基于等效方野法的M6型射波刀辐射质指数测定研究

Study on determining the radiation quality index of M6 CyberKnife based on equivalent square field method

    摘要
  • 摘要:
    目的  使用BJR-25方法和等效方野（ESFS）法测定一台M6型射波刀（CK-M6）的辐射质指数（QI），并比较相关的剂量学参数。
    方法  首先使用BJR-25方法计算CK-M6的QI。将其源至电离室有效测量点距离（SCD）调整至1000 mm，以水为测量介质，开野（W×H）=83.2 mm×77.0 mm，即ESFS为10 cm×10 cm时，CK-M6出束200 MU，分别测量组织体模比（TPR）20、TPR10，并进行比值计算（TPR20/TPR10， \mathrmT\mathrmP\mathrmR_10^20 ），即ESFS法获得的QI。分别用2种方法获得的QI查找电离室的PU、Sw/Sa、Kq。然后测量非均整 （FFF）模式下80 mm×80 mm及90 mm×90 mm各3组矩形野的剂量以验证ESFS公式在FFF模式下的有效性。相对实验组间数据采用单因素方差检验进行统计学分析。
    结果 ESFS法计算的QI为0.655，与传统的BJR-25方法计算出的QI为0.667比较，二者相差−1.80%。 ESFS法和BJR-25方法对应的PU、Sw/Sa、Kq分别为0.993、1.122 5、0.996 8，0.994、1.120 5、0.995 6，偏差分别为−0.01%、0.18%、0.05%。FFF模式下80 mm×80 mm 3组矩形野分别为（97.75±0.43）、（97.77±0.24）、（97.79±0.31） cGy（F=6.15，P=0.06）；90 mm×90 mm 3组矩形野分别为（99.03±0.04）、（99.10±0.02）、（99.04±0.35） cGy（F=1.83，P=0.18），显示ESFS法也适用于FFF模式。
    结论  2种方法得出的QI虽然在查找PU、Sw/Sa、Kq值中相差均<0.2%，但不影响射波刀剂量的标定及校准。由于ESFS法的QI是直接测量得出，避开了BJR-25方法中引入标准直线加速器一些物理参数的转换造成的不确定性，使其更方便、准确获得。

     

    Abstract:
    Objective To determine the radiation quality index (QI) of the M6 CyberKnife system using the equivalent square field method.
    Methods The Source-to-Chamber Distance (SCD) was set to 100 cm, with water as the measurement medium. The field size was configured as 83.2 mm×77 mm, equivalent to a 10 cm×10 cm square field. A total of 200 monitor units (MU) was delivered from the CyberKnife, and TPR20 and TPR10 were measured. Their ratio (TPR20/TPR10, \mathrmT\mathrmP\mathrmR_10^20 ) was then calculated to obtain the QI.
    Results In the Flattening Filter Free (FFF) mode, there was no significant difference between the three groups of rectangular fields of 80 mm×80 mm and 90 mm×90 mm (P>0.05). Compared with the QI value obtained via the BJR-25 method (QI2=0.667), the QI determined by the equivalent square field method (QI1=0.655) was 1.80% lower.
    Conclusion Although both approaches yielded negligible differences (<0.2%) in key parameters (PU, Sw/Sa, and Kq), indicating no significant impact on dose calibration or adjustment. However, the equivalent square field method provides a more direct measurement that circumvents uncertainties associated with converting physical parameters from standard linear accelerators in the BJR-25 method. This approach is therefore more straightforward and accurate for routine calibration and quality assurance of the M6 CyberKnife system.

     

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