Compensation of x-ray beam penumbra in conformal radiotherapy.
Compensation of x-ray beam penumbra in conformal radiotherapy.
复制标题
适形放射治疗中 X 射线束半影的补偿。
DOI：
10.1118/1.1287283
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发表时间：
2000
期刊：
影响因子：
3.8
通讯作者：
Wong,JW
中科院分区：
文献类型：
--
作者：
Sharpe,MB;Miller,BM;Wong,JW
在放射治疗中，大体肿瘤体积被临床确定的边缘包围，以允许未检测到的恶性细胞的存在。增加额外的边距以适应定位不确定性和器官运动，从而创建规划目标体积或PTV。最后，在PTV周围的射束孔中包括一个边缘，以说明射束边缘的剂量下降(即“半影”)。对于更高能量的光束和邻近PTV的低密度组织，应该增加光束孔径裕度，以考虑到散射光子和电子的范围增加。然而，边际的增加也会增加正常组织的照射体积。在这项工作中，通过使用滤光片和多叶准直器(MLC)技术来产生增加光束强度的补偿光圈来减小光束孔径裕度。对6 mV和18 mV X射线的补偿技术进行了评估，计算了半影区宽度作为皮内光束强度、皮层宽度和组织密度的函数。剂量计算是使用3D叠加算法进行的，其中包括焦外源模型。用胶片剂量法对计算结果进行了实验验证。结果表明，在10 mm宽的范围内，当照射强度增加20%时，6 MV X射线在肺部体模中的95%~50%等剂量线之间的距离从11 mm减少到4 mm。在18 mV时，该距离从16 mm减小到6 mm，且剥皮强度增加20%，但需要15 mm宽的剥皮。在所有情况下，半影补偿没有导致场界外的任何明显的散射剂量增加。这些结果表明，半影补偿是一种实用的控制光束孔径裕度的方法。
In radiotherapy, the gross tumor volume is surrounded by a clinically defined margin to allow for the presence of undetected malignant cells. Additional margins are added to accommodate positioning uncertainties and organ motion, creating a planning target volume, or PTV. Finally, a margin is included in the beam apertures surrounding the PTV to account for the dose fall‐off at the beam edges (i.e., the “penumbra”). For higher energy beams and for low density tissues adjacent to the PTV, the beam aperture margin should be increased to account for the increased range of scattered photons and electrons. However, increased margins also increase the volume of normal tissue irradiated. In this work, the beam aperture margin is reduced by using filters and multileaf collimator (MLC) techniques to create compensating rinds of increased beam intensity. These compensation techniques were evaluated for 6 and 18 MV x rays by calculating penumbral widths as a function of the increased beam intensity in the rind, the rind width, and tissue density. Dose calculations were performed using a 3D superposition algorithm, which includes an extrafocal source model. Calculations were validated experimentally with film dosimetry. Results show the distance between the 95%–50% isodose lines is reduced from 11 mm to 4 mm for 6 MV x rays in the lung phantom, when the beam intensity is increased by 20% in a 10 mm wide rind. At 18 MV, this distance is reduced from 16 mm to 6 mm with a 20% increase in rind intensity, but a 15 mm wide rind is required. In all cases, penumbra compensation did not result in any appreciable increase in scatter dose outside the field boundaries. These results suggest that penumbra compensation is a practical means of controlling the beam aperture margin.