THE OPTIMIZATION AND INHERENT LIMITATIONS OF 3D CONFORMAL RADIOTHERAPY TREATMENT PLANS OF THE PROSTATE

This paper describes the applications of an inverse planning optimizat ion algorithm to the real clinical problem of prostate cancer. The alg orithm has been designed to compute optimized beam-weights taking full account of three-dimensional spatial information of dose inside the p atient. The algorithm is based on fast simulated annealing, utilizing a cost-function containing both linear and quadratic terms. The linear part of the cost function allows for the implementation of ''short-cu ts'' in the cost-function computation, which reduces the calculation t ime by a factor of about 30. It has been applied to compute optimized beam-weights for a three-field and a seven-field prostate treatment pl an. It is shown for the three-field plan that the optimization algorit hm can reproduce, and even slightly improve on, the results of an expe rienced human planner. For the seven-field plan, the human planner exp erienced difficulty finding beam-weights that gave an acceptable dose distribution. It is shown that the optimization algorithm can achieve good results in this case. The outcome of the optimization of the seve n-field plan prompted an investigation into the best results that coul d be achieved by an ''ideal'' conformal radiotherapy technique. The re sults of this investigation are presented and it is shown that the lim iting factor for conformal therapy of the prostate is the size of the overlap volume between the planning target volume (PTV) and the rectum . Finally, the efficiency and accuracy of fast simulated annealing is compared with that of classical simulated annealing. The former was fo und to be at least 10 times faster for the problem studied.