螺旋断层调强放疗靶区外沿纵向剂量跌落及影响因素研究

目的研究螺旋断层调强放疗(HT)靶区外沿纵向剂量跌落的变化规律及影响因素,以便对临床关于计划衔接、靶区预处理以及执行效率等方面的应用进行指导。方法回顾性选取2019年12月份郑州大学第一附属医院放射治疗部收治的8例头颈部肿瘤患者资料作为研究对象,使用德国西门子SOMATOM Definition AS大孔径定位CT在获得层厚为1 mm的头颈部图像中勾画计划靶区和剂量跌落结构。在计划设计时射野宽度(FW)分别选择5.0、2.5和1.0 cm,螺距分布选择0.430、0.287和0.215,调制因子使用默认值1.8,剂量计算网格选择最精细0.195 cm×0.195 cm,其余计划参数都保持一致。按照不同参数分别进行计划设计,并将结果进行单因素方差统计分析。结果研究显示同一射野宽度下不同螺距变化曲线重合,因而螺距对剂量跌落没有影响,不同射野宽度变化曲线相互独立说明射野宽度对纵轴双向剂量跌落有影响,靶区外沿纵向的剂量跌落速度与射野宽度成反相关系:即射野宽度越大剂量跌落速度越慢,半影区越大;反之射野宽度越小剂量跌落速度越快,半影区越小。当剂量跌落至处方剂量50%时距靶区纵向边界的距离近似约等于射野宽度一半,而对于距靶区边界不同距离处的剂量值可通过拟合公式计算得到。射野宽度和螺距对靶区的适形度指数(CI)和均匀性指数(HI)影响较小,相对而言射野宽度为2.5 cm时靶区最佳。总治疗出束时间随射野宽度和螺距的增大而逐步降低。结论当靶区分段治疗需要考虑计划衔接、执行效率及沿纵向剂量跌落控制时,可选择最佳的射野宽度、螺距等计划参数或根据纵向剂量跌落公式对衔接处靶区进行内收处理,以达到理想的剂量分布。

The impact factors of longitudinal dose fall-off outside the target with helical tomotherapy

Objective To study the changing characteristics and impact factors of helical tomotherapy (HT)for longitudinal dose fall-off outside the target, in order to guide the plan junction or pretreatment target and implementation efficiency in clinical. Methods Eight patients with head and neck tumors admitted to the Department of Oncology Radiotherapy of the First Affiliated Hospital of Zhengzhou University in December 2019 were retrospectively selected as the research objects. The planning target area and dose drop structure were outlined in the head and neck images with a thickness of 1 mm obtained by Siemens SOMATOM Definition AS positioning computerized tomography (CT).Different field widths (FW, 5.0 cm/2.5 cm/1.0 cm) and pitches (0.430/0.287/0.215) were assembled for planning with the same modulation factor (1.8), finest does calculation grid (0.195 cm×0.195 cm) and other planning parameters were consistent. The plans were designed by different parameters, and the result was analyzed by univariate analysis. Results The that different pitch curves coincided under the same field width by comparative analyzing, so pitchs had no effect on dose drop. The different field width curves were independent of each other, indicating that the field width had an effect on dose drop in the head and foot direction. The relationship between the longitudinal dose drop speed outside the target and the change of the field width was inversely correlated:the larger field widths meant the slower dose fall-off and the larger penumbra, while the smaller field widths meant the faster fall-off and the smaller penumbra. When the dose fall-off to 50% of the prescribed dose, the distance from the target was approximately equal to half the field widths, and the pitchs had not affect the rate of dose-drop, while the dose at different distances from the target boundary could be calculated by the fitting formulas. The field widths and pitchs had little effect on the CI and HI index of the target, relatively, the target area was best when the field width was 2.5 cm. The total beam-on time gradually decreased with the increase of the field widths and pitches. Conclusions When segment target therapy needs to consider planning junction, execution efficiency, and controlling longitudinal dose fall-off and considered the execution, the optimal planned parameters such as field widths and pitches could be selected or the target at the junction regions could be adducted according to the longitudinal dose drop formula, so as to achieve the ideal dose distribution.