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Two accurate algorithms for calculating the energy fluence profile in inverse radiation therapy planning

Two accurate algorithms for calculating the required incident energy fluence distributions from the optimal irradiation density distribution in inverse radiation therapy planning have been developed. The algorithms are characterized by a high speed and accuracy and an ability to handle both divergent and parallel beams even for extremely heterogeneous target volumes. The fastest algorithm is based on a longitudinal distance weighting method, whereas the slower but more accurate algorithm uses an area weighting method which has the advantage that it also works very well at low spatial resolutions. Both algorithms have been inverted for forward calculation of the delivered absorbed dose distribution from known fluence profiles. This is essential for the validation of the resultant dose distributions since the projected irradiation density has been determined from dose distributions calculated with convolution methods. A shortcoming of the longitudinal distance weighting algorithm for forward calculation is that it requires a higher spatial resolution in the energy fluence to minimize the discretization noise caused by interpolation. The area weighting algorithm on the other hand requires a longer calculation time. When the forward calculation time is also taken into account, the more accurate area weighting method is the most advantageous algorithm from the point of view of total calculation time.