Department of Radiation Oncology, Massachusetts General Hospital and Harvard Medical School, Boston, MA 02114, USA.
Phys Med Biol. 2012 Mar 7;57(5):1159-72. doi: 10.1088/0031-9155/57/5/1159. Epub 2012 Feb 14.
Traditionally, dose in proton radiotherapy is prescribed as Gy(RBE) by scaling up the physical dose by 10%. The relative biological effectiveness (RBE) of protons is considered to vary with dose-averaged linear energy transfer (LET(d)), dose (d) and (α/β)(x). The increase of RBE with depth causes a shift of the falloff of the beam, i.e. a change of the beam range. The magnitude of this shift will depend on dose and (α/β)(x). The aim of this project was to quantify the dependence of the range shift on these parameters. Three double-scattered beams of different ranges incident on a computational phantom consisting of different regions of interest (ROIs) were used. Each ROI was assigned with (α/β)(x) values between 0.5 and 20 Gy. The distribution of LET(d) within each ROI was obtained from a Monte Carlo simulation. The LET(d) distribution depends on the beam energy and thus its nominal range. The RBE values within the ROIs were calculated for doses between 1 and 15 Gy using an in-house developed biophysical model. Dose-volume histograms of the RBE-weighted doses were extracted for each ROI for a 'fixed RBE' (RBE = 1.1) and a 'variable RBE' (RBE = f (d, α/β, LET(d))), and the percentage difference in range was obtained from the difference of the percentage volumes at the distal 80% of the dose. Range differences in normal tissue ((α/β)(x) = 3 Gy) of the order of 3-2 mm were obtained, respectively, for a shallow (physical range 4.8 cm) and a deep (physical range 12.8 cm) beam, when a dose of 1 Gy normalized to the mid-SOBP was delivered. As the dose increased to 15 Gy, the variable RBE decreases below 1.1 which induces ranges of about 1 mm shorter than those obtained with an RBE of 1.1. The shift in the range of an SOBP when comparing biological dose distributions obtained with a fixed or a variable RBE was quantified as a function of dose, (α/β)(x) and physical range (as a surrogate of the initial beam energy). The shift increases with the physical range but decreases with increasing dose or (α/β)(x). The results of our study allow a quantitative consideration of RBE-caused range uncertainties as a function of treatment site and dose in treatment planning.
传统上，质子放射治疗中的剂量是通过将物理剂量提高 10%来规定为 Gy(RBE)。质子的相对生物效应 (RBE) 被认为随剂量平均线性能量传递 (LET(d))、剂量 (d) 和 (α/β)(x) 而变化。RBE 的增加会导致束的衰减深度发生偏移，即束射程发生变化。这种偏移的幅度将取决于剂量和 (α/β)(x)。本项目的目的是量化这些参数对射程偏移的依赖关系。使用三个不同射程的双散射束照射由不同感兴趣区域 (ROI) 组成的计算体模。每个 ROI 被分配了 0.5 到 20 Gy 之间的 (α/β)(x) 值。每个 ROI 内的 LET(d) 分布是通过蒙特卡罗模拟获得的。LET(d) 分布取决于束能量，因此也取决于其标称射程。使用内部开发的生物物理模型，在 1 到 15 Gy 之间为 ROI 内的 RBE 值进行了计算。对于“固定 RBE”(RBE = 1.1) 和“可变 RBE”(RBE = f(d,α/β,LET(d)))，提取了每个 ROI 的 RBE 加权剂量的剂量-体积直方图，并从剂量的远端 80%的体积百分比差异中获得射程差异。当以归一化为中 SOBP 的 1 Gy 剂量进行照射时，分别在浅层（物理射程 4.8 cm）和深层（物理射程 12.8 cm）束中获得了正常组织 ((α/β)(x) = 3 Gy) 的射程差异约为 3-2 mm。当剂量增加到 15 Gy 时，可变 RBE 降低到 1.1 以下，这会导致射程比使用 1.1 的 RBE 获得的射程短约 1 mm。当比较使用固定或可变 RBE 获得的生物剂量分布时，SOBP 射程的偏移量被量化为剂量、(α/β)(x) 和物理射程（作为初始束能量的替代物）的函数。偏移量随物理射程的增加而增加，但随剂量或 (α/β)(x) 的增加而减小。我们的研究结果允许根据治疗部位和剂量在治疗计划中定量考虑 RBE 引起的射程不确定性。
