Background: We tested the hypothesis that the shorter half-life of Pd-103 versus I-125 results in different late radiation-related morbidities following prostate brachytherapy.
Methods: As of June 14th, 2002, 352 of a planned total of 600 patients with 1997 American Joint Committee on Cancer (AJCC) clinical stage T1c-T2a prostatic carcinoma (Gleason grade 2-6, PSA 4-10 ng/mL) had been randomized to implantation with I-125 (144 Gy, TG-43) or Pd-103 (125 Gy, NIST-99). Treatment-related morbidity was monitored by questionnaires based on standard American Urologic Association (AUA) and Radiation Therapy Oncology Group (RTOG) criteria that were mailed at 1, 3, 6, 12, 18, and 24 months after implant. The use of alpha-blockers to relieve obstructive symptoms was not controlled for but was noted at each follow-up point. All patients reported here had a minimum follow-up of 2 years. Dosimetric parameters analyzed included the V100, which was defined as the percentage of the postimplant prostate volume covered by 100% of the prescription dose. Rectal doses were expressed as the R100, defined as the rectal volume (cc) that received at least 100% of the prescription dose. Statistical comparisons were by Student's unpaired t-test at specified follow-up times.
Results: The AUA scores peaked at the 1-month postimplant time point for both isotopes and gradually declined. The difference in AUA scores between patients who received I-125 versus those who received Pd-103 was greatest at 1 and 6 months following implantation. At 1 month, I-125 patients had a mean AUA score of 14.8 (+/-9.5) compared with 18.6 (+/-9.8) for the Pd-103 patients (P = 0.0009). By 6 months, mean AUA scores for the I-125 patients had decreased to 12.0 (+/-9.1) compared with 9.9 (+/-8.7) for the Pd-103 patients (P = 0.04). The use of alpha-blockers was similar between groups at all time points. Radiation proctitis (persistent bleeding) occurred in 29 of 314 patients (9%). There was an overall trend toward more proctitis in I-125 patients (P = 0.21). However, only four of the 163 patients (2%) with an R100 below the recommended 1.0 cc developed bleeding, which did not differ between isotopes (P = 0.49).
Discussion: Patients treated with Pd-103 had more intense radiation prostatitis in the first month after implantation, but they recovered from their radiation-related symptoms sooner than I-125 patients, consistent with palladium's shorter half-life. The trend toward more proctitis in the I-125 patient group likely reflects their higher R100 values due to less rapid dose fall-off that can be overcome with judicious treatment planning and implant execution.