2 ACCURATE ALGORITHMS FOR CALCULATING THE ENERGY FLUENCE PROFILE IN INVERSE RADIATION-THERAPY PLANNING

Two accurate algorithms for calculating the required incident energy f luence distributions from the optimal irradiation density distribution in inverse radiation therapy planning have been developed. The algori thms are characterized by a high speed and accuracy and an ability to handle both divergent and parallel beams even for extremely heterogene ous target volumes. The fastest algorithm is based on a longitudinal d istance weighting method, whereas the slower but more acurate algorith m uses an area weighting method which has the advantage that it also w orks very well at low spatial resolutions. Both algorithms have been i nverted for forward calculation of the delivered absorbed dose distrib ution from known fluence profiles. This is essential for the validatio n of the resultant dose distributions since the projected irradiation density has been determined from dose distributions calculated with co nvolution methods. A shortcoming of the longitudinal distance weightin g algorithm for forward calculation is that it requires a higher spati al resolution in the energy fluence to minimize the discretization noi se 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 meth od is the most advantageous algorithm from the point of view of total calculation time.