Hi—ART螺旋断层治疗机剂量学参数的测量

Measurement of Dosimetric Parameters for Hi-ART Helical Tomotherapy Unit

目的探讨Hi—ART螺旋断层治疗机照射野剂量学参数测量的内容和方法。方法用断层治疗机专门配置的微型扫描水箱在治疗条件下测量了6MVX线的百分深度剂量和射野离轴比，并与常规Primus加速器6MVX线进行比较。根据AAPMTG51号报告用Tomotrometer剂量仪和A1sL电离室在源皮距85cm、照射野40cm×5cm、1．5cm深度条件下对断层治疗机进行输出剂量刻度，并对剂量线性和重复性进行测量分析。输出剂量率随机架角的变化分别用0．6cm^3电离室和Unidos剂量仪在直径为3cm有机玻璃体模中测量和用治疗机自身的MVCT探测器测量。设置不同的照射范围，在固体水组织等效材料中对多叶准直器照射野输出因子进行测量。结果Hi-ART断层治疗机6MVX线百分深度剂量的最大剂量点在1．0cm左右。Hi—ART断层治疗机和Primus6MVX线在源皮距85cm、深度10cm处的百分深度剂量分别为59．6％和64．7％。单个照射野内剂量分布是不均匀的，在人体左右方向剂量分布呈锥形，在人体头脚方向剂量分布和照射野的宽度有关，40cm×5cm照射野的输出剂量率为848．38cGy／min。剂量仪的读数R和照射时间t的关系为R=-0．017＋0．256t，线性相关系数为0．999。重复测量的输出剂量率的最大偏差为1．6％，标准偏差〈0．5％；输出剂量率随机架角度变化的最大偏差为1．1％，标准偏差〈0．5％。多叶准直器相邻叶片对单个叶片照射野的剂量贡献比较大，继续增加叶片数目输出因子基本保持不变。结论Hi—ART断层治疗机的输出剂量率高，照射野剂量分布不均匀。独特的设计和剂量学特性使其剂量计算模型和调强实现方式更加简单、高效。

Objective To develop a measurement method of dosimetric parameters for Hi-ART to- motherapy unit. Methods Percentage depth doses and beam profiles were measured using the dedicated mini water phantom, and compared to the results of 6 MV X-ray from Primus accelerator. Following the AAPM TG51 protocol,absolute dose calibration was carried out under SSD of 85 cm at depth of 1.5 cm for field of 5 cm × 40 cm. The output linearity and reproducibility were evaluated. The output variation with the gantry rotation was also investigated using 0.6 cm^3 ion chamber in cylindrical perplex phantom and on-board MVCT detectors. Leaf fluence output factors were quantified for the leaf of interest and its adjacent leaves. Results The buildup depth was around 1.0 cm. The PDD values at 10 cm for Hi-ART and Primus were 59.7% and 64.7%, respectively. Varying with the field width, the lateral and longitudinal beam profiles were not so homogeneous as the Primus fields. The measured dose rate was 848.38 cGy/min. The fitted linear function between the readings of dosimeter and the irradiated time was R（nC） = - 0. 017 ＋ 0. 256 · t（sec） ,with a relative coefficient of 0. 999. The maximum deviation and standard deviation of output were 1.6% and less than 0.5% ; The maximum deviation and standard deviation of output changed by gantry angle were 1.1% and 0.5% , respectively. Leaf fluence output factors did not increase significantly when leaves were opened beyond the two adjacent leaves. Conclusions Hi-ART Tomotherapy unit has a very high dose output and inbomogeneous beam profiles owing to its special design of the treatment head. This may be useful in dose calculation and treatment delivery.