兆伏锥形束CT在头颈部肿瘤精确放疗摆位误差的研究

Megavoltage cone-beam CT in the evaluation of set-up errors in head and neck cancers treated with precision radiotherapy

目的 分析兆伏锥形束CT（MVCBCT）引导的头颈部肿瘤精确放疗中摆位误差.方法 头颈部肿瘤共22例（鼻咽癌15例,腮腺癌4例,脑胶质瘤术后3例）,其中接受三维适形放疗7例、调强放疗15例,照射剂量为56.0～70.4 Gy分28～32次6～7周完成.19例用头颈肩面罩固定,3例头部面罩固定.所有患者均行CT模拟,勾画靶区和危及器官.每周治疗前拍1次MVCBCT图像,每位患者接受4～7次MVCBCT.分析相对于计划CT上中心在头脚、垂直、左右方向上的摆位误差.结果 左右、前后和头脚方向中任一方向〉0.3 cm 28次,〉0.4 cm 15次,〉0.5 cm 9次,头脚方向要比左右、前后方向超过近0.1 cm.头脚和左右中心偏移方向的频次差别不大,而在垂直方向则更多的向后偏移达4：1.相对来说头脚方向摆位误差较左右、前后方向大,随着放疗时间延长各方向偏移的平均值均有增大趋势.结论 头颈部肿瘤精确放疗时CTV至PTV边界外扩并不需均匀外放,垂直方向上治疗中心向前偏移概率明显超过向后,头脚方向比左右、前后方向摆位误差大;随着放疗的进行3个方向上的摆位误差都有增大趋势.

Objective To evaluate set-up errors by megavoltage cone-beam CT in head and neck cancers treated with precision radiotherapy. Methods From April 2007 to March 2008, 22 patients with nasopharyngeal carcinoma （15 patients）, parotid carcinoma （4 patients） and brain glioma （3 patients） were enrolled, among whom 7 patients underwent three-dimensional conformal radiotherapy （3DCRT） and 15 received intensity modulated radiotherapy （IMRT）. The radiation dose was 56.0 - 70. 4 Gy in 28 - 32 fractions within 6 -7 weeks. Megavohage cone-beam scan was performed weekly before treatment. The isocenter displacement was calculated by comparing megavoltage cone-beam CT and planning CT in left-right （LR）, cranio-caudal （CC）, and anterior-posterior （AP） directions. Results Totally 129 sets of megavohage cone-beam CT images were obtained for 22 patients. The frequency of isocenter shift more than 0. 3 cm,0. 4 cm and 0. 5 em were 28,15 and 9, respectively. The maximum mean set-up error was found in CC, which was about 0. 1 em more than that in LR and AP. The frequency of isocenter shift direction was almost identical in LR and CC, which was more frequent （about 75% ） in the posterior direction. Conclusions During the course of radiation of brain tumor and head and neck cancer, the enlarging tendency of set-up errors has been found in all the three directions. The isocenter shift in AP was more frequent to the posterior direction.