Calculation methods and their accuracy of dose calculation for irregular irradiation fields using multi leaf collimator

在恶性肿瘤的放射治疗中，常采用多叶准直器的不规则照射野，将高剂量累积到原发灶，而减少对周围正常组织的剂量。对不规则射野进行剂量测量，以寻找最佳剂量计算方法及其准确性。对于选定的9个不规则射野，根据每个不规则射野和参考射野（10 × 10 cm）中心参考深度的剂量测量值估计输出因子（射野因子）。评价了A/P法、A/Pe法、Ac/P法、MD-EAC法、克拉克森法、叠加法对不规则射野的剂量计算。MD-EAC法、克拉克森法和叠加法的剂量计算误差均在1 ~ 2%以内，但测量点被屏蔽的射野除外。对6个不规则射野的剂量计算结果令人满意，误差为1%。与其它方法相比，Kahn法对3个不规则野的误差最小，对2个不规则野的误差次之。对于被测点被屏蔽的场地，Kahn方法也和其他方法一样存在较大的误差。对于中心点附近被屏蔽的区域，克拉克森法和卡恩法的误差较大，叠加法的误差最小，证明卡恩法的误差很小。

In the radiotherapy of malignant tumors, the irregular irradiation fields with the multi leaf collimator are often used to accumulate the high dose to the primary lesion and to reduce the dose to the normal tissue around the lesion. Dose measurements were done to find the best dose calculation method for the irregular fields and its accuracy. For the selected 9 irregular fields, Output Factors (Field Factors) were estimated from the dose measurements in the reference depth at the center of each irregular field and of the reference field (10 × 10 cm). The dose calculation of A/P method, A/Pe method, Ac/P method, MD-EAC method, Clarkson's Method, Superposition method were evaluated for irregular fields. The errors of dose calculation of MD-EAC method, Clarkson's method and Superposition method are within 1〜2% except for the field where the measured point was shielded.In order to improve the accuracy of dose calculation, Kahn's method which deals the scatter dose divided to collimator and phantom, was examined. The results of dose calculation were satisfactory for 6 irregular fields with 1% error. By comparing with other methods, the error of Kahn's method was the smallest for 3 irregular fields and secondly smaller for 2 fields. For the field where the measured point is shielded, Kahn's method showed also the large error as other methods. For the field where the point near the center was shielded, the errors of Clarkson's method and Kahn's method were larger than other methods and the error of Superposition method was the smallest.It was confirmed that the error of Kahn's method is totally small.