瓦里安加速器高分辨多叶准直器位置偏移对前列腺癌调强放射治疗剂量分布的影响

Influence of position deviation of Varian accelerators high definition multi leaf collimator on dose distribution in intensity modulated radiotherapy of prostate cancer

目的评价高分辨多叶准直器(HDMLC)位置偏移对前列腺癌调强放射治疗(IMRT)在不同剂量率下剂量分布的影响,为多叶准直器(MLC)的日常质量控制提供参考。方法选取2014年2月至2015年4月32例前列腺癌患者,使用9个照射野IMRT计划进行放射治疗,射野间隔40°,使用瓦里安Turebeam STX加速器HDMLC、Eclipse治疗计划系统,模拟每个计划在下列条件下引入固定数值(0.3、0.5、1.0、2.0 mm)的MLC位置偏差:(1)MLC X1X2方向同时扩大;(2)MLC X1X2方向同时缩小。每个计划结果的比较使用以下2个指标:(1)95%等剂量线适形指数(95%CI);(2)直肠最高剂量(Rectum Dmax)。结果 (1)如果MLC沿X1X2准直器方向扩大偏移大于或等于0.3 mm,则95%CI下降,Rectum Dmax增加,并且这种影响与剂量率和偏移量的增大成正比;(2)如果MLC沿X1X2准直器方向缩小偏移大于或等于0.3 mm时,95%CI先增加到峰值;如果MLC缩小偏移大于或等于1.0 mm时,95%CI迅速下降,且剂量率越高下降越明显;然而,Rectum Dmax呈明显减少趋势,并且这种减少与剂量率和偏移量的增大成正比。结论 HDMLC的叶片位置误差大于0.3 mm时,前列腺癌的IMRT计划会发生大量的剂量偏差,并且这种偏差随着剂量率的增加更加明显,因此MLC日常质量保证应将MLC平均位置误差控制在0.3 mm以内。

Objective To evaluate the influence of position deviation of high definition multi leaf collimator（HDMLC） on the dose distribution under different dose rates in the intensity modulated radiotherapy（IMRT） of prostate cancer. Methods 32 patients with prostate cancer from February 2014 to April 2015 were selected and performed the radiotherapy by using the IMRT plan-ning with 9 irradiation fields,the irradiation interval of 40°,Varian Truebeam accelerator HDMLC,Eclipse treatment planning system,the MLC position deviation of introduced fixed numerical values（0.3,0.5,1.0,2.0 mm） for each planning was under the following conditions：（1）simultaneous enlargement of MLC X1 and X2 directions;（2）simultaneous shrink of MLC X1 and X2 directions.The two indexes were used for the comparison of each planning results：（1）the 95% isodose line conformal index（95%CI）;（2） the rectum maximum dose（rectum Dmax）. Results（1） If MLC amplified deviation at the MLC X1X2 collimator directions≥0.3 mm,95% CI was decreased,while rectum Dmax was increased,moreover this influence was directly proportional to the enlargement of dose rate and deviation amount;（2）if the MLC shrunk deviation at the MLC X1X2 collimator directions ≥0.3 mm,95%CI was firstly increased to the peak value;if MLC shrunk deviation ≥1.0 mm,95%CI was rapidly dropped,moreover the higher the dose,the more obvious the drop;however,rectum Dmax showed a obviously decreasing trend and this reduce was directly proportional to the dose rate and deviation amount. Conclusion If the leaf deviation of HDMLC is more than 0.3 mm,a large number of dose deviation could occur in the IMRT planning of prostate cancer,moreover this deviation could be more obvious with the increase of dose rate,therefore the MLC daily quality assurance should control the average MLC position deviation within 0.3 mm.