Department of Clinical Oncology, The University of Hong Kong, Tuen Mun Hospital, Hong Kong Special Administrative Region.
Med Phys. 2013 Apr;40(4):041712. doi: 10.1118/1.4794505.
Due to the complexity of 4D target tracking radiotherapy, the accuracy of this treatment strategy should be experimentally validated against established standard 3D technique. This work compared the accuracy of 3D and 4D dose calculations in respiration tracking stereotactic body radiotherapy (SBRT).
Using the 4D planning module of the CyberKnife treatment planning system, treatment plans for a moving target and a static off-target cord structure were created on different four-dimensional computed tomography (4D-CT) datasets of a thorax phantom moving in different ranges. The 4D planning system used B-splines deformable image registrations (DIR) to accumulate dose distributions calculated on different breathing geometries, each corresponding to a static 3D-CT image of the 4D-CT dataset, onto a reference image to compose a 4D dose distribution. For each motion, 4D optimization was performed to generate a 4D treatment plan of the moving target. For comparison with standard 3D planning, each 4D plan was copied to the reference end-exhale images and a standard 3D dose calculation was followed. Treatment plans of the off-target structure were first obtained by standard 3D optimization on the end-exhale images. Subsequently, they were applied to recalculate the 4D dose distributions using DIRs. All dose distributions that were initially obtained using the ray-tracing algorithm with equivalent path-length heterogeneity correction (3D EPL and 4D EPL) were recalculated by a Monte Carlo algorithm (3D MC and 4D MC) to further investigate the effects of dose calculation algorithms. The calculated 3D EPL, 3D MC, 4D EPL, and 4D MC dose distributions were compared to measurements by Gafchromic EBT2 films in the axial and coronal planes of the moving target object, and the coronal plane for the static off-target object based on the γ metric at 5%/3mm criteria (γ5%/3mm). Treatment plans were considered acceptable if the percentage of pixels passing γ5%/3mm (Pγ<1) ≥ 90%.
The averaged Pγ<1 values of the 3D EPL, 3D MC, 4D EPL, and 4D MC dose calculation methods for the moving target plans are 95%, 95%, 94%, and 95% for reproducible motion, and 95%, 96%, 94%, and 93% for nonreproducible motion during actual treatment delivery. The overall measured target dose distributions are in better agreement with the 3DMC dose distributions than the 4DMC dose distributions. Conversely, measured dose distributions agree much better with the 4D EPL/MC than the 3D EPL/MC dose distributions in the static off-target structure, resulting in higher Pγ<1 values with 4D EPL/MC (91%) vs 3D EPL (24%) and 3D MC (25%). Systematic changes of target motion reduced the averaged Pγ<1 to 47% and 53% for 4D EPL and 4D MC dose calculations, and 22% for 3D EPL/MC dose calculations in the off-target films.
In robotic tracking SBRT, 4D treatment planning was found to yield better prediction of the dose distributions in the off-target structure, but not necessarily in the moving target, compared to standard 3D treatment planning, for reproducible and nonreproducible target motion. It is important to ensure on a patient-by-patient basis that the cumulative uncertainty associated with the 4D-CT artifacts, deformable image registration, and motion variability is significantly smaller than the cumulative uncertainty occurred in standard 3D planning in order to make 4D planning a justified option.
由于 4D 目标跟踪放射治疗的复杂性，应通过实验验证该治疗策略的准确性，将其与既定的标准 3D 技术进行比较。本研究比较了呼吸跟踪立体定向体部放射治疗（SBRT）中 3D 和 4D 剂量计算的准确性。
使用 CyberKnife 治疗计划系统的 4D 计划模块，在不同的胸部体模的 4DCT 数据集上创建移动目标和静态的靶外结构的治疗计划，这些体模在不同的范围内移动。4D 计划系统使用 B 样条变形图像配准（DIR）将不同呼吸几何形状上计算的剂量分布累积到参考图像上，每个呼吸几何形状对应于 4DCT 数据集的静态 3DCT 图像，从而构成 4D 剂量分布。对于每个运动，进行 4D 优化以生成移动目标的 4D 治疗计划。为了与标准 3D 计划进行比较，将每个 4D 计划复制到参考端呼气图像上，并进行标准 3D 剂量计算。首先通过在端呼气图像上进行标准 3D 优化获得靶外结构的治疗计划。随后，使用 DIR 重新计算 4D 剂量分布。最初使用等效路径长度不均匀性校正的射线追踪算法（3D EPL 和 4D EPL）获得的所有剂量分布均通过蒙特卡罗算法（3D MC 和 4D MC）重新计算，以进一步研究剂量计算算法的影响。在移动目标物体的轴向和冠状平面以及静态靶外物体的冠状平面上，通过 Gafchromic EBT2 胶片测量 3D EPL、3D MC、4D EPL 和 4D MC 剂量分布，并根据 5%/3mm 标准（γ5%/3mm）的γ 度量进行比较。如果通过 γ5%/3mm（Pγ<1）的像素百分比≥90%，则认为治疗计划是可接受的。
在可重复性运动中，移动目标计划的 3D EPL、3D MC、4D EPL 和 4D MC 剂量计算方法的平均 Pγ<1 值分别为 95%、95%、94%和 95%，而在实际治疗输送过程中不可重复性运动的平均 Pγ<1 值分别为 95%、96%、94%和 93%。与 4D MC 剂量分布相比，总体测量的目标剂量分布与 3D MC 剂量分布更吻合。相反，在静态靶外结构中，测量的剂量分布与 4D EPL/MC 吻合得更好，而与 3D EPL/MC 吻合得更差，导致 4D EPL/MC（91%）与 3D EPL（24%）和 3D MC（25%）相比，Pγ<1 值更高。目标运动的系统变化将靶外胶片中 4D EPL 和 4D MC 剂量计算的平均 Pγ<1 值降低到 47%和 53%，而 3D EPL/MC 剂量计算的平均 Pγ<1 值降低到 22%。
在机器人跟踪 SBRT 中，与标准 3D 治疗计划相比，4D 治疗计划发现可以更好地预测靶外结构的剂量分布，但对于可重复性和不可重复性的靶标运动，并不一定能够更好地预测移动目标的剂量分布。重要的是，为了使 4D 计划成为合理的选择，需要在患者个体基础上确保与 4D-CT 伪影、变形图像配准和运动变异性相关的累积不确定性明显小于标准 3D 计划中的累积不确定性。
