Objective To investigate the dose distribution and radiation risk of Varian thoracic cone beam computed tomography (CBCT) with default parameters with reference to Monte Carlo simulation and International Commission on Radiological Protection (ICRP) report 110.Methods EGSnrc/BEAMnrc code was used to simulate the material, thickness, and geometry of the kV CBCT source (kVS) to establish the kVS model.A benchmarked MCSIM code was applied to calculate the dose distribution in the ICRP phantom after the scan with the standard thoracic parameters (110 kV, 20 mA, and 262 mAs), and the conversion coefficient of absolute dose was obtained in a spherical phantom following the TG-61 protocol.The results of Monte Carlo simulation were validated by PDD and Profile in a water phantom and the measurement of the absolute dose in the computed tomography dose index (CTDI) phantom and Alderson phantom.The models including BEIR VⅡ were used to evaluate the radiation risks.Results With reference to the criterion of 3%/1 mm, the uncertainties of PDD and Profile were less than 2%.The difference between the measured and calculated values was<2.9% in the CTDI phantom and ≤0.05 cGy in the Alderson phantom.In the ICRP110 phantom, the doses to the left lung, right lung, left breast, right breast, heart, thyroid, trachea, cancellous bone, and cortical bone were 1.28, 1.39, 1.74, 1.80, 1.46, 0.48, 0.88, 0.85, and 1.84 cGy, respectively.The relative risks of ischemic heart disease, breast cancer, lung cancer, thyroid cancer, and tracheal cancer in a standard scan were1.001 , 1.009, 1.019, 1.000, and 1.008, respectively. Conclusions The accumulated dose and long-term risks of CBCT during image-guided thoracic radiotherapy cannot be neglected and should be effectively controlled.%目的：利用蒙特卡洛模拟和 ICRP110标准女性模体定量研究瓦里安 CBCT 默认参数胸部扫描的剂量分布和辐射风险。方法利用 EGSnrc／BEAMnrc 模拟 CBCT 千伏射线源材料、厚度、结构等建立模型。标定 MCSIM 程序以计算 ICRP 模体在标准胸部参数扫描（110 kV、20 mA、262 mAs）后的剂量分布，其中绝对剂量转换系数依照 TG－61规范在球形模体中获得。通过水模体中的PDD、Profile、CTDI 以及 Alderson 仿真模体中的点剂量测量验证蒙特卡洛模拟结果。利用 BEIR Ⅶ等模型评估辐射风险。结果按照3％／1 mm 标准，PDD 和 Profile 的不确定度均＜2％。 CTDI 模体测量与计算值差异＜2.9％，Alderson 模体内差异≤0.05 cGy。 ICRP110模体的左肺、右肺、左乳、右乳、心脏、甲状腺、气管、松质骨、密质骨的 Dmean 分别为1.28、1.39、1.74、1.80、1.46、0.48、0.88、0.85、1.84 cGy。单次扫描对应的缺血性心脏病、乳腺癌、肺癌、甲状腺癌、气管癌的相对风险值分别为1.001、1.009、1.019、1.000、1.008。结论CBCT 成像在影像引导胸部放疗过程中的累积剂量及其远期风险不可忽略且应被合理管控。
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