Brost Eric Edward, Watanabe Yoichi
Department of Radiation Oncology, University of Minnesota, 420 Delaware St. SE, MMC-494, Minneapolis, MN, USA.
Med Phys. 2018 Jun 1. doi: 10.1002/mp.13021.
Cerenkov photons are created by high-energy radiation beams used for radiation therapy. In this study, we developed a Cerenkov light dosimetry technique to obtain a two-dimensional dose distribution in a superficial region of medium from the images of Cerenkov photons by using a deconvolution method.
An integral equation was derived to represent the Cerenkov photon image acquired by a camera for a given incident high-energy photon beam by using convolution kernels. Subsequently, an equation relating the planar dose at a depth to a Cerenkov photon image using the well-known relationship between the incident beam fluence and the dose distribution in a medium was obtained. The final equation contained a convolution kernel called the Cerenkov dose scatter function (CDSF). The CDSF function was obtained by deconvolving the Cerenkov scatter function (CSF) with the dose scatter function (DSF). The GAMOS (Geant4-based Architecture for Medicine-Oriented Simulations) Monte Carlo particle simulation software was used to obtain the CSF and DSF. The dose distribution was calculated from the Cerenkov photon intensity data using an iterative deconvolution method with the CDSF. The theoretical formulation was experimentally evaluated by using an optical phantom irradiated by high-energy photon beams.
The intensity of the deconvolved Cerenkov photon image showed linear dependence on the dose rate and the photon beam energy. The relative intensity showed a field size dependence similar to the beam output factor. Deconvolved Cerenkov images showed improvement in dose profiles compared with the raw image data. In particular, the deconvolution significantly improved the agreement in the high dose gradient region, such as in the penumbra. Deconvolution with a single iteration was found to provide the most accurate solution of the dose. Two-dimensional dose distributions of the deconvolved Cerenkov images agreed well with the reference distributions for both square fields and a multileaf collimator (MLC) defined, irregularly shaped field.
The proposed technique improved the accuracy of the Cerenkov photon dosimetry in the penumbra region. The results of this study showed initial validation of the deconvolution method for beam profile measurements in a homogeneous media. The new formulation accounted for the physical processes of Cerenkov photon transport in the medium more accurately than previously published methods.
切伦科夫光子由用于放射治疗的高能辐射束产生。在本研究中，我们开发了一种切伦科夫光剂量测定技术，通过去卷积方法从切伦科夫光子图像中获取介质浅表区域的二维剂量分布。
利用卷积核推导了一个积分方程，用于表示相机针对给定入射高能光子束获取的切伦科夫光子图像。随后，利用入射束注量与介质中剂量分布之间的知名关系，得到了一个将深度处的平面剂量与切伦科夫光子图像相关联的方程。最终方程包含一个称为切伦科夫剂量散射函数（CDSF）的卷积核。通过将切伦科夫散射函数（CSF）与剂量散射函数（DSF）进行去卷积来获得CDSF函数。使用GAMOS（基于Geant4的面向医学模拟的架构）蒙特卡罗粒子模拟软件来获得CSF和DSF。使用具有CDSF的迭代去卷积方法从切伦科夫光子强度数据计算剂量分布。通过使用高能光子束照射的光学体模对理论公式进行了实验评估。
去卷积后的切伦科夫光子图像强度显示出与剂量率和光子束能量呈线性依赖关系。相对强度显示出与束输出因子类似的射野大小依赖性。与原始图像数据相比，去卷积后的切伦科夫图像在剂量分布上有所改善。特别是，去卷积显著改善了高剂量梯度区域（如半影区）的一致性。发现单次迭代去卷积可提供最准确的剂量解。去卷积后的切伦科夫图像的二维剂量分布与方形射野和多叶准直器（MLC）定义的不规则形状射野的参考分布吻合良好。
所提出的技术提高了切伦科夫光子剂量测定在半影区的准确性。本研究结果显示了去卷积方法在均匀介质中束轮廓测量方面的初步验证。新公式比以前发表的方法更准确地考虑了切伦科夫光子在介质中的传输物理过程。
