Objective 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. Resutls 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, Dmin of the visual pathway and Dmean or Dmin of the eyeball in group B were significantly different between Fixed and Iris plans (all P<0.05), while in group A, only Dmin of 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). Conclutions 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.
Wang Jinyuan,Qu Baolin,Xu Shouping et al. Evaluation of the quality and efficiency of SRT plans for intracranial tumors using Fixed and Iris collimators of CyberKnife[J]. Chinese Journal of Radiation Oncology, 2017, 26(11): 1292-1297.
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2017, Vol. 26 
