RESUMO
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 =0.5and foam core=0.1) in the Monaco treatment planning system for the iBEAM(R) 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.
RESUMO
Objective To evaluate the effect of carbon fiber couch on dose distribution of radiotherapy planning and verification pass rate.Methods Establishing the carbon fiber treatment couch model in Pinnacle8.0m Treatment Planning system (TPS),and then this model was used to correct dose calculations of oblique fields in the treatment plans of 10 cases of nasopharyngeal carcinoma,10 cases of breast cancer and 10 cases of lung cancer and evaluate the effect of carbon fiber couch on the whole dose distribution of the plans.Then these plans were measured by three-dimensional dose verification equipment Delta4 to confirm the improvement extent of Gamma pass rate after considering the carbon fiber treatment couch.Results For the majority of plans,when the carbon fiber couch was taken into consideration,the target doses was significantly reduced (4772 cGy-7266 cGy vs.4859 cGy-7347 cGy,P=0.000-0.002) and the relative deviation of D95 was 1％ to 3％.Measurement results of Delta4 showed that Gamma pass rate (3 mm/3％ criteria) increased in all plans (96.4％-98.8％ vs.93.4％-97.3％,P =0.000),some of that were up to 5 percentage when the couch model was applied.Conclusions Target doses will be overestimated if the treatment couch is ignored in TPS measurement.,However it should arouse enough attention when the disease with smaller doses corresponding gradient.