Department of Radiation Oncology (MAASTRO), GROW-School for Oncology and Developmental Biology, Maastricht University Medical Center, Maastricht 6201 BN, The Netherlands.
Phys Med Biol. 2013 May 21;58(10):3377-95. doi: 10.1088/0031-9155/58/10/3377. Epub 2013 Apr 25.
Precision image-guided small animal radiotherapy is rapidly advancing through the use of dedicated micro-irradiation devices. However, precise modeling of these devices in model-based dose-calculation algorithms such as Monte Carlo (MC) simulations continue to present challenges due to a combination of very small beams, low mechanical tolerances on beam collimation, positioning and long calculation times. The specific intent of this investigation is to introduce and demonstrate the viability of a fast analytical source model (AM) for use in either investigating improvements in collimator design or for use in faster dose calculations. MC models using BEAMnrc were developed for circular and square fields sizes from 1 to 25 mm in diameter (or side) that incorporated the intensity distribution of the focal spot modeled after an experimental pinhole image. These MC models were used to generate phase space files (PSFMC) at the exit of the collimators. An AM was developed that included the intensity distribution of the focal spot, a pre-calculated x-ray spectrum, and the collimator-specific entrance and exit apertures. The AM was used to generate photon fluence intensity distributions (ΦAM) and PSFAM containing photons radiating at angles according to the focal spot intensity distribution. MC dose calculations using DOSXYZnrc in a water and mouse phantom differing only by source used (PSFMC versus PSFAM) were found to agree within 7% and 4% for the smallest 1 and 2 mm collimator, respectively, and within 1% for all other field sizes based on depth dose profiles. PSF generation times were approximately 1200 times faster for the smallest beam and 19 times faster for the largest beam. The influence of the focal spot intensity distribution on output and on beam shape was quantified and found to play a significant role in calculated dose distributions. Beam profile differences due to collimator alignment were found in both small and large collimators sensitive to shifts of 1 mm with respect to the central axis.
精准影像引导小动物放射治疗技术通过使用专用的微照射设备得到了快速发展。然而，由于束流非常小、光束准直的机械公差低、定位精度要求高以及计算时间长等因素，这些设备在基于模型的剂量计算算法（如蒙特卡罗（MC）模拟）中的精确建模仍然具有挑战性。本研究的目的是引入并证明一种快速分析源模型（AM）的可行性，该模型可用于研究准直器设计的改进，也可用于更快速的剂量计算。使用 BEAMnrc 开发了圆形和方形射野尺寸从 1 到 25 毫米（直径或边长）的 MC 模型，这些模型纳入了实验针孔图像模拟的焦点强度分布。这些 MC 模型用于在准直器出口处生成相空间文件（PSFMC）。开发了一种 AM，其中包括焦点强度分布、预先计算的 X 射线光谱以及准直器特定的入口和出口孔径。AM 用于生成光子通量强度分布（ΦAM）和 PSFAM，其中包含根据焦点强度分布辐射的光子。在水和小鼠模型中使用 DOSXYZnrc 进行 MC 剂量计算，仅在源使用方面（PSFMC 与 PSFAM）有所不同，发现对于最小的 1 和 2 毫米准直器，分别在 7%和 4%以内一致，对于所有其他射野尺寸，基于深度剂量分布，在 1%以内一致。对于最小束流，PSF 生成时间大约快 1200 倍，对于最大束流，PSF 生成时间大约快 19 倍。焦点强度分布对输出和束形状的影响进行了量化，发现其对计算剂量分布有重要作用。在小准直器和大准直器中都发现了由于准直器对准引起的射束轮廓差异，对于相对于中心轴的 1 毫米的偏移量非常敏感。
