Objective: To discuss the scan condition of megavoltage computed tomography (MVCT) of helical-tomotherapy (HT) in the radiotherapy of head and neck neoplasm. Methods: 11 patients with head and neck neoplasm were enrolled in the research, and different combinations of thickness and range were adopted to implement scan of MVCT. After the image registration of scan and plan was achieved, the deviation value at four directions included of longitudinal (LNG), lateral (LAT), vertical (VRT) and rotational (ROLL) on the registration position were further analyzed. Results: For patients with long planning target volume (PTV), such as nasal pharyngeal cancer (NPC), the differences of two thickness (4mm and 6mm) at four directions (LAT, LNG, VRT and ROLL) were not significant when their scan ranges were same. When the scan ranges were basis of skull-chin and throat-supraclavicular position, respectively, the differences of the registration between two thickness at three directions (LAT, VRT and ROLL) were significant (t=-5.48, t=-2.56, t=-3.82, P<0.05). For other patients with shorter PTV, such as hypophysoma and others, the difference of registration at different scan thickness and range were not significantly. Conclusion: For the patients with long PTV, such as head and neck neoplasm, the wider thickness should be chosen to reduce the scanning time, and the difference of different registration region should be pay more attention. While for the patients with shorter PTV, the narrower thickness should be adopted to enhance image quality so as to easily achieve clinical registration.%目的:探讨螺旋断层放射治疗(HT)系统兆伏级CT(MVCT)在头颈部肿瘤放射治疗中扫描条件的设定.方法:选取11例头颈部肿瘤患者,行MVCT扫描时采用不同层厚与范围组合,将扫描和计划图像配准后,对配准部位的进出(LNG)、左右(LAT)、升降(VRT)及旋转(ROLL)4个方向的偏差值进行分析.结果:对于鼻咽癌等靶区长的患者,相同扫描范围4 mm和6 mm的不同扫描层厚在LAT、LNG、VRT和ROLL的4个方向偏差值均无统计学差异;当不同扫描范围为颅底-下颏及喉-锁骨上时,在LAT、VRT和ROLL的3个方向的偏差值有明显统计学差异(t=-5.48,t=-2.56,t=-3.82;P<0.05).对于垂体瘤等靶区短的患者,采用不同的扫描层厚及范围,配准结果无统计学差异.结论:靶区较长的头颈部肿瘤,可选择较宽层厚减少扫描时间,需关注不同配准区域形成结果的差异;对于靶区短的肿瘤,可采用较窄的层厚提高图像质量,使其易于临床配准.
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