Prostate brachytherapy in patients with prostate volumes ≥ 50 cm: dosimetic analysis of implant quality

Objectives: Permanent implantation with ¹²⁵I in patients with localized prostate cancer who have prostate volumes ≥ 50 cm³ is often technically difficult owing to pubic arch interference. The objective of this study was to describe dosimetry outcomes in a group of patients who were implanted using the real-time ultrasound-guided technique who had prostate volumes ≥ 50 cm³.

Materials and Methods: A total of 331 patients received an ¹²⁵I prostate seed implant from January 1, 1995, to June 1, 1999, of whom 66 (20%) had prostate volumes ≥ 50 cm³ at the time of the procedure. The real-time seed implant method was used in all patients and consisted of intraoperative planning and real-time seed placement using a combination of axial and sagittal ultrasound imaging. Pubic arch interference was managed using an extended lithotomy position or by angling the tip of the ultrasound probe in an anterior direction. No preimplant pubic arch CT scan study was performed and no patients were excluded from treatment because of prostate size. Implant quality was assessed using CT-based dosimetry performed 1 month postimplant. Dose–volume histograms for the prostate, bladder, rectum, and urethra volumes were generated. The target dose for these implants was 160 Gy and an adequate implant was defined as the dose delivered to 90% of the prostate (D90) ≥ 140 Gy. The dose delivered to 95% of the prostate (D95) and doses to 30% of the rectal (DRECT30) and urethral (DURE30) volumes were also calculated.

Results: Prostate volumes in the 66 patients ranged from 50 to 93 cm³ (median 57, mean 61 cm³). Total activity implanted was 27.8–89.1 mCi (median 57 mCi), with a range in activity per seed of 0.36–0.56 mCi (median 0.4 mCi). The prostate D90s and D95s ranged from 13,245 to 22,637 cGy (median 18,750) and 11,856 to 20,853 cGy (median 16,725), respectively. Only one patient (1.5%) had a D90 < 140 Gy. The DURE30 values ranged from 15,014 to 27,800 cGy (median 20,410) and the DRECT30 values were 3137–9910 cGy (median 5515).

Conclusion: Implantation of the large prostate can be accomplished using the real-time method. A total of 98.5% of the patients receive a high-quality implant. In addition, these implants should not put patients at increased risk for significant urinary and bowel complications because urethral and rectal doses can be kept at acceptable levels.