Institute of Radiation Protection, Helmholtz Zentrum München (HMGU), Neuherberg, Germany.
Department of Physik, Technical University of Munich, Munich, Germany.
Med Phys. 2017 May;44(5):1912-1920. doi: 10.1002/mp.12206. Epub 2017 Apr 20.
Systematic investigation of the energy and angular dependence of secondary neutron fluence energy distributions and ambient dose equivalents values (H*(10)) inside a pencil beam scanning proton therapy treatment room using a gantry.
Neutron fluence energy distributions were measured with an extended-range Bonner sphere spectrometer featuring ³He proportional counters, at four positions at 0°, 45°, 90°, and 135° with respect to beam direction and at a distance of 2 m from the isocenter. The energy distribution of secondary neutrons was investigated for initial proton beam energies of 75 MeV, 140 MeV, and 200 MeV, respectively, using a 2D scanned irradiation field of 11 × 11 cm² delivered to a 30 × 30 × 30 cm³ PMMA phantom. Additional measurements were performed at a proton energy of 118 MeV including a 5 cm range-shifter (PMMA), yielding a Bragg peak position similar to that of 75 MeV protons.
Ambient dose equivalent values from 0.3 μSv/Gy (75 MeV; 90°) to 24 μSv/Gy (200 MeV; 0°) were measured inside the treatment room at a distance of 2 m from the isocenter. H*(10) values were lower (by factors of up to 7.2 (at 45°)) at 75 MeV compared to those at 118 MeV with the 5 cm range-shifter. At 0° and 45°, an evaporation peak was found in the measured neutron fluence energy distributions, at neutron energies around MeV, which contributes about 50% to total H*(10) values, for all investigated proton beam energies.
This study showed a pronounced increase of secondary neutron H*(10) values inside the proton treatment room with increasing proton energy without beam modifiers. For example, in beam direction this increase was about a factor of 50 when protons of 75 MeV and 200 MeV were compared. The existence of a peak of secondary neutrons in the MeV region was demonstrated in beam direction (0°). This peak is due to evaporation neutrons produced in the existing surrounding materials such as those used for the gantry. Therefore, any simulation of the secondary neutrons within a proton treatment room must take these materials into account. In addition, the results obtained here show that the use of a range-shifter increases the production of secondary neutrons inside the treatment room. Using a range-shifter, the higher neutron doses observed mainly result from the higher incident proton energy (118 MeV instead of 75 MeV when no range-shifter was used), due to higher neutron production cross-sections.
使用旋转机架系统地研究了铅笔束扫描质子治疗室中二次中子注量能谱和周围剂量当量值（H*(10)）在能量和角度上的依赖性。
在距等中心 2 m 处，使用配备有³He 正比计数器的扩展范围 Bonner 球谱仪，分别在 0°、45°、90°和 135°相对于束方向的四个位置测量了中子注量能谱。使用 2D 扫描照射场（面积为 11×11 cm²）对初始质子束能量分别为 75 MeV、140 MeV 和 200 MeV 的二次中子能谱进行了研究，照射到 30×30×30 cm³ PMMA 体模上。在质子能量为 118 MeV 时进行了附加测量，包括一个 5 cm 的射程移动器（PMMA），产生的布拉格峰位置类似于 75 MeV 质子的位置。
在距等中心 2 m 处的治疗室内，测量到的周围剂量当量值在 0.3 μSv/Gy（75 MeV；90°）至 24 μSv/Gy（200 MeV；0°）之间。与使用 5 cm 射程移动器时的 118 MeV 相比，75 MeV 时的 H*(10)值更低（低至 7.2 倍（45°））。在 0°和 45°，在 MeV 左右的中子能区发现了测量中子注量能谱中的蒸发峰，对于所有研究的质子束能量，其对总 H*(10)值的贡献约为 50%。
这项研究表明，在没有束修正器的情况下，质子治疗室内的二次中子 H*(10)值随着质子能量的增加而显著增加。例如，当将 75 MeV 和 200 MeV 的质子进行比较时，在束方向上的这种增加约为 50 倍。在束方向（0°）证明了 MeV 区存在二次中子峰。该峰是由于现有周围材料（例如用于旋转机架的材料）中产生的蒸发中子引起的。因此，在质子治疗室内进行二次中子的任何模拟都必须考虑这些材料。此外，这里获得的结果表明，使用射程移动器会增加治疗室内的二次中子剂量。使用射程移动器时，观察到的较高中子剂量主要是由于更高的入射质子能量（当不使用射程移动器时为 118 MeV，而不是 75 MeV）所致，这是由于更高的中子产生截面。
