OPTIMIZING THE MOVEMENT OF A SINGLE ABSORBER FOR 1D NONUNIFORM DOSE DELIVERY BY (FAST) SIMULATED ANNEALING

A new simplified technique for 1D non-uniform dose delivery using a si ngle dynamic absorber, driven by a computer system, has been recently proposed together with a simple analytic algorithm. This technique use s an optimized 'stepped' absorber's speed profile and the generated fl uence profile is an approximation of the desired radiation beam. In th e case of non-uniform beam profiles with multiple maxima/minima, the o riginal proposed 'stepping algorithm' has some limitations and produce s a too rough approximation of the desired profiles. In order to incre ase the agreement between desired and generated profiles, more sophist icated optimization schemes are required. In this paper we have applie d a variant of simulated annealing (SA) as a statistical optimization algorithm to further investigate the possibilities and the limits of t he single-absorber technique in the field of 1D intensity modulation. In the current application the cost function used is the mean square r oot of the percentage differences between desired and generated profil es, the absorber's resting times have been chosen as optimization vari ables and at each iteration just one variable is randomly changed, add ing an incremental 'grain'. A Cauchy generating function is used, diff erent cooling schedules are evaluated; constraints related to our appa ratus are introduced and starting annealing parameters are set after s ome initial optimization tests. The method is tested in reproducing th eoretical non-uniform beams, by comparing desired modulated fluence pr ofiles with calculated fluence profiles obtainable with the single abs orber after the derivation of optimized speed profiles by the proposed SA approach. The results of these simulations show that the applicati on of the SA method optimizes the single absorber's performance and th at clinically important modulated beams useful for conformal radiother apy can be accurately reproduced.