摘要目的 研究调强放疗中剂量率和多叶光栅的叶片位置容差对点吸收剂量(绝对剂量)的影响.方法 选取2例前列腺癌患者的调强治疗计划.将该计划移到封闭小水箱(水模)上,在剂量率分别为100、200、300、400、500 MU/min情况下,利用电离室测量点吸收剂量.调整水箱位置使电离室位于剂量梯度比较小区域,以使剂量梯度对测量结果的影响降到最低.在测量多叶光栅叶片位置容差对点吸收剂量影响时,剂量率不变,叶片位置容差分别为1、2、3、4 mm,调用该治疗计划进行实际测量.治疗计划系统为瓦里安Eclipse,实际测量用瓦里安加速器23EX.结果 随剂量率增大,点吸收剂量测量偏差也增大,最大值和最小值相差1.2%.在叶片控制系统正常工作情况下,叶片位置容差对点吸收剂量影响很小,测量结果相近.结论 因实际治疗时点吸收剂量(绝对剂量)偏差会随剂量率增大而增大,为提高治疗速度并考虑到剂量率对生物效应的影响,在提高剂量率同时也应尽量避免高剂量率所带来的误差,选择合适的剂量率进行治疗.叶片位置容差对点吸收剂量影响不大,但该数值不应设置太大,是为保证实际叶片位置尽可能接近于MLC文件中给出的数值,使实际剂量分布无论在剂量梯度大或剂量梯度小的区域都能与计划所给出的分布尽可能相近,如果叶片控制系统出现故障也可能尽早发现.

abstractsObjective To evaluate the effect of dose-rate and leaf position tolerance on the point dose in step-and-shoot intensity-modulated radiotherapy(IMRT). Methods IMRT plans of 2 prostate cancer patients were selected and recalculated for a water phantom.An ionization chamber was used to measure the dose of some points in the phantom at five nominal dose-rates:100 MU/min,200 MU/min,300 MU/min,400 MU/min and 500 MU/min.It was necessary to adiust the position of the water phantom to locate the ionization chamber in region where the dose gradient was very low in order to minimize the effect of dose gradient on the measured results.When measuring the effect of leaf position tolerance on point dose,the dose-rate was kept constant and the values of tolerance were 1 mm,2 mm,3 mm and 4 mm.This work was conducted on a Varian 23EX equipped with a Millennium 120-leaf multi-leat collimator(MLC).The treatment planning system was Varian Eclipse. Results As the dose-rate increased,the error between the measured dose and the calculated dose also enlarged.The difference between the maximum and the minimum was 1.2%.When MLC control system was working normally,tlle effect of leaf position tolerance on the measured point dose was very little and negligible. Conclusions The dose-rate must be selected suitably to ensure that the delivery can be finished in a short time with the radiobiological effect taking into account.It should be noted that the error between the measured dose and the calculated dose increases with the trend of inereasing more rapidly at higher dose rates.The value of leaf position tolerance should not be set too large,in order to minimize the difference between the measured dose and the planned dose in the region of steep gradient and find as early as possible when the multi-leaf collimator control system performs improperly.