The impact of field shape optimization: A feasibility study

The impact of field shape optimization is studied for prostate type geometry. For this study, 76 and 81 Gy plans were generated. Dose distributions for wedged plans and Intensity Modulated (IM) plans for three and seven fields were compared for a quadratic cost function. For wedged plans, a Simulated Annealing Algorithm (SAA) was used to optimize gantry angles, wedge angles, beam weights and field shapes. Two kinds of wedged plans were generated: 1) field sizes were determined by the requirement of full target coverage in the beam's-eye-view (fixed fields) and 2) the field shape, in particular at the critical organ target overlap region was also among the variables optimized. For IM plans the SAA was used to optimize gantry angles and a conjugate gradient algorithm was used to optimize the IM beam fluences. Both the field shape optimized wedged plans and IM plans had significantly superior dose area histograms of the target, rectum and the bladder and cost function values compared to the fixed field optimized wedged plans.