Development of techniques for increasing the accuracy of dose calculation in a convolution method used for body stereotactic radiotherapy

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The following studies have been performed:・High-energy x-ray spectra sets from six linear accelerators were reconstructed using about ten energy bins. Each set can be applied accurately to attenuators from low Z materials like water to high Z materials like lead. It was analyzed how each set varies with the off-axis distance.・A pair of primary and scatter dose kernels in water was constructed for monochromatic radiation at each energy bin using Monte Carlo simulations on the basis of the differential method. The primary dose kernel indicates relatively smaller values around the photon interaction point, when compared with a primary dose kernel yielded with a photon interaction point at the center of a large water phantom. This fact may leads to the increase of dose calculation accuracy around the jaw and MLC field borders when using primary dose kernels based on the differential method.・The in-water doses, calculated using a convolution method with sets of primary and scatter dose kernels yielded for respective energy bins, are generally within ±1% when compared with measurements. Namely, it can be understood that when the convolution is performed for monochromatic radiation at each energy bin, the beam hardening effect does not need to be taken into account.・With respect to the calculation of in-air x-ray intensity at a point, a method of using error functions on the x-ray target and flattening filter planes has been developed. For an open jaw field, the error functions are used in the area visual via the jaw collimators from the point. For an MLC field, a leaf-field output subtraction method is introduced using the error functions in the area visual via the jaw collimators from the point and using the error functions in the are a visual via each MLC leaf from the point.