Optimization of planar high-dose-rate implants

Purpose: Brachytherapy has long been used to deliver localized radiation to the breast and other cancer sites. For interstitial implants, proper sourc e positioning is critical in obtaining satisfactory dose distributions. The present work examines techniques for optimizing source guide placement in high-dose-rate (HDR) biplanar implants, and examines the effects of subopti mal catheter placement. Methods and Materials: Control of individual dwell times in HDR implants al lows a high degree of dose uniformity in planes parallel to the implant pla nes. Biplanar HDR implants can be considered optimized when the dose at the implant center is equal to the dose at the symmetric target boundaries. It is shown that this optimal dose uniformity is achieved when the interplana r separation is related to the target thickness T through the direct propor tionality, s = T/root 2. To quantify the significance of source positioning , the average dose and a related quantity, equivalent uniform dose (EUD), m ere calculated inside the treatment volume for two conditions of suboptimal catheter geometry. In one case, the interplanar spacing was varied from 1 cm up to the target thickness T, while a second study examined the effects of off-center placement of the implant planes. Results: Both the average dose and EUD mere minimized when the interplanar spacing satisfied the relationship s = T/root 2. EUD, however, was signific antly smaller than the average dose, indicating a reduced relative cell kil ling in the high dose regions near the dwell points. It was also noted that in contrast to the average dose, the EUD is a relatively weak function of catheter misplacement, suggesting that the biological consequences of subop timal implant geometry may be less significant than is indicated by the inc rease in average dose. Conclusion: A concise formula can be used to determine the interplanar sepa ration needed for optimal dose uniformity in Manchester-type implants. Devi ations from optimal source geometry result in an increase in the average do se inside the treatment volume, but the weaker dependence of the EUD sugges ts that the surviving fraction of cells may not be not strongly affected by suboptimal source geometry. (C) 1999 Elsevier Science Inc.