Fjæra Lars Fredrik, Li Zuofeng, Ytre-Hauge Kristian S, Muren Ludvig P, Indelicato Daniel J, Lassen-Ramshad Yasmin, Engeseth Grete May, Brydøy Marianne, Mairani Andrea, Flampouri Stella, Dahl Olav, Stokkevåg Camilla H
a Department of Physics and Technology , University of Bergen , Bergen , Norway.
b Department of Oncology and Medical Physics , Haukeland University Hospital , Bergen , Norway.
Acta Oncol. 2017 Jun;56(6):763-768. doi: 10.1080/0284186X.2017.1314007. Epub 2017 Apr 19.
For tumours near organs at risk, there is concern about unintended increase in biological dose from elevated linear energy transfer (LET) at the distal end of treatment fields. The objective of this study was therefore to investigate how different paediatric posterior fossa tumour locations impact LET and biological dose to the brainstem during intensity-modulated proton therapy (IMPT).
Multiple IMPT plans were generated for four different simulated tumour locations relative to the brainstem for a five-year-old male patient. A prescribed dose of 59.4 Gy(RBE) was applied to the planning target volumes (PTVs). Plans with two lateral and one posterior non-coplanar fields were created, along with plans with modified field arrangements. The dose-averaged LET (LET) and the physical dose × RBE (D × RBE), where RBE=1+c × LET, were calculated using the FLUKA Monte Carlo code. A scaling parameter c was applied to make the RBE represent variations in the biological effect due to LET.
High LET values surrounded parts of the PTV and encompassed portions of the brainstem. Mean LET values in the brainstem were 3.2-6.6 keV/μm. The highest absolute brainstem LET values were seen with the tumour located most distant from the brainstem, whereas lower and more homogeneous LET values were seen when the tumour invaded the brainstem. In contrast, the highest mean D × RBE values were found in the latter case (54.0 Gy(RBE)), while the case with largest distance between tumour and brainstem had a mean D × RBE of 1.8 Gy(RBE).
Using IMPT to treat posterior fossa tumours may result in high LET values within the brainstem, particularly if the tumour volume is separated from the brainstem. However, the D × RBE was greater for tumours that approached or invaded the brainstem. Changing field angles showed a reduction of LET and D × RBE in the brainstem.
对于靠近危及器官的肿瘤，人们担心治疗野远端的线性能量传递（LET）升高会意外增加生物剂量。因此，本研究的目的是调查不同的小儿后颅窝肿瘤位置在调强质子治疗（IMPT）期间如何影响脑干的LET和生物剂量。
为一名5岁男性患者生成了相对于脑干的四种不同模拟肿瘤位置的多个IMPT计划。向计划靶区（PTV）施加59.4 Gy（RBE）的处方剂量。创建了具有两个侧野和一个后非共面野的计划，以及具有修改后的野排列的计划。使用FLUKA蒙特卡罗代码计算剂量平均LET（LET）和物理剂量×RBE（D×RBE），其中RBE = 1 + c×LET，应用缩放参数c以使RBE代表由于LET引起的生物效应变化。
高LET值围绕PTV的部分区域并包含脑干的部分区域。脑干中的平均LET值为3.2 - 6.6 keV/μm。肿瘤位于离脑干最远时，脑干的绝对LET值最高，而当肿瘤侵犯脑干时，LET值较低且更均匀。相比之下，后一种情况的平均D×RBE值最高（54.0 Gy（RBE）），而肿瘤与脑干之间距离最大的情况的平均D×RBE为1.8 Gy（RBE）。
使用IMPT治疗后颅窝肿瘤可能会导致脑干内出现高LET值，特别是如果肿瘤体积与脑干分离。然而，对于接近或侵犯脑干的肿瘤，D×RBE更大。改变野角度可使脑干中的LET和D×RBE降低。
