INTRAOPERATIVE OPTIMIZATION OF NEEDLE PLACEMENT AND DWELL TIMES FOR CONFORMAL PROSTATE BRACHYTHERAPY

Purpose: Traditionally, transperineal prostate brachytherapy has been heavily operator dependent. To overcome this limitation, a treatment p lanning method was developed for intraoperative planning, guidance, an d evaluation. In this setting, reliability, speed, and ease of underst anding are primary considerations. This planning method has been imple mented for ultrasound guided implants of the prostate, but can be exte nded for use in other body sites. Methods and Materials: The length an d cross-section of the target(prostate) location of urethra and rectum are determined intraoperatively from live ultrasound imaging. The pla nning program then automatically generates a ''reference plan'' contai ning needle locations, dwell times, and the resulting isodose distribu tion. As needles are placed, this information is corrected to account for any deviation of needle placement or movement of the prostate. Onc e all needles are in place, the normalization is adjusted to reconcile remaining hot-spots with coverage of the target volume. Optimization is performed in three separate stages. Each stage works to enhance onl y a subset of the implant parameters. (a) Pattern Optimization attempt s to find the most appropriate placement for the needles or catheters. It is based on the transverse contour of the target volume. Needles a re placed uniformly around the perimeter, and interior needle position s are determined from the cross-sectional area and shape. Critical str uctures such as the urethra are explicitly avoided. This step provides the overall framework for the implant, and is not generally repeated. (b) Relative Dwell Time Optimization selects relative dwell times tha t will give the best uniformity of dose. It works by setting the relat ive dwell time in each source position inversely proportional to the d ose delivered to that point by the other source positions. It is used in the reference plan, and is repeated as each needle is inserted. Thi s provides dosimetric feedback to the physician, who can judge the eff ect of deviations from the reference plan. (c) Relative Volume Optimiz ation is an interactive method for fine tuning the normalization based upon volume analysis. The volume analysis is presented in tabular and graphical form, both being updated rapidly as the normalization is ad justed. The information is formatted to help the operator judge covera ge and uniformity. Special functions are provided that allow the opera tor to ''jump'' to special normalization values based on several indic es of uniformity or uniformity/coverage.Results: This system overcomes some conventional brachytherapy limitations. Rather than depend on th e operator's intuitive judgement of where the needles should be placed , a global plan is generated and validated with full dose calculations . Immediate feedback is provided concerning the adequacy of placement and avoidance of critical structures. This information is provided in terms of actual tissue doses to the target volume and critical structu res using point doses, isodose distributions, and volume analysis. Sin ce the new method was introduced in January 1994, 33 implants have bee n performed. The needle placement method has been reliable in the clin ic, with different doctors producing similar results on subsequent fra ctions for the same patient. Conclusion: The method of decomposing the optimization problem into several simple steps is capable of into sev eral rapidly, consistently, and reliably designing conformal treatment plans of high uniformity. Operator dependence has been significantly reduced. We are adapting the method for other anatomic sites.