宫颈癌调强计划在水与介质中蒙特卡罗计算的剂量差异

Comparison of absorbed doses between dose-to-medium and dose-to-water calculated by Monte Carlo method for cervical cancer IMRT

目的探讨宫颈癌调强放疗计划设计时,蒙特卡罗算法中选择对水与介质两种算法造成的剂量差异.方法随机选取19例宫颈癌调强计划,采用蒙特卡罗算法计算两种介质的计划,一种是计算对实际介质的吸收剂量D_(m),另一种是计算对水的吸收剂量D_(w);通过剂量体积直方图比较两种剂量算法造成的差异;在30cm×30cm×30cm均匀水模体中,从上到下每间隔2cm插入7个电子密度分别为0.4、0.6、0.8、1.0、1.2、1.4和1.6的虚拟组织.设计10cm×10cm照射野,100MU,0cm开始间隔0.2cm设置剂量感兴趣点,分别计算水和介质的两种计划,记录每个兴趣点剂量值,比较D_(m)和D_(w)的偏差.结果19例宫颈癌靶区剂量体积参数的偏差结果都在1%以内,均有统计性差异(P<0.05).危及器官中,Rectum的D_(2%)和Femoral的D_(2%)和D_(50%),D_(w)均比D_(m)要高,差异均有统计学差异(P<0.05),其中股骨头的剂量相对偏差接近3%,而肠道、膀胱等组织差别则相对要小一些(<1%).在模体中当相对电子密度(RED)<1.0,D_(m)数值要大于D_(w);当相对电子密度(RED)=1.0,D_(m)与D_(w)无差别;当相对电子密度(RED)>1.0,D_(m)小于D_(w).相对电子密度越大,二者的剂量偏差越大,特别是在RED=1.6,D_(w)比D_(m)高接近10%.结论在宫颈癌调强计划设计中,因为肿瘤附近存在较多骨组织等高密度组织,所以应选择D_(m)的蒙卡方法进行剂量计算.

Objective To evaluate the difference of absorbed doses calculated to medium and to water by Monte Carlo algorithm for intensity-modulated radiotherapy of cervical cancer patients.Methods For 19 cervical cancer patients participated in this retrospective study,plans were calculated for dose to water(D_(w))and dose to medium respectively.The differences between D_(w)and D_(m)were qualitatively evaluated by dose-volume parameters.With the 30cm×30cm×30cm uniform water phantom established in TPS,7 virtual tissues with different electron densities were inserted at 2cm intervals.A 10cm×10cm irradiation field was developed in the isocenter of the phantom,the source skin distance was 100cm,100MU,and the dose interest points are set at 0.2cm intervals from the isocenter surface depth of 0cm.The comparison between D_(w)and Dtn was performed evaluating quantity essential for respective volumes of interest.Results The deviation results of 19 cervical cancer patients to evaluate the dose to the water and the medium in the target were both within 1%,and there are statistical differences(P<0.05).Among the organs at risk,the absorbed dose of D„was much higher than D_(m),no matter the dose of D_(2%)for rectum,or the dose of D2,and D_(50%)for femoral.The comparative dose deviation of the femoral head was close to 3%,was statistically significant(P<0.05),while the difference of the intestinal and bladder tissues was less than 1%.Conclusion In the design of IMRT for cervical cancer,because there are many high-density tissues such as bone tissue near the tumor,the MC method of D_(m)should be selected for dose calculation.