CT模拟定位减少鼻咽癌放射治疗中的视神经剂量

Dose at the optic nerve can be reduced with CT-simulation in treatment planning for nasopharyngeal carcinoma

目的 比较CT模拟定位治疗计划和常规普通模拟定位治疗计划治疗鼻咽癌时视神经的放射剂量。方法 对 33例T1～T4期的鼻咽癌用PickerPQ5 0 0 0螺旋CT及AcQPlan 4 .1.1软件系统进行模拟定位。首先利用治疗计划系统上的数字重建的射线影像 (DRR) ,根据患者的CT(或MRI)和临床检查结果 ,按照常规使用普通X射线模拟机拍摄定位片的定位方法设计出照射野。然后在所有CT层面逐层勾画出鼻咽肿瘤的大体肿瘤体积 (GTV)、临床靶体积 (CTV)和计划靶体积 (PTV)。同时逐个勾画出周围重要器官轮廓 ,特别小心勾画出眼球后视神经至视交叉的行程。根据肿瘤和周围重要器官之间在三维空间的相互关系设计合理的照射野。分别对两种治疗计划进行剂量计算 ,将靶中心剂量归一为 10 0 % ,主要比较 2种计划的肿瘤靶区、眼球和视神经最大放射剂量、平均剂量和中位剂量。同时比较 2种计划放射野内视神经的长度。结果 不论是CT模拟计划还是常规模拟计划 ,鼻咽GTV和CTV均可有满意的剂量分布。但眼球、视神经和视交叉所接受的最大剂量、平均剂量和中位剂量都有显著差别。CT模拟计划中上述器官的受量明显低于常规计划。CT模拟计划可将更多的视神经保护在照射野外 ,避免不必要的照射。

Objective To compare and evaluate dose at optic structures (eyeballs, optic nerves and chiasma) between conventional and CT simulated treatment planning for nasopharyngeal carcinoma. Methods Thirty three patients with T1 to T4 nasopharyngeal carcinoma underwent simulated spiral CT with picker PQ 5000 and AcQ plan 4.1.1 software system. At first irradiation portals were delinated by the simulation films taken on the conventional X ray simulator using the digital reconstruction (DRR) of the treatment planning system (TPS) supplemented by the patient's CT (or MRI) and clinical findings. Then, the gross tumor volume (GTV),clinical target volume (CTV) and planned target volume (PTV) were delineated on every section of the CT scans. At the same time, the silhouette of the nearby important organs was drawn out ,paying special attention to the backward passage of the retrobulbar optic nerves up to the optic chiasma. The final portals were designed by the 3 dimensional relation between the tumor and its surrounding organs. Then dose distribution was separately calculated by two independent TPSs,with 100% at the target center. Finally,the maximum dose (D max ),average dose (D ave ) and median dose (D med ) of the tumor target area, eye ball,optic nerve and also the length of optic nerve included within the portal of these two technics were compared.Results The dose distribution in nasopharyngeal GTV and CTV were satisfactory both in CT simulated and conventional planning. However, the D max ,D ave and D med all showed significant differences. Those by CT simulated planning were obviously lower than those by conventional planning. CT simulated planning was able to spare more optic nerve by leaving it out of the irradiation portal,thereby,avoiding unwanted radiation.Conclusion CT simulated treatment planning is able to reduce the superfluous radiation to the optic nerve in external irradiation of nasophargngeal carcinoma.