鼻咽癌调强放疗中解剖及剂量学改变再次计划必要性研究

Necessity of replanning for the change of anatomy and dosimetry during intensity modulated radiotherapy for nasopharyngeal carcinoma

目的探讨鼻咽癌调强放疗（IMRT）中解剖学动态变化规律，以及这些变化对剂量学分布的影响，客观评价放疗重新修改物理计划的必要性。方法搜集12例Ⅲ～Ⅳ期鼻咽癌患者进行相关临床前瞻性研究，所有患者均接受同步放化疗。疗前常规螺旋CT扫描，由临床医生进行靶区及危及器官勾画。IMRT计划完成后再次螺旋CT扫描校正治疗中心，开始治疗后的每周按治疗中心重复进行螺旋CT扫描，然后将获取的CT图像和原计划CT图像融合。首先在系统融合界面就变化的PTV1及正常器官（腮腺、轮廓）进行重新修改，并计算出靶区及正常器官体积变化范围，从中寻找再次计划的最佳时间段。其次按照首次计划条件在重新修改的靶区上再次计算，得出靶区及正常器官剂量学参数后与首次计划对比观察其变化。结果IMRT治疗中头颈部外轮廓、腮腺体积逐步缩小，放疗至5周左右腮腺及轮廓体积变化达顶峰，因而选择第5周CT和首次CT作为剂量学研究对象。再次计算及配对设计比较发现治疗中和治疗前PTV1 D99、D95，脊髓Dmax、Dmean，脑干Dmax、Dean，下颌骨Dmax、Dmean相似（P值均〉0．05），而双侧腮腺D50不同（P左=0．03，P右=0．01）。结论IMRT治疗过程中鼻咽癌患者出现腮腺缩小、轮廓改变和Prv缩小。放疗至5周左右相关体积变化达顶峰。第5周再次计划与原计划相比脊髓、脑干、下颌骨、PTV1各项剂量学参数值变化不大，但腮腺剂量增加较为明显。

Objective To study the dynamic change of anatomy and dosimetry distribution and its influence during intensity modulated radiation therapy（ IMRT）for nasopharyngeal carcinoma（NPC）. Methods From June 2006 to August 2007,12 patients with stage Ⅲ-Ⅳ NPC receiving initial IMRT concurrently combined with chemotherapy were included in the study. The target volumes and involving organs were contoured on the first set of spiral CT images. When IMRT planning was finished, a second CT scans was acquired to rectify the treatment center. Weekly spiral CT images acquired during the treatment period according to the same treatment center were fused with the first CT images. In order to determine the best time of replanning,modified plan target volume（ PTV1 ） and normal organs（ parotids and outline） were contoured on the fusion interface by a single physician. Changes of each structure throughout treatment period were measured by a system software. Then the dose distributions were computed and evaluated for replanning CT using the same beams arrangement in the original plan. Cumulative dose was estimated compared with the original plan. Results The volume of outlines and parotids decreased gradually,and the change came to peak in the fifth week. So the fifth and first week CT scans were selected as research objectives. No significant changes in maximal and mean dose was observed in the brainstem,spinal cord or mandible. Despite volume changes, D99 and D95 of the PTVl did not change significantly（P 〉0.05）. D95 of the bilateral parotids increased significantly（PL =0.03,PR =0.01）. Conclusions During IMRT for NPC,the volume of PTVl ,the out-lines and parotids decrease,and the change come to peak in the fifth week. Comparing to the first treatment plan, the dose parameters of the parotids increase significantly in the second plan based on the fifth week CT, but those of the brainstem, cord ,mandible and PTV1 change slightly.