带金属植入物患者调强放射治疗计划中射野角度设置对剂量计算准确度的影响

Effect of field angle setting on dose calculation accuracy in intensity-modulated radiotherapy plans for patients with metal implants

目的:对金属植入物及其伪影在放疗计划中产生的剂量计算偏差进行测试,评估射野角度设置对调强放射治疗(IMRT)计划剂量计算准确度的影响。方法:模拟髋关节置换患者,在CIRS调强模体中插入两根不锈钢金属棒。将指形电离室分别置于金属棒所在平面内中间位置的3个点,采用带金属伪影消减技术的CT模拟定位机获取未经校正和校正后的模体图像。在Monaco计划系统中利用两种CT图像,在0o^360o内每隔5o设置一个射野(10 cm×10 cm,100 MU),检测不同入射路径射野的剂量计算偏差。勾画靶区和危及器官,设计5野和7野IMRT计划,每种计划分别设有0、1或2个射野的入射路径通过金属区域,检测射野角度设置对IMRT计划剂量计算偏差的影响。结果:入射路径没有通过金属区域的单个射野在未校正及校正后图像中的剂量计算偏差分别为3.24%和1.56%,入射路径通过金属区域时剂量计算偏差分别达到5.51%~72.14%和5.32%~48.19%。在5野IMRT计划中,当有0、1和2个入射路径通过金属区域的射野时,未校正图像的计划剂量计算偏差分别为3.15%、8.75%和13.33%,校正图像的计划剂量计算偏差分别为1.54%、5.93%和9.06%;7野IMRT计划中,未校正图像的计划剂量计算偏差分别为3.03%、5.28%和10.71%,校正图像的计划剂量计算偏差分别为1.29%、4.38%和7.75%。结论:放射治疗计划中入射路径通过金属区域的射野会严重影响剂量计算的准确度,应尽量避免使用这种射野。虽然采用金属伪影消减技术校正CT图像能够改善这种影响,但在IMRT计划中存在两个及以上的这种射野可能导致临床上不可接受的剂量计算偏差。

Objective To measure the dose calculation deviation caused by metal implants and CT image artifacts,and to evaluate the effect of field angle setting on the dose calculation accuracy of intensity-modulated radiotherapy(IMRT)plans for patients with metal implants.Methods With the ionization chamber was placed at 3 points in the middle of the plane,a CIRS phantom with bilateral metal cylindrical steel inserts to simulate patient receiving hip replacement was scanned with CT simulator with metal artifact reduction(MAR)to obtain two image series,namely uncorrected phantom image and phantom image corrected with MAR.In Monaco planning system,a 10 cm×10 cm field with 100 MU was set up every 5°in the range from 0°to 360°to measure the dose calculation deviation caused by different incident paths.After target areas and organs-at-risk were delineated,5-and 7-field IMRT plans were designed,and each plan had 0,1 or 2 fields in which incident paths passed through the metal region.The effect of field angle setting on dose calculation deviation was analyzed.Results When the incident path didn't passed through the metal region,the dose calculation deviation on the uncorrected image and the image corrected with MAR was 3.24%and 1.56%,respectively;when it passed through the metal region,the deviation was 5.51%-72.14%and 5.32%-48.19%,respectively.For 5-field IMRT plan,when the number of fields passing through the metal region was 0,1,2,respectively,the dose calculation deviation was 3.15%,8.75%,13.33%on uncorrected image,and 1.54%,5.93%,9.06%on the image corrected with MAR.For 7-field IMRT plan,the dose calculation deviation was 3.03%,5.28%,10.71%on the uncorrected image,and 1.29%,4.38%and 7.75%on the image corrected with MAR.Conclusion The incident path of the radiation field passing through the metal region which will significantly affect the accuracy of dose calculation should be avoided in IMRT plan.Although the effect can be improved by MAR,the presence of 2 or more such fields in IMRT plan can result in clinically unacceptable dose calculation deviations.