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 探究BJR-25方法和等效方野（ESFS）法测定一台M6型射波刀（简称CK-M6）的辐射质指数（QI），并比较相关的剂量学参数。
    方法 采用BJR-25方法计算CK-M6的QI。将CK-M6的源至电离室有效测量点的距离（SCD）调整至1000 mm，以水为测量介质，照射野（宽×高）=83.2 mm×77.0 mm，即ESFS为10 cm×10 cm时，CK-M6出束200 MU，分别测量水下20 cm、10 cm处的组织模体比（TPR），并计算TPR20/TPR10比值（ \mathrmT\mathrmP\mathrmR_10^20 ），即ESFS法测得的QI。分别用2种方法获得的QI查找电离室相应的扰动因子（PU）、水对空气的阻止本领比（Sw/Sa）、射线束转换因子（Kq）。测量非均整 （FFF）模式下SCD=1 000 mm时分别等效于80 mm×80 mm及90 mm×90 mm的3组矩形射野的剂量以验证ESFS法在FFF模式下的等效性。符合正态分布的计量资料的组间比较采用单因素方差分析。
    结果 ESFS法计算的QI为0.655，与传统的BJR-25方法计算出的QI（0.667）比较，偏差为−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和0.994、1.120 5、0.995 6，偏差分别为−0.10%、0.18%、0.12%。FFF模式下在SCD=1 000 mm处等效于80 mm×80 mm 3组矩形射野的测量剂量分别为（97.75±0.43）、（97.77±0.24）、（97.79±0.31） cGy （F=6.15，P=0.06）；90 mm×90 mm 3组矩形射野的测量剂量分别为（99.03±0.04）、（99.10±0.02）、（99.04±0.35） cGy（F=1.83，P=0.18），这表明ESFS法也适用于FFF模式。
    结论  2种方法得出的QI查找的PU、Sw/Sa、Kq值的偏差均<0.2%，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，避免了BJR-25方法中引入标准直线加速器物理参数的转换造成的不确定性，使其获得更方便、准确。

     

    Abstract:
    Objective To determine the radiation quality index (QI) of an M6 CyberKnife (referred to as CK-M6) with the BJR-25 method and the equivalent square field size (ESFS) and compares related dosimetric parameters.
    Method The QI of the CK-M6 was calculated using the BJR-25 method. The distance from the CK-M6 source to the effective measurement point (SCD) of the ionization chamber was adjusted to 1 000 mm. Water was used as the measurement medium, and the irradiation field was set to dimensions of 83.2 mm×77.0 mm (width×height). When the ESFS was 10 cm×10 cm, the CK-M6 beam output was 200 MU. The tissue phantom ratios (TPR) were measured at water depths of 20 (TPR20) and 10 cm (TPR10), and the TPR20/TPR10 ratio (\mathrmT\mathrmP\mathrmR_10^20 ) was calculated and used as the QI for the ESFS. The QI obtained by the two methods were used in determining the corresponding disturbance factor (PU), stopping power ratio of water to air (Sw/Sa), and beam conversion factor (Kq) of the ionization chamber. The equivalence of the ESFS method in the flattening filter free (FFF) mode was verified by measuring doses for three groups of 80 mm×80 mm rectangular fields and doses for three groups of 90 mm×90 mm rectangular fields at an SCD=1 000 mm. One-way analysis of variance was used for comparisons of measurement data conforming to normal distribution between groups.
    Results The QI calculated using the ESFS method was 0.655, with a deviation of −1.80% when compared with the QI calculated using the conventional BJR-25 method (0.667). The corresponding PU, Sw/Sa, and Kq were respectively 0.993, 1.122 5, and 0.996 8 for the ESFS method and 0.994, 1.120 5, and 0.995 6 for the BJR-25 method, with deviations of −0.10%, 0.18%, and 0.12%, respectively. The equivalence of the FFF mode with an SCD=1 000 mm was 80 mm×80 mm, and the measured doses of the three groups of rectangular fields were (97.75±0.43), (97.77±0.24), and (97.79±0.31) cGy, respectively (F=6.15, P=0.06). The measured doses of the three groups of 90 mm×90 mm rectangular fields were (99.03±0.04), (99.10±0.02), and (99.04±0.35) cGy (F=1.83, P=0.18). These results suggest that the ESFS method is suitable for application to the FFF mode.
    Conclusions The QI obtained by the two methods differed by less than 0.2% in the search for PU, Sw/Sa, and Kq values but did not affect the calibration and adjustment of the dose of the CyberKnife system. The direct measurement of QI of the ESFS method eliminated uncertainties due to the conversion of some physical parameters of the standard linear accelerator introduced in the BJR-25 method, offering a convenient and accurate method for QI determination.

     

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