Qi Zhen-Yu, Huang Shao-Min, Deng Xiao-Wu
State Key Laboratory of Oncology in Southern China, Guangzhou, Guangdong 510060, P. R. China.
Ai Zheng. 2006 Jan;25(1):110-4.
BACKGROUND & OBJECTIVE: CT values are the basis that radiation treatment planning system (TPS) used to calculate the dose distribution of an irradiation treatment plan. However, the CT values of a certain tissue could be very different when it is scanned with dissimilar parameters or acquisition modes, and would add errors to the planning dose calculation. This study was designed to investigate the factors that may impact the calibration of quantitative CT used for radiation treatment planning, and determine the relative importance of these factors.
CT values of different tissue substitute rods under different CT scanning conditions were measured and compared using RMI 467 phantom. Scanning conditions were classified into 3 modes: (1) evaluating the effect of CT parameters on CT density map by choosing different voltage, slice thickness and acquisition modalities; (2) evaluating the geometric effects of patient size, shape and position in the field of view (FOV) by changing different position of tissue substitute rods in the phantom; (3) evaluating the effect of patient support table on quantitative CT by comparing CT values acquired on patient support table top with those acquired in air. The relative importance leading to CT value error of the 3 modes was also analyzed.
For high-density cortical bone, the applied voltage was the most relevant factor resulting in difference to the reconstructed Hounsfield values of about 150 units, which was 12.7% when scanning voltage changed from 120 KV to 150 KV. The geometric disturbance from different patient sizes and shapes in the FOV could be minimized (less than 10 units for cortical bone). The CT values acquired on the table top varied about 26.34% for water and 293.84% for solid water when compared with those acquired in air.
CT scan voltage and patient support table top scattering make the most impacts to CT values, which may result in error of TPS dose calculation. Inhomogeneous phantoms like RMI 467 can be used to perform QA tests, and calibrations for quantitative CT and reduce the dose error of radiation treatment. It is advisable that a complete set of CT parameters and image acquisition modalities should be determined as part of radiotherapy planning protocols. Patient setup and immobilization modality during CT acquisition should be the same as those during treatment delivery, so as to increase the accuracy of the CT density map for TPS.
CT值是放射治疗计划系统（TPS）用于计算照射治疗计划剂量分布的基础。然而，当用不同参数或采集模式扫描某一组织时，其CT值可能会有很大差异，这会给计划剂量计算增加误差。本研究旨在调查可能影响用于放射治疗计划的定量CT校准的因素，并确定这些因素的相对重要性。
使用RMI 467体模测量并比较不同CT扫描条件下不同组织替代棒的CT值。扫描条件分为3种模式：（1）通过选择不同的电压、层厚和采集方式评估CT参数对CT密度图的影响；（2）通过改变体模中组织替代棒的不同位置评估患者大小、形状和视野（FOV）内位置的几何效应；（3）通过比较在患者支撑台面上获取的CT值与在空气中获取的CT值评估患者支撑台对定量CT的影响。还分析了导致3种模式CT值误差的相对重要性。
对于高密度皮质骨，施加的电压是导致重建的亨氏值差异最大的相关因素，约为150个单位，当扫描电压从120 kV变为150 kV时，差异为12.7%。视野内不同患者大小和形状产生的几何干扰可降至最低（皮质骨小于10个单位）。与在空气中获取的CT值相比，在台面上获取的水的CT值变化约26.34%，固体水的CT值变化约293.84%。
CT扫描电压和患者支撑台面散射对CT值影响最大，这可能导致TPS剂量计算误差。像RMI 467这样的非均匀体模可用于进行质量保证测试、定量CT校准并减少放射治疗的剂量误差。建议将一整套CT参数和图像采集模式作为放射治疗计划方案的一部分确定下来。CT采集期间的患者摆位和固定方式应与治疗时相同，以提高用于TPS的CT密度图的准确性。
