IPHC, UNISTRA, CNRS, 23 rue du Loess, 67200, Strasbourg, France.
INFN Sezione di Milano, Milano, Italy.
Med Phys. 2017 Aug;44(8):4276-4286. doi: 10.1002/mp.12348. Epub 2017 Jun 30.
The real-time monitoring of the spread-out Bragg peak would allow the planned dose delivered during treatment to be directly verified, but this poses a major challenge in modern ion beam therapy. A possible method to achieve this goal is to exploit the production of secondary particles by the nuclear reactions of the beam with the patient and correlate their emission profile to the planned target volume position. In this study, we present both the production rate and energy spectra of the prompt-γ produced by the interactions of the C ion beam with a polymethyl methacrylate (PMMA) target. We also assess three different Monte Carlo models for prompt-γ simulation based on our experimental data.
The experiment was carried out at the GSI Helmholtz Centre for Heavy Ion Research, Darmstadt, Germany with a 220 MeV/u C ions beam impinging on a 5× 5× 20 cm polymethyl methacrylate beam stopping target, with the prompt-γ being detected by a hexagonally-shaped barium fluoride scintillator with a circumscribed radius of 5.4 cm and a length of 14 cm, placed at 60° and 90° with respect to the beam direction. Monte Carlo simulations were carried out with three different hadronic models from the Geant4 code: binary ion cascade (BIC), quantum molecular dynamics (QMD), and Liege intranuclear cascade (INCL ).
An experimental prompt-γ yield of 1.06 × 10 sr was measured at 90°. A good agreement was observed between the shapes of the experimental and simulated energy spectra, especially with the INCL physics list. The prompt-γ yield obtained with this physics list was compatible with the measurement within 2σ, with a relative difference of 26% on average. BIC and QMD physics lists proved to be less accurate than INCL , with the difference between the measured and simulated yields exceeding 100%. The differences between the three physics lists were ascribed to important discrepancies between the models of the physical processes producing prompt-γ emissions.
In conclusion, this study provides prompt-γ yield values in agreement with previously published results for different carbon ions energies. This work demonstrates that the INCL physics list from Geant4 is more accurate than BIC and QMD to reproduce prompt-γ emission properties.
扩展布拉格峰的实时监测将允许直接验证治疗过程中所给予的计划剂量，但这在现代离子束治疗中是一个重大挑战。实现这一目标的一种可能方法是利用束流与患者相互作用产生的次级粒子，并将其发射谱与计划靶区位置相关联。在这项研究中，我们介绍了 C 离子束与聚甲基丙烯酸甲酯（PMMA）靶相互作用产生的瞬发γ射线的产生率和能谱。我们还根据实验数据评估了三种不同的用于瞬发γ射线模拟的蒙特卡罗模型。
该实验在德国达姆施塔特的 GSI 重离子研究中心进行，使用 220 MeV/u 的 C 离子束撞击 5×5×20 cm 的聚甲基丙烯酸甲酯束流停止靶，用一个 5.4 cm 外接圆半径和 14 cm 长的六边型氟化钡闪烁体探测瞬发γ射线，放置在与束流方向成 60°和 90°的位置。蒙特卡罗模拟使用 Geant4 代码中的三种不同的强子模型进行：二进制离子级联（BIC）、量子分子动力学（QMD）和 Liege 核内级联（INCL）。
在 90°处测量到 1.06×10 sr 的实验瞬发γ射线产额。实验和模拟能谱的形状吻合较好，特别是与 INCL 物理列表。用这个物理列表得到的瞬发γ射线产额与测量值在 2σ范围内一致，平均相差 26%。BIC 和 QMD 物理列表的精度不如 INCL，测量值与模拟值的差异超过 100%。这三个物理列表之间的差异归因于产生瞬发γ射线发射的物理过程模型之间存在重要差异。
总之，本研究提供了与先前发表的不同碳离子能量下的瞬发γ射线产额值一致的结果。这项工作表明，Geant4 的 INCL 物理列表比 BIC 和 QMD 更准确地再现了瞬发γ射线发射特性。
