Ulrich Silke, Wieser Hans-Peter, Cao Wenhua, Mohan Radhe, Bangert Mark
a Department of Medical Physics in Radiation Therapy , German Cancer Research Center (DKFZ) , Heidelberg , Germany.
b Heidelberg Institute for Radiation Oncology (HIRO) , Heidelberg , Germany.
Acta Oncol. 2017 Nov;56(11):1420-1427. doi: 10.1080/0284186X.2017.1354131. Epub 2017 Aug 22.
Organ motion during radiation therapy with scanned protons leads to deviations between the planned and the delivered physical dose. Using a constant relative biological effectiveness (RBE) of 1.1 linearly maps these deviations into RBE-weighted dose. However, a constant value cannot account for potential nonlinear variations in RBE suggested by variable RBE models. Here, we study the impact of motion on recalculations of RBE-weighted dose distributions using a phenomenological variable RBE model.
4D-dose calculation including variable RBE was implemented in the open source treatment planning toolkit matRad. Four scenarios were compared for one field and two field proton treatments for a liver cancer patient assuming (α∕β) = 2 Gy and (α∕β) = 10 Gy: (A) the optimized static dose distribution with constant RBE, (B) a static recalculation with variable RBE, (C) a 4D-dose recalculation with constant RBE and (D) a 4D-dose recalculation with variable RBE. For (B) and (D), the variable RBE was calculated by the model proposed by McNamara. For (C), the physical dose was accumulated with direct dose mapping; for (D), dose-weighted radio-sensitivity parameters of the linear quadratic model were accumulated to model synergistic irradiation effects on RBE.
Dose recalculation with variable RBE led to an elevated biological dose at the end of the proton field, while 4D-dose recalculation exhibited random deviations everywhere in the radiation field depending on the interplay of beam delivery and organ motion. For a single beam treatment assuming (α∕β) = 2 Gy, D was 1.98 Gy (RBE) (A), 2.15 Gy (RBE) (B), 1.81 Gy (RBE) (C) and 1.98 Gy (RBE) (D). The homogeneity index was 1.04 (A), 1.08 (B), 1.23 (C) and 1.25 (D).
For the studied liver case, intrafractional motion did not reduce the modulation of the RBE-weighted dose postulated by variable RBE models for proton treatments.
在扫描质子放射治疗期间，器官运动导致计划物理剂量与实际交付物理剂量之间出现偏差。使用恒定相对生物效应（RBE）值1.1可将这些偏差线性映射为RBE加权剂量。然而，恒定值无法解释可变RBE模型所提示的RBE潜在非线性变化。在此，我们使用一种现象学可变RBE模型研究运动对RBE加权剂量分布重新计算的影响。
在开源治疗计划工具包matRad中实现了包括可变RBE的4D剂量计算。针对一名肝癌患者的单野和双野质子治疗，比较了四种情况，假设（α∕β） = 2 Gy和（α∕β） = 10 Gy：（A）具有恒定RBE的优化静态剂量分布，（B）使用可变RBE的静态重新计算，（C）具有恒定RBE的4D剂量重新计算，以及（D）使用可变RBE的4D剂量重新计算。对于（B）和（D），可变RBE由麦克纳马拉提出的模型计算得出。对于（C），通过直接剂量映射累积物理剂量；对于（D），累积线性二次模型的剂量加权放射敏感性参数以模拟对RBE的协同照射效应。
使用可变RBE进行剂量重新计算导致质子射野末端的生物剂量升高，而4D剂量重新计算在辐射野各处呈现随机偏差，这取决于束流输送与器官运动的相互作用。对于假设（α∕β） = 2 Gy的单束治疗，（A）的剂量为1.98 Gy（RBE），（B）为2.15 Gy（RBE），（C）为1.81 Gy（RBE），（D）为1.98 Gy（RBE）。均匀性指数分别为1.04（A）、1.08（B）、1.23（C）和1.25（D）。
对于所研究的肝脏病例，分次内运动并未降低可变RBE模型所假定的质子治疗RBE加权剂量的调制。
