IU Health Proton Therapy Center, Bloomington, Indiana 47408, USA.
Acta Oncol. 2013 Apr;52(3):553-60. doi: 10.3109/0284186X.2012.681065. Epub 2012 May 4.
To investigate the moving gap region dosimetry in proton beam cranio-spinal irradiation (CSI) to provide optimal dose uniformity across the treatment volume.
Proton beams of ranges 11.6 cm and 16 cm are used for the spine and the brain fields, respectively. Beam profiles for a 30 cm snout are first matched at the 50% level (hot match) on the computer. Feathering is simulated by shifting the dose profiles by a known distance two successive times to simulate a 2 × feathering scheme. The process is repeated for 2 mm and 4 mm gaps. Similar procedures are used to determine the dose profiles in the moving gap for a series of gap widths, 0-10 mm, and feathering step sizes, 4-10 mm, for a Varian iX 6MV beam. The proton and photon dose profiles in the moving gap region are compared.
The dose profiles in the moving gap exhibit valleys and peaks in both proton and photon beam CSI. The dose in the moving gap for protons is around 100% or higher for 0 mm gap, for both 5 and 10 mm feathering step sizes. When the field gap is comparable or larger than the penumbra, dose minima as low as 66% is obtained. The dosimetric characteristics for 6 MV photon beams can be made similar to those of the protons by appropriately combining gap width and feathering step size.
The dose in the moving gap region is determined by the lateral penumbras, the width of the gap and the feathering step size. The dose decreases with increasing gap width or decreasing feathering step size. The dosimetric characteristics are similar for photon and proton beams. However, proton CSI has virtually no exit dose and is beneficial for pediatric patients, whereas with photon beams the whole lung and abdomen receive non-negligible exit dose.
研究质子束颅脊柱照射（CSI）中移动间隙区域剂量学，以在整个治疗体积内提供最佳的剂量均匀性。
分别使用射程为 11.6 厘米和 16 厘米的质子束用于脊柱和脑部场。首先在计算机上将 30 厘米长的探头的束流轮廓在 50%水平（热匹配）处匹配。通过已知距离两次连续移动剂量分布来模拟羽毛，以模拟 2×羽毛方案。对于 2 毫米和 4 毫米的间隙重复该过程。类似的过程用于确定一系列间隙宽度（0-10 毫米）和羽毛步长（4-10 毫米）的移动间隙中的剂量分布，对于瓦里安 iX 6MV 光束。比较质子和光子在移动间隙区域的剂量分布。
在质子和光子 CSI 的移动间隙中，剂量分布呈现出谷值和峰值。对于质子，对于 0 毫米间隙和 5 毫米和 10 毫米羽毛步长，移动间隙中的剂量约为 100%或更高。当射野间隙与半影可比或更大时，会得到低至 66%的剂量最小值。通过适当组合间隙宽度和羽毛步长，可以使 6MV 光子束的剂量学特性类似于质子的特性。
移动间隙区域的剂量由侧向半影、间隙宽度和羽毛步长决定。随着间隙宽度的增加或羽毛步长的减小，剂量会降低。光子和质子束的剂量学特性相似。然而，质子 CSI 几乎没有出口剂量，对儿科患者有益，而光子束则使整个肺和腹部接收到不可忽视的出口剂量。
