准直器角度对鼻咽癌容积旋转调强计划及剂量验证的影响

目的:通过旋转准直器，研究准直器的旋转对鼻咽癌容积旋转调强（VMAT）计划与剂量验证的影响。方法:选择10名T3期鼻咽癌患者，每位患者分别设计10个VMAT计划，10个VMAT计划准直器角度分别为0°、5°、10°、15°、20°、25°、30°、35°、40°、45°，比较分析不同计划中靶区剂量、危及器官和正常组织的受照剂量以及机器跳数，并对每个计划进行剂量验证。结果:准直器角度为10°的VMAT计划，PGTVnx、PTV1以及PTV2的HI均值最小，CI均值最大（P<0.05）。PGTVnd的HI均值在所有计划中变化不大（P>0.05），CI均值在5°~30°最大（P<0.05）。脑干、脊髓以及眼球Dmax在5°~20°时较小（P<0.05），视交叉和下颌骨Dmax在15°~25°时较小，腮腺V30在35°时最小。靶区外正常组织，在低剂量区V5~V20时，0°和5°受照体积最低，在高剂量区V25~V50，10°最低。在不同的准直器角度，机器跳数平均值最小的是准直器为0°的计划，最大的是30°（P<0.05）。所有计划的γ通过率均在98%以上，其中准直器在20°时通过率最高。结论:在进行鼻咽癌VMAT计划设计时，可以将准直器角度设置在10°~20°来获取更好的靶区剂量分布，减少危及器官以及正常组织的受照剂量，同时不会降低剂量验证的通过率。【关键词】鼻咽癌；容积旋转调强；准直器角度；剂量验证

Effects of collimator angles on planning and dose verification of volumetric modulated arc therapy for nasopharyngeal carcinoma

Objective To study the effects of collimator rotation on the planning and dosimetric verification of volumetric modulated arc therapy (VMAT) for nasopharyngeal carcinoma. Methods Ten patients with stage T3 nasopharyngeal carcinoma were selected and 10 VMAT plans with collimator angles of 0°, 5°, 10°, 15°, 20°, 25°, 30°, 35°, 40°and 45° were designed for each patient. Dosimetric parameters, such as target coverage, the dose to organs-at-risk and normal tissues as well as monitor units, were compared among different plans, and dose verification was performed on each plan. Results The PGTVnx, PTV1, and PTV2 in VMAT plan with a collimator angle of 10° had the lowest average homogeneity index and the largest average conformity index (P<0.05). The differences in average homogeneity index of PGTVnd among all plans was trivial (P>0.05), and the maximum average conformity index occurred at 5°-30° (P<0.05). The maximum doses to brain stem, spinal cord and eyeball were relatively lower at 5°-20° (P<0.05); the maximum doses to optic chiasm and mandible were relatively smaller at 15°-25°; and the smallest parotid gland V30 was at 35°. For the normal tissue outside the target areas (B-P), the minimum radiation volume in low-dose region (V5, V10, V15 and V20) was at 0°and 5°, while that in high dose region (from V25 to V50) was at 10°. Among all plans, the average monitor unit is minimum for the plan with collimator angles of 0° and was maximum for the plan with collimator angles of 30° (P<0.05). The average gamma passing rates for all plans were above 98%, and the plan with collimator angles of 20° had the highest gamma passing rate. Conclusion In VMAT planning for nasopharyngeal carcinoma, collimator angle can be set at 10°-20° to obtain better target dose distribution, reduce the doses to organs-at-risk and normal tissues while maintaining the passing rate of dose verification.