Department of Radiation Oncology, St. Jude Children's Research Hospital, Memphis, TN, 38105-3678, USA.
Med Phys. 2018 Apr;45(4):1677-1683. doi: 10.1002/mp.12822. Epub 2018 Mar 13.
To evaluate the feasibility of using on-treatment magnetic resonance imaging (MRI) to detect proton range changes during treatment courses.
MRI-based virtual computed tomography (vCT) was generated to calculate water-equivalent path length (WEPL) at the distal surface of a clinical target volume. T2-weighted MR images with and without fat suppression were processed by thresholding and fuzzy c-mean clustering to assign a bulk HU (Hounsfield Unit) to each segment of fat, water, and air. Bony tissues in vCT were transferred from planning CT via a region-based registration. We validated this method using images from three patients (aged 9, 12, and 17 yr old) with pelvic sarcomas who underwent proton therapy. MRI-based WEPL was evaluated against those of repeat CT (rCT) and cone beam CT (CBCT).
The vCT agreed well with the rCT, wherein the use of bulk HU and imperfect bone registration contributed to discrepancies. The 99th percentile error of the MRI-based WEPL was up to 3.3 mm for posterior fields when we compensated the effect of mobile air. The gamma index (2-mm WEPL/2-mm pixel distance) was greater than 99% for those fields. We observed larger errors for anterior fields, which were due to bowel gas movement, mismatched respiratory motion, and differences in patient posture between the vCT and rCT. Applied to multiple on-treatment MRI of a patient, the estimated WEPL demonstrated an underrange trend, which was consistent with the CBCT results and the increased patient body circumference.
The MRI-based vCT produced highly accurate WEPL estimates, demonstrating the potential of using on-treatment MRI for detecting changes in proton ranges.
评估在治疗过程中使用治疗中磁共振成像（MRI）来检测质子射程变化的可行性。
生成基于 MRI 的虚拟计算机断层扫描（vCT），以计算临床靶区远端表面的水等效路径长度（WEPL）。对带和不带脂肪抑制的 T2 加权 MR 图像进行阈值处理和模糊 c-均值聚类，以将每个脂肪、水和空气段的体素 Hounsfield 单位（HU）分配给它们。vCT 中的骨组织通过基于区域的配准从计划 CT 转移过来。我们使用三名患有骨盆肉瘤的患者（年龄分别为 9、12 和 17 岁）的图像对该方法进行了验证，这些患者接受了质子治疗。评估了基于 MRI 的 WEPL 与重复 CT（rCT）和锥形束 CT（CBCT）的相关性。
vCT 与 rCT 吻合良好，体素 HU 的使用和不完美的骨骼配准导致了差异。当我们补偿移动空气的影响时，基于 MRI 的 WEPL 的第 99 个百分位误差对于后野可达 3.3 毫米。对于那些野，伽玛指数（2 毫米 WEPL/2 毫米像素距离）大于 99%。我们观察到前野的误差较大，这是由于肠气运动、呼吸运动不匹配以及 vCT 和 rCT 之间的患者体位差异所致。应用于一名患者的多次治疗中 MRI，估计的 WEPL 显示出低于范围的趋势，这与 CBCT 结果和患者身体周长的增加一致。
基于 MRI 的 vCT 产生了高度准确的 WEPL 估计，表明在治疗过程中使用 MRI 检测质子射程变化具有潜力。
