摘要目的 探讨与评价运用第5代CyberKnife VSI系统中固定(Fixed)和可变(Iris)准直器实施颅内肿瘤SRT时计划质量的剂量学特性.方法 选择20例颅内肿瘤患者分为A、B两组,A、B组分别为小、大体积靶区组,每组10例,统一靶区剂量21 Gy分3次.分别对每例患者选择Fixed和Iris准直器进行治疗计划设计.通过靶区CI、HI、GI、GSI与OAR等剂量学参数,分析评估SRT计划优化质量及实施效率,从而评价两套准直器的束流特性情况.采用配对t检验分析其差异.结果 Iris计划执行的平均时间明显优于Fixed计划(A组P=0.001;B组P=0.000).B组中低剂量区(20%、10%处方剂量)体积Fixed计划明显低于Iris计划(P=0.001、0.009).对于OAR,B组中视通路D min及眼球D mean、D min具有统计学差异(P均<0.05),A组中仅视交叉D min具有统计学意义(P=0.043).对于靶区相关参数两组均相近(P均>0.05).结论 利用第5代CyberKnife VSI系统治疗颅内小体积肿瘤时,Iris与Fixed准直器相比,除治疗时间明显缩短外无明显剂量学特性差异.对体积较大且形状复杂的肿瘤靶区,Iris计划虽然正常器官限量在允许范围内,但其低剂量区体积要明显略高于Fixed计划.而对于两种准直器更深层次剂量学特性,仍需更多病例进一步研究可能.

abstractsObjective To discuss and evaluate the dosimetric characteristics of different plans implementing stereotactic radiotherapy(SRT)for intracranial tumors using Fixed and Iris collimators of CyberKnife VSI.Methods Twenty patients with intracranial tumors were selected and divided into group A with a small target volume(≤30 cm3)and group B with a large target volume(≥30 cm3). There were 10 patients in each group,and the prescribed dose to the target was 21 Gy in 3 fractions. For each patient, two treatment plans were designed using Fixed and Iris collimators. By analyzing the dosimetric parameters such as conformity index(CI),homogeneity index(HI), gradient index(GI), gradient score index(GSI), and organs at risk (OAR),the quality and efficiency of the plans were evaluated in order to discuss the beam characteristics for two sets of collimators. The difference was analyzed with the paired t-test. Results The mean time of Iris plan for delivering was significantly less than that of Fixed plan(group A:P=0.001;group B:P=0.000). In group B,the peripheral dose(20% and 10% of the prescribed dose)volumes of Fixed plan were significantly less than those of Iris plan(P=0.001 and 0.009). For OAR,D minof the visual pathway and D meanor D minof the eyeball in group B were significantly different between Fixed and Iris plans(all P<0.05), while in group A, only D minof the optic chiasm was significantly different between the two plans(P=0.043). For the other parameters of targets,there were no significant differences between Fixed and Iris plans in both groups(all P>0.05). Conclusions Apart from less treatment time in the Iris plan, there are no significant dosimetric differences between the two collimator plans of CyberKnife VSI in treating small intracranial tumor. For the large and complex tumor,although Iris plan meets the requirement for OAR dose constraints,its low-dose volumes are larger than those of Fixed plan. Further studies of the dosimetric characteristics in CyberKnife should be done.