Comparison of two treatment couch model in Monaco treatment planning system

Objective: To construct the uniform electron density couch model (model A ED = 0.25) and two components non uniform electron density couch model (model B FD = 0.5 and foam core = 0.1) in the Monaco treatment planning system for the iBEAM ^® evo Extension 415, and to compare which model can better quantify the treatment couch influence on radiation dose. Methods: Phantom was positioned in the center of the couch, the attenuation of the couch was evaluated with 6 MV for a field size of 10 cm × 10 cm. Dose measurements of couch attenuation were performed at gantry angles from 180.0 ° to 122.8 °, using a 0.125cc semiflex ionization chamber (PTW), isocentrically placed in the center of a homogeneous cylindrical phantom. Each experimental setup was first measured on the linear accelerator and then reproduced in the TPS. By adjusting the relative-to water electron density (ED) values of the couch, the measured attenuation was replicated. The model accuracies of the model A and model B were evaluated by comparing the measured and calculated results at the minimum computational grid (2 mm) and maximum computing grid (5 mm), respectively. Results: The maximum measured and calculated percentage deviation for the central phantom position was 4.01%. The couch model was included in the TPS with a uniform ED of 0.25 or a 2 component model with a fiber ED = 0.5 and foam core ED = 0.1. For model A and B under 2 and 5 mm voxel grid size, the mean absorbed dose with couch was reduced to 0.61%, 0.84%, 0.71% and 0.92% from 2.8% without couch. Conclusions: Model A has a good agreement between measured and calculated dose distributions for all different voxel grid sizes and gantry angles. It can accurately describes the dose perturbations due to the presence of the couch and should therefore be used during treatment planning. (authors)