An initial systematic study of the linear energy transfer distributions of a proton beam under a transverse magnetic field
An initial systematic study of the linear energy transfer distributions of a proton beam under a transverse magnetic field
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横向磁场下质子束线性能量传递分布的初步系统研究
DOI：
10.1002/mp.15478
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发表时间：
2022
期刊：
影响因子：
--
通讯作者：
Matsuura T.
中科院分区：
文献类型：
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作者：
Fujii Y;Ueda H;Umegaki K;Matsuura T.
目的为了评价磁共振（MR）引导下质子束治疗的生物学效果，需要全面表征磁场下的剂量和剂量平均线性能量转移（LETd）分布。虽然详细的分析，弯曲的光束路径和扭曲的剂量分布的特点，磁场对LETd的影响也应探讨，以确定质子的相对生物学效应（RBE）。因此，本初步研究的目的是提出一个基本的分析，在磁场的存在下，使用Monte Carlo模拟（MCS）。方法Geant 4 MCS（版本10.1.p01）进行计算质子束的LETd分布。入射束能量设置为70.2、140.8和220 MeV，模拟零发射率和有限发射率笔形束以及扫描场。在放置于等中心的水模体内施加0-3 T的横向磁场，并计算模体内的三维剂量和LETd分布。分析了在不同能量和磁场作用下，沿弯曲轨迹的LETd深度分布和布拉格峰（BP）深度处的横向LETd分布。此外，对于零发射度和有限发射度射束，分析了剂量和LETd分布之间的横向不对称性的相关性。最后，展开布拉格峰（SOBP）领域进行了模拟，以评估的LETd distributions.ResultsA横向磁场扭曲的横向LETd分布的笔形束在接近BP的深度依赖的不对称性，和在BP的失真的幅度增加更高的能量束和更大的磁场。对于零发射度射束，左和右D20位置之间的LETd差异相对较大;在1.5 T的磁场下，LETd在140.8和220 MeV处的差异分别为1.5和2.3 keV/μm。这些不对称性在剂量不对称性较大的位置处明显。对于有限发射束，不对称的大小不太重要，对于扫描场甚至更小。然而，在相同磁场下，220 MeV射束的扫描场半影的左右D20位置之间仍然存在1.5-keV/μm的差异。对于SOBP场，发现SOBP的远端区域具有最高的LETd失真，其次是中等尺寸SOBP的中心和近端区域（5 × 5 × 5立方厘米），而对于10 × 10 × 10-cm 3 SOBP野，LETd畸变的程度随深度变化不大。结论我们的结果表明，不仅要考虑剂量，还要考虑LETd畸变，以准确评估磁共振引导质子束治疗的生物学有效性。
PurposeTo evaluate the biological effectiveness of magnetic resonance (MR)‐guided proton beam therapy, comprehensively characterizing the dose and dose‐averaged linear energy transfer (LETd) distributions under a magnetic field is necessary. Although detailed analysis has characterized curved beam paths and distorted dose distributions, the impact of a magnetic field on LETdshould also be explored to determine the proton relative biological effectiveness (RBE). Hence, this initial study aims to present a basic analysis of LETddistributions in the presence of a magnetic field using Monte Carlo simulation (MCS).MethodsGeant4 MCS (version 10.1.p01) was performed to calculate the LETddistribution of proton beams. The incident beam energies were set to 70.2, 140.8, and 220 MeV, and both zero‐ and finite‐emittance pencil beams as well as scanned field were simulated. A transverse magnetic field of 0–3 T was applied within a water phantom placed at the isocenter, and the three‐dimensional dose and LETddistributions in the phantom were calculated. Then, the depth profiles of LETdalong the curved trajectory and the lateral LETdprofile at the Bragg peak (BP) depth were analyzed under changing energies and magnetic fields. In addition, for zero‐ and finite‐emittance beams, the correlation of the lateral asymmetries between the dose and LETddistributions were analyzed. Finally, spread‐out Bragg peak (SOBP) fields were simulated to assess the depth‐dependent asymmetry of the LETddistributions.ResultsA transverse magnetic field distorted the lateral LETddistribution of a pencil beam at close to the BP, and the magnitude of the distortion at the BP increased for higher energy beams and larger magnetic fields. For a zero‐emittance beam, the differences in LETdbetween the left and right D20positions were relatively large; the difference in LETdwas 1.5 and 2.3 keV/μm at 140.8 and 220 MeV, respectively, at a magnetic field of 1.5 T. These asymmetries were pronounced at positions where the dose asymmetries were large. The size of the asymmetry was less substantial for a finite‐emittance beam and even less for a scanned field. However, a 1.5‐keV/μm difference still remained between the left and right D20positions of a scanned field penumbra for a 220 MeV beam under the same magnetic field. For the SOBP field, it was found that the distal region of SOBP had the highest LETddistortions, followed by the central and proximal regions for the middle‐sized SOBP (5 × 5 × 5 cm3), whereas the degree of LETddistortion did not vary much with depth for the 10 × 10 × 10‐cm3SOBP field.ConclusionOur results indicate that not only the dose but also LETddistortions should be considered to accurately evaluate the biological effectiveness of MR‐guided proton beam therapy.