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，常采用多叶准直仪的不规则照射场，将高剂量累积到原发病灶，减少对病灶周围正常组织的剂量。通过剂量测量，找出了不规则场的最佳剂量计算方法及其精度。对选取的9个不规则场，根据每个不规则场中心参考深度和参考场（10 × 10 cm）的剂量测量值估算输出因子（场因子）。对不规则场的A/P法、A/Pe法、Ac/P法、MD-EAC法、克拉克森法、叠加法进行剂量计算评价。MD-EAC法、克拉克森法和叠加法的剂量计算误差在1 ~ 2%之间，除测点被屏蔽的场外。为了提高剂量计算的精度，研究了将散射剂量分成准直器和幻体的Kahn方法。6个不规则场的剂量计算结果令人满意，误差为1%。与其他方法相比，Kahn方法对3个不规则场误差最小，对2个不规则场误差次之。对于被遮挡的测点，Kahn的方法与其他方法一样误差较大。对于近中心点被遮挡的场，Clarkson法和Kahn法的误差较大，叠加法的误差最小。结果表明，卡恩方法的误差很小。

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.