不同CT扫描条件模拟定位对放射治疗计划的影响

Evaluation of scanning parameters in CT simulation on radiotherapy planning

目的:利用不均匀等效模体,探讨不同CT扫描条件对CT值及照射跳数(MU)的影响。方法:采用大小2种CT孔径(80和70cm)、2种模体几何摆放顺序及2种扫描电压(120和140KV),测量不同组合,比较各自的CT值,建立相应的CT-电子密度(ED)转换曲线,选取盆腔、胸部、头颈部各10例患者的CT图像模拟适形计划(CRT)和调强计划(IMRT),分析照射MU数值的偏差。结果:对于小孔径CT,无论扫描电压、模体几何位置如何变化,其MU数值相差均≤0.1%;大孔径CT扫描电压改变对MU值无影响,模体几何摆放顺序的改变有影响,但<0.3%;CRT计划和IMRT计划各自偏差值相同。结论:对于精确放疗计划系统,CT扫描条件改变和测量模体位置改变均可引起照射MU数值误差。若使用大孔径CT进行模拟定位,需根据其特性建立合理的电子密度曲线,并应用在计划制定中。

OBJECTIVE：To study the influence of different CT scanning conditions on the CT value and radiation do ses by using heterogeneous equivalent phantoms. METHODS： Two kinds of CT bore （80 cm and 70 cm）,geometrical ar rangements of phantom and scanning voltage （120 KV and 140 KV） were adopted respectively. The CT values of different combinations were measured and compared respectively. The CT-electron density （CT-ED） conversion curve was estab- lished. The CT images from thirty patients composed of ten cases of pelvic cavity, ten cases of thorax and ten cases of head and neck region respectively were used tO make the CRT treatment planning and the IMRT planning respectively. The MU deviations of the treatment plannings were analyzed. RESULTS： For the smaller bore CT with any changes of scanning voltage and geometrical arrangements of phantom,the MU deviations of was less than 0.1%. For the larger bore CT, the change of scanning voltage had not influence on the MU value,the arrangements of phantom affected the MU value and the MU deviations was less than 0.3 %. Both the CRT treatment planning and the IMRT planning had the same MU devi ations. CONCLUSIONS： For the accurate radiotherapy planning system, the radiation MU value deviation may be caused by the CT-ED conversion which is caused by the changes of CT scanning condition and measuring phantom placement. In the simulation with big core CT,comprehensive ED curves should be built in TPS.