Abstract Purpose The number for clinical systems of proton beam dedicated to the tumor irradiation therapy has been increasing in recent years. In particular, these new systems are likely added with an imaging unit of cone beam CT (CBCT) for image guidance to reduce patient positioning error. In these CBCT systems, the x-ray source to image-receptor distance (SID) of the CBCT unit mounted on the proton beam gantry may be extensively longer than that of CBCT units on linear accelerators. To characterize the scattering effect for these CBCT units with a long SID, we used Monte Carlo computer simulation to calculate the quantity of scatter-to-primary ratio (SPR). Materials and methods A general-purpose radiation transport code, PENELOPE, was used in our Monte Carlo computer simulation. Analytical uniform and non-uniform phantoms of different materials were used for SPR calculations, including adipose (density: 0.92 g/cm3, relative electron density: 0.49), water (1.00 g/cm3, 1), muscle (1.04 g/cm3, 1.043), and bone (1.85 g/cm3, 1.095). The phantom size was 50 × 50 × 20 cm3 in length, width, and height, respectively. The receiving area of flat-panel detector was 40 × 40 cm2. Cone angle was used to represent the maximum angle that the x-ray of cone beam can cover the entire receiving area of x-ray detector. Different SID geometries of 150, 220, and 300 cm with different cone beam x-ray energies were included in this work. Results In the phantom study with different materials, SPR decreased as either the x-ray energy increased, density of material decreased, or SID increased. The cone angle was 7.59° for 150 cm of SID; 5.19° for 220 cm of SID; 3.81° for 300 cm of SID. A negatively linear relationship was characterized between the cone angle and SID. Our Monte Carlo calculations showed that SPR decreased as the cone angle decreased. A positive correlation was shown between the SPR and cone angle.. Conclusions We used different CBCT systems to determine SPRs and SPR decreased for the CBCT unit with a longer SID..

中文翻译：

---

用于图像引导质子束治疗系统的具有扩展源到图像受体距离的锥形束计算机断层扫描中的散射与原色比

摘要 目的近年来，专用于肿瘤放射治疗的质子束临床系统数量不断增加。特别是，这些新系统可能会添加锥形束 CT (CBCT) 成像单元，用于图像引导，以减少患者定位误差。在这些 CBCT 系统中，安装在质子束机架上的 CBCT 单元的 X 射线源到图像接收器的距离 (SID) 可能比直线加速器上的 CBCT 单元长得多。为了表征这些具有长 SID 的 CBCT 单元的散射效应，我们使用 Monte Carlo 计算机模拟来计算散射与主比 (SPR) 的数量。材料和方法 在我们的蒙特卡罗计算机模拟中使用了通用辐射传输代码 PENELOPE。不同材料的分析均匀和非均匀体模用于 SPR 计算，包括脂肪（密度：0.92 g/cm3，相对电子密度：0.49），水（1.00 g/cm3，1），肌肉（1.04 g/cm3， 1.043) 和骨 (1.85 g/cm3, 1.095)。体模的长、宽、高分别为 50 × 50 × 20 cm3。平板探测器的接收面积为 40 × 40 cm2。锥角用来表示锥形束的x射线可以覆盖x射线探测器整个接收区域的最大角度。这项工作包括具有不同锥形束 X 射线能量的 150、220 和 300 cm 的不同 SID 几何形状。结果 在使用不同材料的体模研究中，SPR 随着 X 射线能量增加、材料密度降低或 SID 增加而降低。对于 150 厘米的 SID，锥角为 7.59°；5. 220 厘米 SID 为 19°；3.81° 对于 300 厘米的 SID。锥角和 SID 之间呈现负线性关系。我们的蒙特卡罗计算表明 SPR 随着锥角的减小而减小。SPR 和锥角之间显示出正相关。结论我们使用不同的 CBCT 系统来确定 SPR，对于具有较长 SID 的 CBCT 单元，SPR 降低。