中文摘要：

目的探讨TrueBeam加速器6MV光子束均整（FF）与非均整（FFF）模式蒙特卡罗模型的绝对剂量刻度与射野输出因子计算方法。方法利用BEAMnrc程序分别建立FF与FFF两种模式在靶到监测电离室（BEAM—up）和监测电离室以下组件部分（BEAM—down）的加速器机头模型，计算入射电子和经次级准直器反射后的粒子在监测电离室的剂量沉积，利用DOSXYZnrc程序计算人射电子在射野中心轴上特定深度处的剂量沉积，结合绝对剂量刻度公式计算标准射野刻度因子和射野输出因子（1cm×1cm～40cm×40cm）。结果FF与FFF模型的10cm×10cm标准辐射野，1MU相当于7．747×10^13±3．099×10^11和3．248×10^13±1．624×10^11电子打靶，在虚拟的加速器监测电离室上产生21．53和35．01cGy剂量；FF与FFF模式射野输出因子模拟值与测量值偏差为0．72％±1．4％和0．56％±0．78％。结论该模型输出因子模拟与测量结果符合度较好，绝对剂量计算精度较高，可以用于临床剂量学研究。

英文摘要：

Objective To explore the Monte Carlo calculation methods for the absolute dose calibration and output factor of 6 MV flattening-filter （FF） and flattening-filter free （FFF） photon beams based on TrueBeam accelerator. Methods The BEAMnrc code was used to model the LINAC head of FF and FFF modes. The BEAM_up covers the components from the target to the monitor chamber, and BEAM_down includes the structures beneath the chamber, the dose deposit to the monitor chamber contributed by the incidence electrons and scattered particles from the secondary collimators were calculated respectively. The incidence electron-induced dose at certain depths on the central axis were simulated by means of the DOSXYZnrc code. By means of dose calibration equation, the calibration factor for the standard field （ 10 cm × 10 cm） and the output factors for various fields （ 1 cm× 1 cm -40 cm×40 cm） were computed respectively. Results For the 6 MV FF and FFF beams under the standard 10 cm × 10 cm field, 1 MU equals to 7. 747 × 10^13 ± 3. 099 ×10^11 and 3. 248 × 10^13 ± 1. 624 × 10^11 electrons to the target, respectively, which deposited 21.53 and 35. 01 cGy to the monitor chamber of the virtual accelerator respectively. The difference between the simulated and calculated output factors were 0. 72% ± 1.4% and 0.56% ±0.78% for FF and FFF, respectively. Conclusions The model generated and measured output factors agree well, indicating the good accuracy of the dose calculation by the model, which would provides basis for further clinical dosimetric studies.