Rutherford Harley, Chacon Andrew, Mohammadi Akram, Takyu Sodai, Tashima Hideaki, Yoshida Eiji, Nishikido Fumihiko, Hofmann Theresa, Pinto Marco, Franklin Daniel R, Yamaya Taiga, Parodi Katia, Rosenfeld Anatoly B, Guatelli Susanna, Safavi-Naeini Mitra
Centre for Medical Radiation Physics, University of Wollongong, Wollongong, NSW 2522, Australia.
Australian Nuclear Science and Technology Organisation (ANSTO), Lucas Heights, NSW 2234, Australia.
Phys Med Biol. 2020 Dec 5;65(23):235052. doi: 10.1088/1361-6560/abaa23.
This work presents an iterative method for the estimation of the absolute dose distribution in patients undergoing carbon ion therapy, via analysis of the distribution of positron annihilations resulting from the decay of positron-emitting fragments created in the target volume. The proposed method relies on the decomposition of the total positron-annihilation distributions into profiles of the three principal positron-emitting fragment species - C, C and O. A library of basis functions is constructed by simulating a range of monoenergetic C ion irradiations of a homogeneous polymethyl methacrylate phantom and measuring the resulting one-dimensional positron-emitting fragment profiles and dose distributions. To estimate the dose delivered during an arbitrary polyenergetic irradiation, a linear combination of factors from the fragment profile library is iteratively fitted to the decomposed positron annihilation profile acquired during the irradiation, and the resulting weights combined with the corresponding monoenergetic dose profiles to estimate the total dose distribution. A total variation regularisation term is incorporated into the fitting process to suppress high-frequency noise. The method was evaluated with 14 different polyenergetic C dose profiles in a polymethyl methacrylate target: one which produces a flat biological dose, 10 with randomised energy weighting factors, and three with distinct dose maxima or minima within the spread-out Bragg peak region. The proposed method is able to calculate the dose profile with mean relative errors of 0.8%, 1.0% and 1.6% from the C, C, O fragment profiles, respectively, and estimate the position of the distal edge of the SOBP to within an average of 0.7 mm, 1.9 mm and 1.2 mm of its true location.
这项工作提出了一种迭代方法，用于通过分析靶区内产生的正电子发射碎片衰变所导致的正电子湮灭分布，来估计接受碳离子治疗患者的绝对剂量分布。所提出的方法依赖于将总正电子湮灭分布分解为三种主要正电子发射碎片种类（(^{11})C、(^{10})C和(^{15})O）的分布曲线。通过模拟一系列单能(^{12})C离子对均匀聚甲基丙烯酸甲酯体模的照射，并测量由此产生的一维正电子发射碎片分布曲线和剂量分布，构建了一个基函数库。为了估计在任意多能照射期间所输送的剂量，将来自碎片分布曲线库的因子的线性组合迭代拟合到照射期间获取的分解后的正电子湮灭分布曲线上，并将所得权重与相应的单能剂量分布曲线相结合，以估计总剂量分布。在拟合过程中纳入了一个总变差正则化项，以抑制高频噪声。该方法在聚甲基丙烯酸甲酯靶中用14种不同的多能(^{12})C剂量分布进行了评估：一种产生平坦的生物剂量，10种具有随机能量加权因子，3种在扩展布拉格峰区域内具有明显的剂量最大值或最小值。所提出的方法能够分别从(^{11})C、(^{10})C、(^{15})O碎片分布曲线计算出平均相对误差为0.8%、1.0%和1.6%的剂量分布曲线，并将扩展布拉格峰远端边缘位置估计到其真实位置平均0.7毫米、1.9毫米和1.2毫米范围内。
