【作者】 王玉；

【导师】 吴宜灿； 胡丽琴；

【作者基本信息】 中国科学技术大学 ， 核科学与技术， 2015， 博士

【摘要】 精准放射治疗技术努力最大限度地杀灭肿瘤细胞,同时最大程度地保护周围正常组织和危及器官。放射治疗过程中,对于肿瘤靶区变形严重的病例,需要及时调整放射治疗计划。基于锥形束CT(Cone Beam Computed Tomography, CBCT)图像进行计划验证和调整具有耗时少、成本低、病人接收剂量低等优点。但CBCT图像存在伪影多、HU值不稳定等缺陷,对剂量计算精度产生很大影响。本文对CBCT图像的散射校正方法进行了深入研究,以解决上述问题并达到自适应放射治疗中基于CBCT图像进行剂量计算实用化的目的。本文首先发展了基于蒙特卡罗的CBCT系统模块化建模方法。该方法可区分出原射线和散射线,并分离出不同部件的散射分布。通过Beamdp程序分析、水模剂量测量实验及模体图像散射成份提取的验证,结果表明该方法可准确模拟射线传播路径并分离出不同部件的散射成份,模型计算得到的剂量与实测误差在2%以内。其次,在笔形束模型计算散射核函数的基础上,本文实现了基于散射核反卷积的CBCT图像散射校正方法,并对该方法涉及到的信号转换模型建立、散射核计算、散射建模等关键环节进行详细研究。采用水模和仿真头模的CBCT图像散射校正进行测试,结果表明CBCT图像质量明显改善,散原射线比(Scatter-to-Primary Ratios, SPR)整体降低一个数量级。最后,在以上研究基础上进一步开展散射校正后CBCT图像应用于剂量计算的研究。采用散射校正后的标准模体CBCT图像建立HU-ED标定曲线。通过对模体和鼻咽癌病例的治疗计划进行验证,与CT-ED标定曲线计算结果进行比较。结果表明基于散射校正后CBCT图像进行计剂量计算的精度大幅度提高,伽马通过率分别从散射校正前的48.1%、75.2%提高至95.3%、97.7%,为实现自适应放射治疗中的基于CBCT图像进行剂量计算提供技术支持。更多还原

【Abstract】 Accurate radiotherapy techniques tried to find a balance between killing the tumor cells and protecting surrounding normal tissues at maximum using advanced equipment and technologies. During the treatment course, the target or organ shape changes severely so that the adjustment of treatment plan is needed. Dose calculationand verification based on CBCT images has the advantages of less time consuming, low cost and lower concomitant dose. However, the usage of CBCT images for dose calculation was hampered by image artifacts mainly caused by scatter radiation, which leads to instable HU values and mis-information. To achieve the clinical practical purpose of dose calculation based on CBCT images in Adaptive Radiotherapy, the scatter correction of CBCT images was investigated in this paper.The module modeling of CBCT system method was developed. EGSnrc program was adopted to simulate the geometry construction of x-ray tube-phantom-detector full system. The method could differentiate the primary and scatter radiation and separate the scatter distribution from phantom and detector. The system model was verified by Beamdp program analysis, dose measurement of water phantom and scatter extraction of simulated CBCT images of three phantoms. The dose error between the calculation and simulation was within2%.On the basis of scatter kernels calculated using Pencil Beam model, the Scatter Kernel Deconvolution method was adopted to correct the scatter in the CBCT images. The key techniques involved were further conducted, including Signal Conversion modeling, Scatter Kernel calculation and scatter distribution modeling. The CBCT images of Water phantom and anthropomorphic head phantom were adopted to verify the method. The results showed that images quality was obviously improved, the overall Scatter-to-Primary Ratios was reduced an order of magnitude.Finally, dose calculation was investigated based on CBCT images after scatter correction. The HU-ED calibration model was established using the standard phantom. The dose calculation for CATphan phantom and nasopharyngeal carcinoma patient were carried out based on calibration model. Compared with dose distribution calculated based on CT-ED calibration curve, the results showed that the precision of dose calculation based on scatter corrected CBCT images had been greatly improved. The pass rates of gamma analysis were95.3%and97.7%respectively, meeting the clinical requirement. The study of dose calculation based on scatter corrected CBCT images would provide technical support for Adaptive Radiotherapy.更多还原