Plan Optimization and Dose Evaluation in Brachytherapy

Today's imaging and computer technology has made significant advances in brachytherapy optimization and evaluation possible. Optimization may be of source strength or of source position (or both). An objective function of achieved versus desired doses is minimized, a global minimum being associated with an analytic solution and a local minimum with an iterative solution. Optimization of source strengths is shown useful in planning low-dose-rate brachytherapy either of cervical cancer, using a remote afterloader with pneumatically loaded cesium 137 sources, or of recurrent enodmetrial cancer, using a vaginal cylinder and iridium 192 seeds in ribbons. Source position optimization, on the other hand, is exemplified by permanent percutaneous implants for prostate cancer or by temporary stereotactic implants for brain tumors with iodine 125 sources of low and high strength, respectively. Evaluation of the true minimum peripheral dose generally requires that three-dimensional images be available. Dose-volume histograms serve to describe both dose uniformity within the treatment volume and the extent to which normal tissues are being irradiated. With respect to the future, deconvolution and simulated annealing techniques are being explored for brachytherapy optimization, and we may expect greater emphasis on the biologically effective dose in brachytherapy evaluation.