A SELF-COLLIMATING CONVOLUTION BACKPROJECTION ALGORITHM FOR OPTIMIZING DOSE DISTRIBUTIONS OF I-125 PROSTATE IMPLANTS

The algorithm presented here for optimizing brachytherapy dose distrib utions is based on the idea that the seed distribution can be modeled as an activity distribution determined analogously to gamma camera ima ging. The peripheral dose to the tumoris converted to a set of uncolli mated projection data that are then filtered and backprojected to prod uce an initial seed distribution. The actual doses resulting from the seed placement are used to correct the initial projection data for att enuation, scatter, and lack of collimation. The corrected projection d ata are backprojected a second time to yield the optimized but unconst rained seed distribution. Clinical constraints such as the number of d ifferent seed activities, the maximum seed activity, the minimum perip heral tumor dose, and the minimum percentage of the volume which recei ves less than a specified dose are then applied to the unconstrained s olution. Through the entire process, the dose calculations are functio ns of source anisotropy, scatter, and attenuation. When applied to a s et of elliptical contours, the algorithm produces elliptical periphera l dose isodose contours and reasonable dose volume histograms for a co nstrained solution. The results for actual patient prostate contours w ere not as good, primarily because of the difficulties encountered in dealing with the irregular geometry of the prostate. However, the algo rithm shows promise for further research. (C) 1997 American Associatio n of Physicists in Medicine.