单纯CT伪影对放疗剂量计算的影响

目的 通过人为制造CT伪影，来研究实际临床操作中单纯伪影对放疗剂量计算的影响。方法 对替换钛合金组件前后的模体进行CT扫描，统计替换前后不同位置的CT值；将钛合金区域的CT值修正为水模体的CT值，并采用Varian的各向异性分析算法（AAA）、Acuros XB （AXB）算法和Pinnacle系统的筒串卷积算法（CCC）3种算法，对替换钛合金组件前后的模体进行剂量计算，统计替换前后不同位置的绝对剂量值，并进行分析。结果 Varian和Pinnacle系统对评价CT值大小比较一致。对于均匀模体，CT值偏差30 HU以下时，3种不同的算法在距离体表0.5 cm时，剂量偏差最大达到12.0%，最小为6.0%；1.5 cm以上偏差的绝对值均<1.0%。对于肺部模体来说，Varian的AAA算法和AXB算法在CT值相差15 HU的情况下，剂量值相差在1.0%左右；但Pinnacle系统的CCC算法在同样情况下剂量值相差较大，相差5.0%左右。结论 CT伪影对放疗剂量计算存在明显影响，导致组织剂量分布发生变化，可能造成浅部肿瘤照射剂量不足，深部肿瘤过量照射。

Effect of CT artifact on calculation of radiotherapy dose

Objective To investigate the effect of simple artifacts on the calculation of radiation dose in actual clinical operations by the aid of artificially caused CT artifacts. Methods The phantom was scanned using CT before and after replacing the titanium alloy component. Then, the CT values were measured at different distances before and after replacement. After correcting the CT value of the titanium alloy region to the CT value of the water phantom, the doses to the phantom were calculated by using Varian''s AAA algorithm, AXB algorithm and Pinnacle system''s CCC algorithm. The absolute dose values at different distances were furtherly analyzed. Results Varian system was consistent with Pinnacle system in evaluating the CT values. When the CT value deviated by less than 30 HU for a uniform phantom, the dose deviations of the three different algorithms were within 6.0%-12.0% at a distance of 0.5 cm from the body surface, and less than 1.0% at a distance of more than 1.5 cm from the body surface. When the CT value deviated by 15 HU for the lung phantom, both Varian''s AAA algorithm and Varian''s AXB algorithm showed about 1.0% dose deviation. However, the CCC algorithm of the Pinnacle system had a significant difference (5.0%) in dose values under the same conditions. Conclusions CT artifacts have noticeable effects on the calculation of radiation dose and change tissue dose distribution which may result in insufficient or excessive exposure doses.