位置重复性对射波刀模体绝对剂量验证的影响

Effect of position repeatability on the verification of absolute dose for CyberKnife

目的:对医院新引进的射波刀按照国际原子能机构(IAEA)TRS-398号报告进行模体绝对剂量测量,进而探索水模体位置的重复性对治疗计划绝对剂量验证通过率的影响,确保输出剂量更加精准地投照到患者肿瘤。方法:选取60、40和25 mm 3种准直器按照正面入射、侧面入射和混合入射方式分别设计3个计划,采用德国PTW公司的UNIDOSEwebline型静电计和TW310100.125cc指形电离室测量绝对剂量。结果:每一组实验3种不同准直器测量对治疗计划绝对剂量的TPS计算值与实际测量值之间的偏差基本一致(小于0.7%)。当射束全部从模体的正面入射时,无论是单射束还是多个射束,验证偏差均在2%左右;随着侧向射束的引入,其值由3%左右逐渐增大到4%左右;甚至当射束全部从侧向入射时,偏差达5%以上;但将模体固定之后再重复测量,即使在侧向射束也存在的条件下,偏差下降为2%左右。结论:模体空间相对位置的重复性是引起计划系统计算结果和实际测量结果之间偏差增大的主要原因。

Objective To measure the absolute dose delivered to phantom on newly introduced CyberKnife according to IAEATRS-398 report for exploring the effect of the position repeatability of water phantom on the passing rate of absolute dose verification,thereby ensuring that the prescription dose is accurately delivered to the tumors.Methods Three types of collimators(60,40 and 25 mm)were used to design 3 different plans based on different incident directions of the beams which included normal incidence,lateral incidence and mixed incidence.The absolute dose was measured by UNIDOSwebline electrometer from PTW,Germany,and TW310100.125cc finger ionization chamber.Results Trivial dose deviations(less than 0.7%)were observed between the absolute dose calculated by treatment planning system and measured results for 3 types of collimators in each group of experiments.The verification deviation was about 2%,when the radiation beams entered the phantom at a normal incidence regardless of single beams or multi beams.With some radiation beams entering the phantom at a lateral incidence,the deviation was gradually increased from 3% to 4%.When all beams entered the phantom at a lateral incidence,the deviation was up to 5%.However,after the phantom was fixed,the deviation was reduced to about 2% even there were radiation beams at a lateral incidence.Conclusion The main reason for the increasing deviation between the results calculated by treatment planning system and the results measured by electrometer is the repeatability of the relative position of phantom.