Objective To investigate the effects of different degrees of bladder filling on the dose distribution in brachytherapy for cervical cancer based on Monte Carlo simulation.Methods A Nucletron-mHDR-V1 192Ir radioactive source model was established with MCNP5 Monte Carlo particle transport software package,and then placed into a three-dimensional voxel model obtained from the conversion of CT images taken from a patient when he had different degrees of bladder filling.The prescription dose of brachytherapy was 60 Gy,and the reference points were selected according to ICRU 38 report.The single tube source device was used.The location of the radioactive source was divided into 18 steps and the distance of each step was 5 mm.Results The absorbed dose to reference point A was 60 Gy when the bladder filling was 54 mL or 328 mL.The absorbed doses to the reference points of the bladder and rectal were 30.21 Gy and 49.05 Gy at the bladder filling of 54 mL,and 33.59 Gy and 50.19 Gy at 328 mL;the absorbed doses to femoral head were found to be 6.33 Gy (left) and 6.39 Gy (fight) at 54 mL,and 6.15 Gy (left) and 6.12 Gy (fight) at 328 mL.The dose volume histogram of the bladder was obtained based on the absorbed doses in the voxels of bladder wall.When the bladder filling was 54 mL,V10,V20,V30,V40,Vs0,V60were100％,39.69％,26.20％,17.89％,14.48％,11.69％,respectively;when the bladder filling was 328 mL,V10,V20,V30,V40,V50,V60 were 86.07％,26.53％,15.58％,9.44％,6.01％,3.72％,respectively.Conclusion No significant differences were found in the absorbed dose to the reference points of the rectal and the femoral head between the bladder filling of 328 mL and that of 54 mL.Although the absorbed dose to the reference point of the bladder increased (3.38 Gy) at the bladder filling of 328 mL as comparison with 54 mL,the irradiated volume of the bladder was significantly reduced (the irradiated volume reduced by 13.16％ when the dose was 20 Gy,8.45％ when the dose was 40 Gy,7.97％ when the dose was 60 Gy).The simulated results in this study provide some guidance for the brachytherapy of cervical cancer.%目的:使用蒙特卡洛方法模拟宫颈癌的近距离放射治疗,探讨不同膀胱充盈度对宫颈癌近距离放射治疗剂量分布的影响.方法:使用MCNP5蒙特卡洛粒子输运软件包建立Nucletron-mHDR-V 192Ir放射源模型,并放入由患者在不同膀胱充盈度时拍摄的CT图像转换得到的三维体素模型,模拟不同膀胱充盈度对宫颈癌近距离放疗的影响.其中处方剂量为60Gy,按ICRU 38号报告推荐方法选取各参考点,采用单管施源器,放射源位置共分为18个步长,每个步长距离为5mm.结果:参考点A点吸收剂量为60Gy,膀胱充盈度为54 mL时:膀胱参考点的吸收剂量为30.21 Gy,直肠参考点的吸收剂量为49.05 Gy,股骨头参考点的吸收剂量为6.33 Gy(左)、6.39 Gy(右);膀胱充盈度为328 mL时:膀胱参考点的吸收剂量为33.59 Gy,直肠参考点的吸收剂量为50.19 Gy,股骨头参考点的吸收剂量为6.15 Gy(左)、6.12 Gy(右).分析膀胱壁上各体素的吸收剂量,得到膀胱的剂量体积直方图,膀胱充盈度为54 mL时:V10=100％,V20=39.69％,V30=26.20％,V40=17.89％,V50=14.48％,V60=1 1.69％;膀胱充盈度为328 mL时:V10=86.07％,V20=26.53％,V30=15.58％,V40=9.44％,V50=6.01％,V60=3.72％.结论:膀胱充盈度为328 mL相对于膀胱充盈度为54 mL时,直肠、股骨头参考点的吸收剂量没有显著变化,虽然膀胱参考点的吸收剂量有所增加(3.38 Gy),但在剂量体积直方图中所有剂量点处,膀胱的受照体积均有比较明显的减少(剂量为20Gy时受照体积减少13.16％,剂量为40Gy时受照体积减少8.45％,剂量为60Gy时受照体积减少7.97％),模拟结果对临床宫颈癌的放射治疗有一定指导意义.
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