螺旋断层放疗MVCT成像剂量测量及其分析

Measurement and analysis of the imaging dose with megavoltage computed tomography for helical tomotherapy

背景与目的:螺旋断层放疗中治疗前/后通常采用MVCT(megavoltage computed tomography,MVCT)来实行图像引导下精确定位,成像过程中势必会增加患者的辐射剂量。通过体部及头部标准模体分别模拟患者MVCT成像条件来实现其剂量学测量及研究。方法:采用螺旋断层治疗机MVCT分别对圆柱形体部模体和自制标准头模体在不同螺距比(1.0、2.0、3.0)、扫描长度(4.8、7.2、9.6、12、14.4cm)及摆位条件下进行扫描,测量模体内各点扫描平均剂量变化,并对其剂量均匀性情况进行定性分析。结果:体部模体内MVCT扫描辐射剂量在0.599～2.876cGy之间;而头部模体点平均剂量为0.913～3.231cGy。测量结果表明,模体内所受照射剂量与螺距比值、扫描长度呈密切相关性;且MVCT扫描剂量与CT螺距值基本成反比关系。不同摆位条件下辐射剂量值沿模体径向大小而相应产生出其变化趋势。结论:实验结果有助于为治疗计划设计及图像引导治疗扫描范围提供较为现实而可靠的临床依据,减小患者治疗期间可能诱发肿瘤所获取的额外剂量。

Background ＆ Objective： During the helical tomotherapy process, megavoltage computed tomography （MVCT） images are usually used for guiding the precise setup of patients before/after treatment delivery, which would certainly increase the total dose for patients. This study was to investigate the imaging dose of MVCT using the body and head phantom on a tomotherapy machine. Methods： A set of cylindrical body and head phantoms was adopted for scanning with different pitch values （1.0/2.0/ 3.0）, lengths （4.8/7.2/9.6/12/14.4 cm） and patient setups on the couch of tomotherapy system. The average MVCT imaging doses were measured using AlSL chambers inserted in the phantoms with preset geometry. The dose uniformity was qualitatively analyzed. Results：The MVCT scanning dose for the body phantom was between 0.599 and 2.876 cGy during each treatment delivery, while the dose for the head phantom was between 0.913 and 3231 cGy. Two major parameters, the assigned pitch numbers and scanning lengths, were the most important impacts to the dose variation. The MVCT dose was inversely proportional to the CT pitch value. With respect to the scanning length, the doses responded differently along the radial direction of the phantoms with different setup criteria. Conclusion. The results may provide a reliable guidance for proper planning design of the scanning region, which is valuable to help minimize the extra doses to patient.