Impact of learning curve and technical changes on dosimetry in low-dose brachytherapy for prostate cancer.

PURPOSE: To assess the impact of experience and technical changes on peri- and postimplantation (1 month later) dosimetry for permanent prostate brachytherapy (PPB). PATIENTS AND METHODS: From July 2003 to May 2010, 150 prostate cancer patients underwent low-dose, loose-seed I(125) PPB as monotherapy with intraoperative planning. Patients were divided into three groups-P1 (n = 64), P2 (n = 45), P3 (n = 41)-according to the technical changes that occurred during the study period: use of an automatic stepper at the beginning of P2 and a high-frequency ultrasound probe in P3. Peri- and postimplantation dosimetric parameters (on day 30) were reported: D90 (dose received by 90% of prostate volume), V100 and V150 (prostate volume receiving, respectively, 100% and 150% of the prescribed dose), D2 cc and D0.1 cc (doses received by 2 cc and 0.1 cc of the rectum), R100 (rectum volume that received 100% of the prescribed dose), and D10 and D30 (doses received by 10% and 30% of the urethra, only during peri-implantation).
RESULTS: We observed a decrease in the number of needles and seeds used over time. The mean peri-implantation D90 was 187.52 Gy without a significant difference between the three periods (p = 0.48). The postimplantation D90, V100, and V150 parameters were, respectively, 168.3 Gy, 91.9%, and 55% with no significant difference between the three periods. The peri-implantation and postimplantation D0.1 cc and R100 significantly decreased over time; on day 30: D0.1 cc P1 = 223.1 Gy vs. D0.1 cc P3 = 190.4 Gy (p = 8.10(-5)) and R100 P1 = 1.06 cc vs. R100 P3 = 0.53 cc (p = 0.0008).
CONCLUSION: We observed a learning curve for the implantation parameters, which led to a significant decrease in the rectal doses without having any impact on the prostate dosimetric parameters.