Evaluation of dose-volume histograms after prostate seed implantation. 4-year experience.

Background and Purpose: Permanent interstitial brachytherapy by seed implantation is a treatment alternative for low-volume low-risk prostate cancer and a complex interdisciplinary treatment with a learning curve. Dose-volume histograms are used to assess postimplant quality. The authors evaluated their learning curve based on dose-volume histograms and analyzed factors influencing implantation quality. Patients and Methods: Since 1999, 38 patients with a minimum follow-up of 6 months were treated at the authors' institution with seed implantation using palladium-103 or iodine-125, initially using the preplan method and later real-time planning. Postimplant CT was performed after 4 weeks. The dose-volume indices D90, V100, V150, the D m a x of pre- and postplans, and the size and position of the volume receiving the prescribed dose (high-dose volume) of the postplans were evaluated. In six patients, postplan imaging both by CT and MRI was used and prostate volumes were compared with preimplant transrectal ultrasound volumes. The first five patients were treated under external supervision. Results: Patients were divided into three consecutive groups for analysis of the learning curve (group 1: n = 5 patients treated under external supervision; group 2: n = 13 patients; group 3: n = 20 patients). D90 p o s t for the three groups were 79.3%, 74.2%, and 99.9%, the V100 p o s t were 78.6%, 73.5%, and 88.2%, respectively. The relationship between high-dose volume and prostate volume showed a similar increase as the D90, while the relationship between high-dose volume lying outside the prostate and prostate volume remained constant. The ratio between prostate volumes from transrectal ultrasound and CT imaging decreased with increasing D90 p o s t , while the preplanning D90 and V100 remained constant. The different isotopes used, the method of planning, and the implanted activity per prostate volume did not influence results. Conclusion: A learning curve characterized by an increase in the D90 p o s t can be observed and results in a stable technique after 18 patients. An important factor influencing the learning curve in addition to the precision of seed positioning is organ volume definition on postimplant imaging.