SIMULTANEOUS-OPTIMIZATION OF DYNAMIC MULTILEAF COLLIMATION AND SCANNING PATTERNS OR COMPENSATION FILTERS USING A GENERALIZED PENCIL BEAM ALGORITHM

A very flexible iterative method for simultaneous optimization of dyna mic multileaf collimation, scanning patterns and compensation filters has been developed. The algorithm can account for and optimize almost all the degrees of freedom available in a modern radiation therapy cli nic. The method has been implemented for three dimensional treatment p lanning. The algorithm has been tested for a number of cases where bot h traditional wedge filters and block collimators, and modern equipmen t such as scanned beams and multileaf collimators are available. It is shown that the algorithm can improve heavily on traditional uniform d ose plans with respect to the probability of achieving tumor control w ithout causing severe complications (P-+) simply by finding the optima l beam weights and block collimator settings. By allowing more complex equipment to deliver the dose and by accounting for their increased f lexibility during the optimization, the dose plan can be substantially improved with respect to the applied objective functions. It is demon strated that flexible lateral collimation combined with compensators o r scanned beams in most cases allow close to optimal dose delivery. He re both the calculation time and the amount of primary computer memory needed has been reduced by performing the dose calculations in a cone beam coordinate system allowing the use of approximately spatially in variant energy deposition kernels. A typical calculation time for opti mization of a two-field technique in a three dimensional volume is abo ut 20 s per iteration step on a Hewlett-Packard 735 workstation. A wel l converged solution is normally obtained within about 50-100 iteratio ns or within 15-30 min.