch model were the EDs that produced the best agreement between Monaco calculations and the measured couch attenuation. Finally, the performance of the two couch models in clinical prostate VMAT plans was compared.