Dose specification for realtime computer-generated implants

Introduction: The prescription of dose in radiotherapy requires an unequivocal definition of what that dose represents. Traditionally, concepts such as Basal Dose points were used for this purpose. This paper describes a type of implant in which needles are not arrayed in neat rows and planes, but rather in an arbitrary arrangement. Definition of basal dose points is not possible. An alternative dose specification method is presented. Methods and Materials: Since November, 1991, we have treated locally advanced prostatic cancer with a combination of external beam and high dose rate interstitial brachytherapy. An intraoperative planning system was developed for this purpose. In this system, needle positions are calculated by the planning system to provide uniform dose and minimize trauma. The needle locations are therefor irregular. The minimum dose in the volume is approximated by generating a large number of dose points (called search points), each one midway between a randomly selected pair of dwell positions. The search point with the lowest dose is assumed to represent the minimum dose in the volume. This point is selected as the reference point, to which the prescription dose is then related. The characteristics of this method and consequences of its use are described. For implants with regular rows and planes, this new method is compared with the Paris System. Results: In 265 implants performed to date, this method produces very stable results for implants with either regular or irregular (i.e. 'optimized') needle spacing. It is rapid and completely automated, making it well suited for intra-operative use. For implants using regular geometry, comparisons have been made with the Paris System. For such cases this method produces results identical to the Paris System