Reconstruction of dose distributions for fine carbon-ion beams using iterative approximation toward carbon-knife
Reconstruction of dose distributions for fine carbon-ion beams using iterative approximation toward carbon-knife
复制标题
使用碳刀迭代近似重建精细碳离子束的剂量分布
DOI：
10.1088/1361-6560/abc131
复制
发表时间：
2020
影响因子：
3.5
通讯作者：
Sakurai Hiroshi
中科院分区：
文献类型：
--
作者：
Tashiro Mutsumi;Souda Hikaru;Yoshida Takuya;Sakurai Hiroshi
对于具有精细碳离子束的碳刀的实际应用，剂量分布的量化是必不可少的，并且对空间分辨率有很高的要求。我们提出了一种新的方法来量化剂量分布，空间分辨率小于剂量计的尺寸。该方法创新了迭代重建技术。利用敏感面积为1mm2的二极管剂量计，测量了∼直径为1 mm、半高全宽(FWHM)的细小碳离子束在表面0.1 mm台阶和布拉格峰深度附近的二维剂量-面积-积分布。然后，从测量的DAP分布重建出空间分辨率为0.1×0.1mm2的剂量分布。然而，在重建的剂量分布中观察到了不自然的高噪声，这被认为源于DAP分布的测量重复性误差估计为2.5%-3%。因此，为了减少重建剂量分布的误差，采用了低通滤波处理。根据感应噪声的幅度估计了低通滤波器的最佳截止频率。利用获得的最佳截止频率的滤波过程，当实际测量误差为3%时，剂量分布被量化，相对于峰值的平均误差约为3%或更小。在重建的剂量率分布中，在表面观察到陡峭的P80-20∼0.2 mm半影，获得了∼90GyBragg−1中心轴处的剂量率和接近布拉格峰的半高宽处的束流尺寸∼1.1 mm。所提出的方法有望用于基于测量的微束模型的确定和碳刀应用的剂量分布计算。
For the practical application of carbon-knife with fine carbon-ion beams, the quantification of the dose distribution is essential and requires a high spatial resolution. We propose a novel method to quantify dose distributions with a spatial resolution smaller than the dosimeter size. The proposed method innovates the iterative reconstruction technique. Using a diode dosimeter with a sensitive area of 1 mm 2, two-dimensional dose-area-product (DAP) distributions were measured at a 0.1 mm step at the surface and near the Bragg peak depths for fine carbon-ion beams of∼ 1 mm size at the full width at half maximum (FWHM). Then, the dose distributions were reconstructed with a spatial resolution of 0.1× 0.1 mm 2 from the measured DAP distributions. However, an unnaturally high noise was observed in the reconstructed dose distributions, which were considered to originate from the measurement reproducibility errors of the DAP distributions estimated to be 2.5%–3%. Therefore, a low-pass filtering process was implemented to reduce the errors on the reconstructed dose distributions. The optimum cut-off frequencies of the low-pass filter were estimated depending on the amplitude of the induced noise. Using the filtering process with the obtained optimum cut-off frequency, the dose distribution was quantified with an average error of approximately 3% or less with respect to the peak value, when the actual measurement had an error of 3%. In the reconstructed dose rate distributions, a steep penumbra P 80-20∼ 0.2 mm was observed at the surface, and a dose rate at the center axis of∼ 90 Gy s− 1 and a beam size of∼ 1.1 mm at FWHM near the Bragg peak were obtained. The proposed method is expected to be useful for the measurement-based determination of microbeam models for commissioning and dose distribution calculations toward carbon-knife applications.