基于Geant4的碳离子治疗三维电离室阵列仿真设计

Simulation Design of Three Dimensional Ionization Chamber Array for Carbon Ion Therapy Based on Geant4

为快速准确地实现碳离子治疗计划的三维剂量验证,采用有机玻璃PMMA(聚甲基丙烯酸甲酯)为电离室室壁和水等效模体,设计了一种三维电离室阵列,并通过Geant4软件对三维电离室阵列的结构设计进行了深入研究与验证。首先通过模拟不同能量碳离子束在水和PMMA模体中沉积的剂量分布,计算了PMMA模体的水等效厚度系数;然后研究了三维电离室阵列中电离腔室间的距离及信号导线对其剂量测量准确度的影响;最后模拟并验证了碳离子束在三维电离室阵列中沉积的剂量分布。结果表明:PMMA模体的水等效厚度系数为1.151;相邻电离腔室间的信号串扰主要来源于前侧的电离腔室,且串扰程度与电离腔室间距呈反比,间距为1 mm时串扰程度占电离腔室内剂量的3%,间距为30 mm时串扰影响可完全消除;信号导线对后侧电离腔室内剂量的干扰影响约为1%。将碳离子束在三维电离室阵列中沉积的剂量分布与PMMA模体中的剂量分布进行对比,碳离子束的射程具有良好的一致性,偏差为0.5 mm。

As an important part of radiation therapy quality assurance, dose verification can verify the accuracy of the treatment dose to the patient’s tumor target area before treatment. At present, the dose verification technology has developed to the stage of three-dimensional dose verification. However, the existing clinical dose verification tools cannot meet the demand for three-dimensional dose verification in heavy ion therapy. In order to quickly and accurately realize three-dimensional dose verification of carbon ion therapy plan, a three-dimensional ionization chamber array was designed by using plexiglass PMMA(polymethyl methacrylate) as the ionization chamber wall and water equivalent phantom. And the structural design of the three-dimensional ionization chamber array was thoroughly studied and verified by Geant4 software. Firstly, by simulating the depth dose distribution of carbon ion beam deposited in water and PMMA phantom with different energies, the distal range ratio of the Bragg peak of the carbon ion beam in water and PMMA phantom was calculated, and the water equivalent thickness factor of PMMA phantom was characterized. Its value is 1.151. Then the effects of the spacing between adjacent ionization chambers and the signal leads on the accuracy of the dose measurement in the three-dimensional ionization chamber array were investigated. The signal crosstalk between adjacent ionization chambers mainly originates from the front ionization chamber, and the degree of crosstalk is inversely proportional to the spacing, the degree of crosstalk accounts for 3% of the dose within the ionization chamber at spacing of 1 mm, and the crosstalk effect can be completely eliminated at spacing of 30 mm. And the interference effect of the signal leads on the dose within the rear ionization chamber is about 1%. Finally, the dose distribution of 100 MeV/u and 200 MeV/u carbon ion beams deposited in the three-dimensional ionization chamber array was simulated, and the distal range of the Bragg peak was obtained in which the carbon ion beam was deposited. It has good agreement compared to the distal range of the Bragg peak in the PMMA phantom, and the deviation is 0.5 mm. The research show that the three-dimensional ionization chamber array can realize the three-dimensional dose verification of heavy ion therapy at the same time, and provides a reference for the development of three-dimensional ionization chamber array. The quality assurance level of carbon ion therapy should be further improved.