Department of Physics, Ryerson University, Toronto, Ontario M5B 2K3, Canada.
Med Phys. 2013 Nov;40(11):111721. doi: 10.1118/1.4824927.
To validate the van Herk margin formula for lung radiation therapy using realistic dose calculation algorithms and respiratory motion modeling. The robustness of the margin formula against variations in lesion size, peak-to-peak motion amplitude, tissue density, treatment technique, and plan conformity was assessed, along with the margin formula assumption of a homogeneous dose distribution with perfect plan conformity.
3DCRT and IMRT lung treatment plans were generated within the ORBIT treatment planning platform (RaySearch Laboratories, Sweden) on 4DCT datasets of virtual phantoms. Random and systematic respiratory motion induced errors were simulated using deformable registration and dose accumulation tools available within ORBIT for simulated cases of varying lesion sizes, peak-to-peak motion amplitudes, tissue densities, and plan conformities. A detailed comparison between the margin formula dose profile model, the planned dose profiles, and penumbra widths was also conducted to test the assumptions of the margin formula. Finally, a correction to account for imperfect plan conformity was tested as well as a novel application of the margin formula that accounts for the patient-specific motion trajectory.
The van Herk margin formula ensured full clinical target volume coverage for all 3DCRT and IMRT plans of all conformities with the exception of small lesions in soft tissue. No dosimetric trends with respect to plan technique or lesion size were observed for the systematic and random error simulations. However, accumulated plans showed that plan conformity decreased with increasing tumor motion amplitude. When comparing dose profiles assumed in the margin formula model to the treatment plans, discrepancies in the low dose regions were observed for the random and systematic error simulations. However, the margin formula respected, in all experiments, the 95% dose coverage required for planning target volume (PTV) margin derivation, as defined by the ICRU; thus, suitable PTV margins were estimated. The penumbra widths calculated in lung tissue for each plan were found to be very similar to the 6.4 mm value assumed by the margin formula model. The plan conformity correction yielded inconsistent results which were largely affected by image and dose grid resolution while the trajectory modified PTV plans yielded a dosimetric benefit over the standard internal target volumes approach with up to a 5% decrease in the V20 value.
The margin formula showed to be robust against variations in tumor size and motion, treatment technique, plan conformity, as well as low tissue density. This was validated by maintaining coverage of all of the derived PTVs by 95% dose level, as required by the formal definition of the PTV. However, the assumption of perfect plan conformity in the margin formula derivation yields conservative margin estimation. Future modifications to the margin formula will require a correction for plan conformity. Plan conformity can also be improved by using the proposed trajectory modified PTV planning approach. This proves especially beneficial for tumors with a large anterior-posterior component of respiratory motion.
使用真实剂量计算算法和呼吸运动建模来验证适用于肺部放射治疗的范赫克边界公式。评估了边界公式对病变大小、峰峰值运动幅度、组织密度、治疗技术和计划一致性的变化的稳健性，以及边界公式假设的具有完美计划一致性的均匀剂量分布的假设。
在 ORBIT 治疗计划平台（瑞典 RaySearch 实验室）中，基于虚拟体的 4DCT 数据集生成 3DCRT 和 IMRT 肺部治疗计划。使用 ORBIT 中提供的可变形注册和剂量积累工具模拟随机和系统呼吸运动引起的误差，用于模拟病变大小、峰峰值运动幅度、组织密度和计划一致性变化的情况。还对边界公式剂量分布模型、计划剂量分布和半影宽度进行了详细比较，以检验边界公式的假设。最后，还测试了一种用于补偿不完美计划一致性的校正方法，以及一种新的应用于考虑患者特定运动轨迹的边界公式的应用。
范赫克边界公式确保了所有 3DCRT 和 IMRT 计划的所有一致性的完全临床靶区体积覆盖，除了软组织中的小病变。系统和随机误差模拟中没有观察到与计划技术或病变大小相关的剂量趋势。然而，累积计划显示，随着肿瘤运动幅度的增加，计划一致性降低。在比较边界公式模型中假设的剂量分布与治疗计划时，在随机和系统误差模拟中观察到低剂量区域的差异。然而，在所有实验中，边界公式都尊重了 ICRU 定义的计划靶区（PTV）边界推导所需的 95%剂量覆盖，因此可以估计合适的 PTV 边界。对于每个计划，在肺组织中计算的半影宽度与边界公式模型中假设的 6.4mm 值非常相似。计划一致性校正产生的结果不一致，主要受图像和剂量网格分辨率的影响，而轨迹修正的 PTV 计划与标准的内部靶区体积方法相比具有剂量效益，V20 值最多可降低 5%。
边界公式显示出对肿瘤大小和运动、治疗技术、计划一致性以及低组织密度的变化具有稳健性。这通过维持所有推导的 PTV 由 95%剂量水平覆盖来验证，这是 PTV 的正式定义所要求的。然而，边界公式推导中对完美计划一致性的假设导致了保守的边界估计。未来对边界公式的修改需要进行计划一致性校正。通过使用建议的轨迹修正 PTV 规划方法，也可以提高计划一致性。这对于具有较大前后向呼吸运动成分的肿瘤尤其有益。
