Pedersen Jesper, Petersen Jørgen B B, Stokkevåg Camilla H, Ytre-Hauge Kristian S, Flampouri Stella, Li Zuofeng, Mendenhall Nancy, Muren Ludvig P
a Department of Medical Physics , Aarhus University Hospital/Aarhus University , Aarhus , Denmark.
b Department of Oncology and Medical Physics , Haukeland University Hospital , Bergen , Norway.
Acta Oncol. 2017 Nov;56(11):1413-1419. doi: 10.1080/0284186X.2017.1373198. Epub 2017 Oct 17.
The increased linear energy transfer (LET) at the end of the Bragg peak causes concern for an elevated and spatially varying relative biological effectiveness (RBE) of proton therapy (PT), often in or close to dose-limiting normal tissues. In this study, we investigated dose-averaged LET (LET) distributions for spot scanning PT of prostate cancer patients using different beam angle configurations. In addition, we derived RBE-weighted (RBE) dose distributions and related normal tissue complication probabilities (NTCPs) for the rectum and bladder.
A total of 21 spot scanning proton plans were created for each of six patients using a prescription dose of 78 Gy(RBE), with each plan using two 'mirrored' beams with gantry angles from 110°/250° to 70°/290°, in steps of 2°. Physical dose and LET distributions were calculated as well as RBE dose distributions using either RBE = 1.1 or three different variable RBE models. The resulting biological dose distributions were used as input to NTCP models for the rectum and bladder.
For anterior oblique (AO) configurations, the rectum LET volume and RBE dose increased with increasing angles off the lateral opposing axis, with the RBE rectum dose being higher than for all posterior oblique (PO) configurations. For PO configurations, the corresponding trend was seen for the bladder. Using variable RBE models, the rectum NTCPs were highest for the AO configurations with up to 3% for the 80°/280° configuration while the bladder NTCPs were highest for the PO configurations with up to 32% for the 100°/260°. The rectum D constraint was fulfilled for most patients/configurations when using uniform RBE but not for any patient/configuration with variable RBE models.
Compared to using constant RBE, the variable RBE models predicted increased biological doses to the rectum, bladder and prostate, which in turn lead to substantially higher estimated rectum and bladder NTCPs.
布拉格峰末端线性能量传递（LET）的增加引发了人们对质子治疗（PT）相对生物效应（RBE）升高且在空间上变化的担忧，这种情况通常发生在或接近剂量限制正常组织处。在本研究中，我们调查了使用不同射束角度配置对前列腺癌患者进行点扫描PT时的剂量平均LET（LET）分布。此外，我们推导了直肠和膀胱的RBE加权（RBE）剂量分布以及相关的正常组织并发症概率（NTCP）。
为6名患者中的每一位创建了总共21个点扫描质子计划，处方剂量为78 Gy（RBE），每个计划使用两个“镜像”射束，机架角度从110°/250°到70°/290°，步长为2°。计算了物理剂量和LET分布以及使用RBE = 1.1或三种不同可变RBE模型的RBE剂量分布。所得的生物剂量分布用作直肠和膀胱NTCP模型的输入。
对于前斜（AO）配置，直肠LET体积和RBE剂量随着与外侧相对轴角度的增加而增加，RBE直肠剂量高于所有后斜（PO）配置。对于PO配置，膀胱呈现出相应趋势。使用可变RBE模型时，AO配置的直肠NTCP最高，80°/280°配置时高达3%，而PO配置的膀胱NTCP最高，100°/260°时高达32%。使用均匀RBE时，大多数患者/配置满足直肠D约束，但使用可变RBE模型时，没有任何患者/配置满足该约束。
与使用恒定RBE相比，可变RBE模型预测直肠、膀胱和前列腺的生物剂量增加，这反过来导致估计的直肠和膀胱NTCP大幅升高。
