Saito Nami, Schmitt Daniela, Bangert Mark
Department of Medical Physics in Radiation Oncology, German Cancer Research Center, Heidelberg, Germany.
National Center for Radiation Research in Oncology (NCRO), Heidelberg, Germany.
J Appl Clin Med Phys. 2018 Jul;19(4):87-97. doi: 10.1002/acm2.12359. Epub 2018 Jun 3.
To retrospectively analyze and estimate the dosimetric benefit of online and offline motion mitigation strategies for prostate IMRT.
Intrafractional motion data of 21 prostate patients receiving intensity-modulated radiotherapy was acquired with an electromagnetic tracking system. Target trajectories of 734 fractions were analyzed per delivered multileaf-collimator segment in five motion metrics: three-dimensional displacement, distance from beam axis (DistToBeam), and three orthogonal components. Time-resolved dose calculations have been performed by shifting the target according to the sampled motion for the following scenarios: without adaptation, online-repositioning with a minimum threshold of 3 mm, and an offline approach using a modified field order applying horizontal before vertical beams. Change of D95 (targets) or V65 (organs at risk) relative to the static case, that is, ΔD95 or ΔV65, was extracted per fraction in percent. Correlation coefficients (CC) between the motion metrics and the dose metrics were extracted. Mean of patient-wise CC was used to evaluate the correlation of motion metric and dosimetric changes. Mean and standard deviation of the patient-wise correlation slopes (in %/mm) were extracted.
For ΔD95 of the prostate, mean DistToBeam per fraction showed the highest correlation for all scenarios with a relative change of -0.6 ± 0.7%/mm without adaptation and -0.4 ± 0.5%/mm for the repositioning and field order strategies. For ΔV65 of the bladder and the rectum, superior-inferior and posterior-anterior motion components per fraction showed the highest correlation, respectively. The slope of bladder (rectum) was 14.6 ± 5.8 (15.1 ± 6.9) %/mm without adaptation, 14.0 ± 4.9 (14.5 ± 7.4) %/mm for repositioning with 3 mm, and 10.6 ± 2.5 (8.1 ± 4.6) %/mm for the field order approach.
The correlation slope is a valuable concept to estimate dosimetric deviations from static plan quality directly based on the observed motion. For the prostate, both mitigation strategies showed comparable benefit. For organs at risk, the field order approach showed less sensitive response regarding motion and reduced interpatient variation.
回顾性分析并评估前列腺调强放疗中在线和离线运动缓解策略的剂量学益处。
使用电磁跟踪系统获取21例接受调强放疗的前列腺患者的分次内运动数据。在五个运动指标中分析每个多叶准直器分段所对应的734个分次的靶区轨迹：三维位移、距射束轴的距离（DistToBeam）以及三个正交分量。通过根据采样运动移动靶区，针对以下情况进行了时间分辨剂量计算：不进行自适应调整、在线重新定位且最小阈值为3毫米，以及离线方法，即采用先水平束后垂直束的修改后的射野顺序。相对于静态情况，即ΔD95或ΔV65，提取每个分次中D95（靶区）或V65（危及器官）的变化百分比。提取运动指标与剂量指标之间的相关系数（CC）。使用患者层面CC的平均值来评估运动指标与剂量学变化的相关性。提取患者层面相关斜率（以%/毫米为单位）的平均值和标准差。
对于前列腺的ΔD95，在所有情况下，每个分次的平均DistToBeam显示出最高的相关性，在不进行自适应调整时相对变化为-0.6±0.7%/毫米，对于重新定位和射野顺序策略为-0.4±0.5%/毫米。对于膀胱和直肠的ΔV65，每个分次的上下和前后运动分量分别显示出最高的相关性。膀胱（直肠）的斜率在不进行自适应调整时为14.6±5.8（15.1±6.9）%/毫米，在3毫米重新定位时为14.0±4.9（14.5±7.4）%/毫米，对于射野顺序方法为10.6±2.5（8.1±4.6）%/毫米。
相关斜率是一个有价值的概念，可直接基于观察到的运动来估计与静态计划质量的剂量学偏差。对于前列腺，两种缓解策略显示出相当的益处。对于危及器官，射野顺序方法在运动方面显示出较不敏感的反应，并减少了患者间的差异。
