准直器角度对颅内两个脑转移瘤容积旋转调强计划的影响

【摘 要】 目的:探讨多叶准直器角度对两个脑转移瘤容积旋转调强（VMAT）计划剂量学参数和机器跳数的影响。 方法:选择10例已放疗的颅内两个脑转移瘤患者CT图像作为研究对象，在准直器角度为0°，调整每条弧的准直器至合适角度，尽量使叶片能够最大遮挡病灶之间的正常脑组织，以VMAT方式分别设计计划，比较两组计划的靶区适形度指数、剂量梯度跌落指数（GI）、剂量均匀性指数（HI）、危及器官剂量分布、靶区周边正常组织受量体积以及机器跳数。 结果:与准直器角度为0°的计划比较，调整每条弧的准直器至合适角度，靶区适形度指数差异无统计学意义（t=1.084, P=0.32），GI和HI差异具有统计学意义（GI:t=3.415, P=0.014；HI:t=3.455, P=0.014）；周边正常组织2和4 Gy剂量体积差异有统计学意义（P=0.018, P=0.045），6、12和16 Gy剂量体积差异无统计学意义（P>0.05）；危及器官最大受照剂量相似，差异无统计学意义（P>0.05）；机器跳数平均减少282 MU。 结论:在两个多发脑转移瘤VMAT计划优化选择准直器运行方向，剂量梯度跌落更陡，剂量分布更均匀，且可降低正常组织低剂量区受量，以及明显减少机器跳数。

Effects of collimator angle on volumetric modulated arc therapy plans for two brain metastases

Abstract: Objective To investigate the effects of multileaf collimator angle on the dosimetric parameters and monitor units (MU) of volumetric modulated arc therapy (VMAT) for patients with two intracranial metastases. Methods The CT images of 10 patients receiving radiotherapy for two intracranial metastases were selected in this study. When the collimator angle remained at zero degree, and the collimator was adjusted to reduce radiation dose to brain tissue in every arc (collimator angle was nonzero degree), VMAT plans were designed for various collimators without changing any optimization parameters. The conformity index, gradient index (GI), homogeneity index (HI), MU, and dose to organs-at-risk and normal tissues around target areas were calculated and analyzed. Results Target conformity index was similar in VMAT plans with collimator angle of zero degree and nonzero degree (t=1.084, P=0.32), but VMAT plans with collimator angle of nonzero degree had lower GI and HI, with statistical significance (GI: t=3.415, P=0.014; HI: t=3.455, P=0.014). Compared with VMAT plans with collimator angle of zero degree, VMAT plans with collimator angle of nonzero degree had smaller volume of normal tissues receiving 2 and 4 Gy (P=0.018, P=0.045), but similar volume of normal tissues receiving 6, 12 and 16 Gy (P>0.05). No statistical differences were found in the maximum dose to organs-at-risk between two plans. Moreover, the comparison between two plans showed that MU was reduced by 282 MU in VMAT plans with collimator angle of nonzero degree. Conclusion The optimal selection of collimator angle in VMAT plans for two brain metastases not only improves GI and dose coverage, but also reduces doses in low dose areas and MU.