调强放疗中金属植入物及伪影的消减技术

Reduction in metal implants and artifacts in intensity modulated radiotherapy

评估金属植入物及其伪影对容积旋转调强计划剂量计算的影响,探讨提高金属植入物患者剂量计算准确性的方法。在调强模体(CIRS model 002H9K,Norfolk,VA)中插入两根不锈钢金属棒来模拟髋关节置换患者。采用带金属伪影消减技术(MAR)的CT模拟定位机获取校正前后的模体图像。在Monaco计划系统中利用两种CT图像勾画靶区,设计全弧和避开金属植入物的分段弧容积旋转调强治疗(VMAT)计划并进行实际测量,分析避开和经过金属区域的VMAT计划剂量计算和实际测量偏差的影响。另外,选取6例椎体中植入金属钢钉的患者,在常规图像CTNorm上设计VMAT计划,复制到CTNorm+DF、CTMAR和CTMAR+DF图像中,评估计划计算结果的差异。在VMAT计划中,常规CT图像中,避开金属区域的分段弧较全弧计划可降低剂量计算差异(4.97%vs.6.14%)。常规CT图像经密度填充后全弧计划计算差异明显改善(2.35%vs.6.14%),避开金属区域的分段弧计划差异变化不明显(4.64%vs.4.97%)。经MAR校正后的图像中,避开金属区域的分段弧计划较全弧计划明显提高了剂量计算精度(0.16%vs.5.57%),采用MAR和密度填充校正后的图像全弧计划和分段弧计划较常规图像的计划显著提高剂量计算的精度(2.14%vs.6.14%,0.42%vs.4.97%)。在临床病例VMAT计划比较中,所有参数中,CTMAR与CTNorm计算结果相当,差异较小;对于Dmin、Dmean、VDp、CTNorm+DF和CTMAR+DF相对CTNorm均减少;CTDF和CTMAR+DF适形性和均匀性相对CTNorm均有所降低。伪影消减技术能够消除图像中大部分的金属伪影,患者体内的金属植入物是影响剂量计算准确度的主要因素,即使在CT模拟机没有伪影消减技术的条件下,如果已知金属植入物密度,单独采用密度填充技术处理金属区域,也能有效提高放疗计划的计算准确度。去伪影消减技术结合密度填充处理可以进一步提高剂量计算的精度。

To evaluate the dose effects of metal implants and their artifacts in volume-modulated arc therapy(VMAT)plans and to explore the methods to improve the accuracy of dose calculations for patients with metal implants.With the ionization chamber placed in the middle of the plane,a IMRT phantom(CIRS model 002H9K,Norfolk,VA)with bilateral metal cylindrical steel inserts to simulate a patient receiving a hip replacement was scanned using a computed tomography(CT)simulator with metal artifact reduction(MAR)to obtain two image series,namely uncorrected phantom images and phantom images corrected with MAR.In the Monaco planning system,after the target area was delineated,full-arc and part-arc VMAT plans avoiding metal implants were designed,and actual measurements were then acquired.The effect of avoiding and passing metal areas on the dose calculation deviation was analyzed.In addition,6 patients with metal nails implanted in the vertebral body were selected;a VMAT plan was designed using the CTNorm image and then copied to the CTNorm+DF,CTMAR,and CTMAR+DF images to evaluate the dose differences.In the VMAT plan,the part-arc plan that avoided metal areas on the CTNorm images reduced the dose calculation difference compared to that of the full-arc plan(4.97%vs.6.14%).After density filling of the CTNorm images,the difference in the calculation of the full-arc plan significantly improved(2.35%vs.6.14%),and the difference in the part-arc plan that avoided the metal area did not change significantly(4.64%vs.4.97%).In the MAR images,the part-arc plan that avoided the metal area significantly improved the dose calculation accuracy compared with that of the full-arc plan(0.16%vs.5.57%).The metal artifact reduction technique and density filling were applied to the corrected image.The full-arc and part-arc plans significantly improved the accuracy of the dose calculations compared with the CTNorm image plan after using MAR combined with density-filling methods(2.14%vs.6.14%and 0.42%vs.4.97%,respectively).In the clinical case,in all planning target volume dose parameter comparisons,the difference between the CTMAR and CTNorm calculation results was small;the Dmin,Dmean,VDp,CTNorm+DF,and CTMAR+DF were reduced relative to the CTNorm;and the CTNorm+DF and CTMAR+DF conformity and uniformity were poor relative to those of the CTNorm.The MAR technique removed most of the metal artifacts on the CT images.Metal implants in patients are the main factor affecting the accuracy of dose calculations.Using a density-filling technique alone can also effectively improve the accuracy of dose calculations with known metal implant densities.The MAR technique combined with density filling can further improve the accuracy of dose calculations.