头颈部兆伏级锥形束CT成像周边剂量测量与评价

摘要

目的 测量兆伏级锥形束CT (MVCBCT)成像在头颈部图像引导放疗的周边剂量(PD),评价PD与MVCBCT成像机器跳数(MU)间关系,探讨MVCBCT扫描射野对PD的影响.方法 将0.65 cm3电离室插放在特设固体水体模中,使用MVCBCT图像采集预案对体模头颈部扫描,测量射野外体模中心垂直平面在不同深度和距射野边缘不同距离的PD.使用相同电离室在体模中参考点上审视MVCBCT成像的MU与PD的线性关系,并在射野外体模横断面上测量PD均匀性.结果 在体模中测量的PD随着射野外距离增加呈指数下降,随着测量深度增加而降低,并随着扫描射野增大而增加.PD与MVCBCT成像MU间呈线性正比关系.PD分布不均匀,高剂量区位于前部.结论 MVCBCT成像时的PD主要依赖于MVCBCT成像MU和扫描射野.临床应用时应尽量考虑在不牺牲图像质量前提下,减少MVCBCT成像MU和缩小扫描射野降低PD.%Objective To evaluate the peripheral dose (PD) from megavoltage cone-beam CT (MVCBCT) imaging for head-and-neck region image-guided radiation therapy,to determine the correlation of PD with monitor unit (MU),and to investigate the impact of imaging field size on the PD.Methods Measurements of PD from MVCBCT were made with a 0.65 cm3 ionization chamber placed in a specially designed phantom at various depths and distances from the field edges.The PD at reference point inside the phantom was measured with the same ionization chamber to investigate the linearity between MU used for MVCBCT and the PD.The homogeneity of PD in the axial plane of the phantom were measured.Results PD from MVCBCT increased with increasing number of MU used for imaging and with increasing the field size.The measured PD in the phantom decreased exponentially as distance from the field edges increased.PD also decreased as the depth from the phantom surface increased.There was a strong linear relationship between PD and MUs used for MVCBCT.The PD was heterogeneous,with higher dose at the anterior than the posterior.Conclusions The PD from MVCBCT depend much on the MVCBCT delivery MU and the scan field size.In clinic,using the smallest number of MU allowable and reducing MVCBCT scanning field size without compromising acquired image quality is an effective method of reducing the PD.