Is there a role for postimplant dosimetry after real-time dynamic permanent prostate brachytherapy?

PURPOSE: To evaluate the correlation of real-time dynamic prostate brachytherapy (RTDPB) dosimetry and traditional postimplant dosimetry for permanent prostate brachytherapy. METHODS AND MATERIALS: A total of 164 patients underwent RTDPB for clinically confined prostate cancer. Of these 164 patients, 45 were implanted with 103Pd and 119 with 125I. Additionally, 44 patients underwent combined external beam radiotherapy and brachytherapy and 120 patients underwent brachytherapy alone. The postimplant dosimetry with computed tomography was performed at 4 weeks and compared with the RTDPB dose plan using the intraclass correlation coefficient. The millicurie/gram of the prostate volume and the percentage of the minimal dose to 90% of the prostate relative to the prescribed implant dose (D90%) of the RTDPB patients was compared with 400 patients treated with a free-seed technique.
RESULTS: The mean D90% achieved in the operating room and on the 3-week dose plan was 109% (range, 93-139%) and 105% (range, 88-140), respectively. The mean percentage of prostate volume receiving 100% of the prescribed minimal peripheral dose (V100) achieved in the operating room and on the 3-week dose plan was 93% (range, 78-98%) and 91% (range, 64-98%), respectively. The intraclass correlation coefficient for each calculated relationship was 0.586 for D90 (p<0.001), 1.19 for V100 (p=0.135), 0.692 for the urethral D90 (p<0.001), 0.602 for the maximal rectal dose (p<0.001), 0.546 for D90 with 125I (p<0.001), and 0.565 for D90 with 103Pd (p<0.001). A 12% decrease was noted in the millicurie/gram of the isotope, with a 2.5% increase in the D90 comparing RTDPB and the free-seed technique.
CONCLUSION: The results of this study demonstrated a correlation between the dose assessment obtained intraoperatively and postoperatively at 3 weeks. With reliable dose data available in the operating room, our results question the need for routine postimplant dose studies. Furthermore, patients treated with RTDPB received less radioactivity per gram of the prostate with a corresponding small increase in the D90. Future analyses will assess variations in the inverse dose planning rules and the clinical correlation of patients undergoing RTDPB vs. older techniques for toxicity and biochemical outcomes.