Isotope selection for patients undergoing prostate brachytherapy

Purpose: Ultrasound-guided transperineal interstitial permanent prostate brachytherapy (TIPPB) is generally performed with either Pd-103 or I-125. The use of I-125 for low Gleason score tumors and Pd-103 for higher Gleason scores has been suggested based on isotope dose rate and cell doubling time observed in in vitro studies. While many centers follow these isotope selection criteria, other centers have elected to use only a single isotope, regardless of Gleason score. No clinical data have been published comparing these isotopes. This study was undertaken to compare outcomes between I-125 and Pd-103 in matched pair analysis for patients undergoing prostate brachytherapy. Methods and Materials: Six hundred forty-eight consecutively treated patients with clinically confined prostate cancer underwent TIPPB between June 1992 and February 1997. Five hundred thirty-two patients underwent TIPPB alone, whereas 116 received pelvic external beam irradiation and TIPPB. Ninety-three patients received androgen deprivation therapy prior to TIPPB. The prescribed doses for TIPPB were 160 Gy for I-125 (pre-TG43) and 120 Gy for Pd-103. Patients treated with combination therapy received 41.4 or 45 Gy (1.8 Gy/fraction) external beam irradiation followed by a 3- to 5-week break and then received either a 120-Gy I-125 or a 90-Gy Pd-103 implant. Until November 1994, all patients underwent an 125I implant after which the isotope selection was based on either Gleason score (Gleason score 2-5:I-125; Gleason 5-8:Pd-103) or. isotope availability. A matched pair analysis was performed to assess any difference between isotopes. Two hundred twenty-two patients were matched according to Gleason score, prostate-specific antigen (PSA), and stage. PSA relapse-free survival (PSA-RFS) was calculated based on the American Society for Therapeutic Radiology and Oncology (ASTRO) Consensus Group definition of failure. Kaplan-Meier actuarial survival curves were compared to assess differences in pretreatment PSA and Gleason score. Results: Univariate analysis of the 648 patients identified Gleason score, pretreatment PSA value, and stage as significant factors to predict PSA-RFS, but failed to identify isotope selection as significant. To address the significance of isotope selection further, the matched pair groupings were performed. The minimum follow-up for all 222 matched patients is 24 months with a median follow-up of 42 months (24-82). The actuarial PSA-RFS at 5 years for all 222 patients is 86.5%. One hundred eleven of the 222 matched patients received a Pd-103 implant with an 87.1% 5-year PSA-RFS. The remaining 111 patients underwent a I-125 implant with an 85.9% 5-year PSA-RFS (p = n.s.). Analysis of Gleason score subgroups 2-4, 5-6, and 7-9 failed to show any significant difference in PSA-RFS comparing isotopes. Pretreatment PSA subgroups of less than or equal to 10 or >10 ng/ml also failed to show any significant difference in PSA-RFS survival comparing isotopes. Analysis of postimplant dosimetry using dose delivered to 90% of the prostate volume (D90) did not identify any difference between the isotope groups. Conclusions: This matched pair analysis failed to demonstrate a difference for I-125 and Pd-103 in PSA-RFS for patients undergoing TIPPB. In addition, there were no observed advantages for either I-125 or Pd-103 in either the low or high Gleason score groups. This data indicates that the role of isotope selection for patients undergoing TIPPB requires further clarification. (C) 1999 Elsevier Science Inc.