Cheng C W, Das I J, Chen D J
Department of Radiation Oncology, University of Arizona, Tucson 85724.
Br J Radiol. 1994 Oct;67(802):1017-22. doi: 10.1259/0007-1285-67-802-1017.
To minimize the risk of overlap of adjoining orthogonal fields in craniospinal irradiation, a skin gap is usually maintained between the caudal margin of the brain fields and the cephalic margin of the spine field. The moving gap technique (feathering) is commonly used to improve the dose in the gap region. Using Kodak XV ready-pack films and a polystyrene phantom, the dosimetry in the moving gap region at the depth of spinal cord for different gap sizes and feathering step sizes was studied. The dose profiles in the moving gap region with half-beam block set-up were also measured. Our results show that the combination of a small skin gap and a larger feathering step size results in a higher dose in the moving gap compared with that with identical gap and feathering step sizes. For example, with a 0.5 cm gap and 1 cm feathering step size, the dose in the moving gap region at depth of 5 cm ranges from 82 to 88% of the dose to the brain field. The corresponding dose range with 1 cm gap and 1 cm feathering step size is about 68-73%. With no gap and 1 cm feathering step, a dose range of 92-98% is achieved in the moving gap. The slightly steeper penumbra of a half-beam block used in the brain fields does not result in any dosimetric advantage in the moving gap compared with the conventional set-up when feathering is employed.
为将颅脊髓照射中相邻正交野重叠的风险降至最低，通常在脑野的尾缘与脊髓野的头缘之间保持一个皮肤间隙。移动间隙技术（羽化）常用于提高间隙区域的剂量。使用柯达XV现成包装胶片和聚苯乙烯体模，研究了不同间隙大小和羽化步长在脊髓深度处移动间隙区域的剂量学。还测量了半束阻挡设置下移动间隙区域的剂量分布。我们的结果表明，与相同间隙和羽化步长相比，小皮肤间隙和较大羽化步长的组合会使移动间隙中的剂量更高。例如，间隙为0.5 cm且羽化步长为1 cm时，5 cm深度处移动间隙区域的剂量为脑野剂量的82%至88%。间隙为1 cm且羽化步长为1 cm时，相应的剂量范围约为68% - 73%。间隙为0且羽化步长为1 cm时，移动间隙中的剂量范围为92% - 98%。当采用羽化时，脑野中使用的半束阻挡稍陡的半值层在移动间隙中与传统设置相比不会带来任何剂量学优势。
