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

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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.