Department of Radiation Physics, The University of Texas MD Anderson Cancer Center, 1515 Holcombe Boulevard, Unit 94, Houston, Texas 77030, USA.
Med Phys. 2013 May;40(5):051714. doi: 10.1118/1.4801910.
Proton dose distributions can potentially be altered by anatomical changes in the beam path despite perfect target alignment using traditional image guidance methods. In this simulation study, the authors explored the use of dosimetric factors instead of only anatomy to set up patients for proton therapy using in-room volumetric computed tomographic (CT) images.
To simulate patient anatomy in a free-breathing treatment condition, weekly time-averaged four-dimensional CT data near the end of treatment for 15 lung cancer patients were used in this study for a dose-based isocenter shift method to correct dosimetric deviations without replanning. The isocenter shift was obtained using the traditional anatomy-based image guidance method as the starting position. Subsequent isocenter shifts were established based on dosimetric criteria using a fast dose approximation method. For each isocenter shift, doses were calculated every 2 mm up to ± 8 mm in each direction. The optimal dose alignment was obtained by imposing a target coverage constraint that at least 99% of the target would receive at least 95% of the prescribed dose and by minimizing the mean dose to the ipsilateral lung.
The authors found that 7 of 15 plans did not meet the target coverage constraint when using only the anatomy-based alignment. After the authors applied dose-based alignment, all met the target coverage constraint. For all but one case in which the target dose was met using both anatomy-based and dose-based alignment, the latter method was able to improve normal tissue sparing.
The authors demonstrated that a dose-based adjustment to the isocenter can improve target coverage and/or reduce dose to nearby normal tissue.
尽管使用传统的图像引导方法可以实现靶区的精确对准，但质子束路径中的解剖结构变化仍可能改变质子剂量分布。在这项模拟研究中，作者探索了使用剂量学因素而不仅仅是解剖结构来为质子治疗设置患者，方法是使用机房内容积 CT 图像。
为了模拟治疗期间自由呼吸的患者解剖结构，本研究使用了 15 例肺癌患者治疗接近尾声时的每周时间平均四维 CT 数据，采用基于剂量的等中心移位方法，无需重新计划，即可纠正剂量学偏差。等中心移位是使用传统的基于解剖结构的图像引导方法作为起始位置获得的。随后，根据剂量学标准使用快速剂量近似方法建立等中心移位。对于每个等中心移位，在每个方向上计算了±8mm 内的每 2mm 的剂量。通过施加目标覆盖约束来获得最佳剂量对准，该约束要求至少 99%的目标接受至少 95%的规定剂量，并使同侧肺的平均剂量最小化。
作者发现，仅使用基于解剖结构的对准时，15 个计划中有 7 个不符合目标覆盖约束。在作者应用基于剂量的对准后，所有计划都满足了目标覆盖约束。在基于解剖结构和基于剂量的对准都满足目标剂量的所有病例中（除了一个病例），后者方法能够更好地保护正常组织。
作者证明，对等中心进行基于剂量的调整可以改善靶区覆盖，或减少邻近正常组织的剂量。
