McNutt T R, Mackie T R, Paliwal B R
Department of Human Oncology, University of Wisconsin-Madison, USA.
Med Phys. 1997 Sep;24(9):1465-76. doi: 10.1118/1.598035.
An iterative convolution/superposition (C/S) algorithm has been created to reconstruct dose distributions in patients from exit dose measurements during a radiotherapy treatment. The method is based on an extended phantom which includes the patient CT representation and an electronic portal imaging device (EPID). The patient CT is assumed to be a true and rigid representation of the patient at the time of treatment. The C/S method computes the dose throughout the extended phantom which allows the exit dose to be predicted in the EPID. The process is then reversed to take the exit dose measurement and infer what the dose distribution must have been to produce the measured exit dose. The dose distribution is modeled without knowledge of the incident intensity distribution, and includes the effects of scatter in the computation. The iterative method begins by assuming that the exit primary energy fluence (PEF) is equal to the exit dose, the PEF is then backprojected through the extended phantom and superposed with the dose deposition kernel to determine a new prediction of the exit dose. The ratio of the computed PEF to exit dose is then multiplied by the measured exit dose image to produce a better representation of the exit PEF. Successive iterations then converge to the exit PEF image that would produce the measured exit dose image. Once convergence is established, the dose distribution is determined by backprojecting the exit PEF followed by superposition with the dose deposition kernel. The method is used to reconstruct the dose from a stimulated dynamic wedge and verified with film. Convergence and termination of the algorithm is then investigated with no noise and in the presence of noise. The method is then expanded to handle multiple treatment beams by separating the representation of the EPID from the patient or phantom representation in the computation process. Investigation of the effects of noise during the process of iterative dose reconstruction is necessary to understand the capabilities of the algorithm using exit dose images that may contain significant amounts of noise. The capability of the algorithm is evaluated for multiple field treatments to a cube phantom and a prostate patient CT representation in the presence of noise. The method is then used to simulate the dose reconstruction process for tomotherapy using 72 intensity-modulated fan beams. Dose reconstruction is shown to be capable of verifying the dose distributions in patients including multiple beams and dynamic collimation, provided the patient CT is known at the time of treatment.
已创建一种迭代卷积/叠加（C/S）算法，用于根据放射治疗期间的出射剂量测量值重建患者体内的剂量分布。该方法基于一个扩展体模，其中包括患者的CT图像和电子射野影像装置（EPID）。假定患者CT是治疗时患者的真实且刚性的影像。C/S方法计算整个扩展体模内的剂量，从而可以在EPID中预测出射剂量。然后过程反过来，采用出射剂量测量值，并推断出为产生测量到的出射剂量所需的剂量分布。在不了解入射强度分布的情况下对剂量分布进行建模，并在计算中纳入散射效应。迭代方法首先假定出射初始能量注量（PEF）等于出射剂量，然后将PEF通过扩展体模进行反向投影，并与剂量沉积核叠加，以确定出射剂量的新预测值。然后将计算得到的PEF与出射剂量的比值乘以测量到的出射剂量图像，以更好地表示出射PEF。随后的连续迭代收敛到能够产生测量到的出射剂量图像的出射PEF图像。一旦确定收敛，通过对出射PEF进行反向投影，然后与剂量沉积核叠加来确定剂量分布。该方法用于从模拟动态楔形板重建剂量，并通过胶片进行验证。然后在无噪声和有噪声的情况下研究算法的收敛和终止情况。然后通过在计算过程中将EPID的表示与患者或体模的表示分开，将该方法扩展以处理多个治疗射束。在迭代剂量重建过程中研究噪声的影响对于了解使用可能包含大量噪声的出射剂量图像的算法能力是必要的。在有噪声的情况下，针对对立方体体模和前列腺患者CT图像进行的多野治疗，评估了该算法的能力。然后该方法用于模拟使用72个调强扇形束的断层放射治疗的剂量重建过程。结果表明，只要在治疗时已知患者CT，剂量重建就能够验证包括多个射束和动态准直在内的患者体内的剂量分布。
