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前鼻咽癌治疗普遍应用调强放疗,与传统三维适形放疗相比,调强放疗对摆位精确度要求更高,剂量更加准确。由于摆位误差需要对靶区进行外扩,本研究旨在探索与确定合适的外扩边界,在保护正常组织的前提下保证靶区剂量的均匀性。应用IGRT图像引导获取12例患者的摆位误差,在治疗计划系统中相应移动等中心,在不改变射野分布和权重的情况下,重新计算剂量分布。分析移动等中心后计划与原计划中各靶区与正常组织的剂量变化,确定适当的靶区外扩边界,以保证靶区剂量的均匀以及正常组织受量。当摆位误差为5 mm时,8.3%的GTV D99及16.7%的CTVD95剂量减少大于6%,当摆位误差为2 mm时,GTV D99、CTV D95剂量减少均小于3%。误差越大对剂量分布的影响越明显。靶区CTV较GTV对于摆位误差更为敏感,按照Stroom等推荐的公式2.0Σ+0.7σ计算GTV外扩边界PGTV:x=4.33 mm;y=2.80 mm;z=4.18 mm,据此生成的计划可有效避免由摆位误差导致的靶区剂量不足。
Pre-treatment of nasopharyngeal carcinoma commonly used intensity-modulated radiotherapy, compared with the traditional three-dimensional conformal radiotherapy, intensity modulated radiotherapy on the positioning accuracy requirements, the dose is more accurate. Because of the error of positioning, the target area needs to be expanded outwardly. This study aims to explore and determine the suitable extended boundary and ensure the uniformity of target dose under the premise of protecting normal tissues. The IGRT images were used to guide the setup error of 12 patients and the center of corresponding movement in the treatment planning system was used to recalculate the dose distribution without changing the field distribution and weight. Analysis of the mobile center after planning and planning of the original target area and the normal tissue dose changes to determine the appropriate target outside the expansion boundary to ensure that the target dose uniformity and normal tissue exposure. When the setup error was 5 mm, the dose reduction of 8.3% of GTV D99 and 16.7% of CTVD95 was more than 6%. When the setup error was 2 mm, the dose reduction of GTV D99 and CTV D95 were all less than 3%. The bigger the error is, the more obvious it is to the dose distribution. The target CTV is more sensitive than the GTV to the setup error. According to the formula 2.0Σ + 0.7σ recommended by Stroom et al., The GTV extended boundary PGTV is calculated as: x = 4.33 mm; y = 2.80 mm; z = 4.18 mm The program can effectively avoid the target dose caused by the positioning error.