Monaco TPS中两种治疗床模型的比较

目的 在Monaco TPS中分别采用均匀电子密度(模型A ED=0.25)和两层非均匀电子密度(模型B FD=0.5和foam core=0.1)构建iBEAM evo Extension415治疗床模型,比较哪种模型可更好量化治疗床对放疗剂量影响。方法　将模体放置在治疗床中间，电离室放置在模体等中心，在10ｃｍ×10ｃｍ 射野下采用6 MV光子线机架从180.0 °～122.8°每间隔10°测量其衰减系数。以实际测量衰减值为参考值，在Monac TPS 中通过调整相对电子密度值确定治疗床模型值。对模型A和模型B分别通过最小计算网格（2ｍｍ）和最大计算网格（5 ｍｍ）模拟结果评估治疗床模型构建的精确性。结果 　不引入治疗床模型时测量和计算的剂量偏差最大可达4.01%，采用2.5 ｍｍ 计算网格进行计算时，测量和计算平均绝对剂量差由无模型时的2,.80％、2，87％分别减少至模型A 的0.61％、0.84％和模型B 的0.71％、0.92％。结论 　在Monaco TPS中采用均匀电子密度治疗床模型，对不同机架角度不同计算网格下计算值和测量值有更好一致性，在计划设计时可以精确构建治疗床对剂量的衰减。

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 =0.5and 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.